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Cool CIS Troubleshooting for Dummies

I am a dummy. You might be a dummy too. At least when it comes to the CIS system used on the '78-'83 911 SC.

I have learned a lot from many people here. Sometimes it's hard to understand what people mean who are trying to be helpful. You will get some pearl of wisdom, but maybe you don't at first understand why someone said this or that. Later on, the a-ha moment sinks in ("so THAT'S what that guy was trying to tell me!") and you feel like even more of a dummy. This being my first Porsche, I didn't have a great basis of comparison for what "normal" is so I asked a lot of REALLY dumb questions. This is the starting point I am assuming. If I can figure this stuff out with a little help, so can you.

I decided that I had been on the receiving end of a lot of great advice, so I decided to try to summarize and write down all the advice and lessons I learned and put it in one concise thread in an attempt to help others out who face similar problems.

LET ME BE CLEAR. I AM NOT PROVIDING NEW INFORMATION HERE. There are a lot of great people on this forum that have contributed a whole lot more than I ever could. There are also a lot of great books, articles and websites for further reading. All I am attempting to do is document and present the information in a way so as to be helpful to others. I will make some mistakes along the way. Please politely point them out and I will correct them. Constructive criticism is very much appreciated.

There is no need to provide kudos directly in the thread as it just takes longer for people to find the information they are looking for. If you find this thread useful, then please you the "Rate Thread" option to upvote it. If you want to follow the progress then please use the "Thread Tools" to subscribe to the thread.

Note (July 1, 2013): I will be adding content over time as I can, so all the content won't be here immediately, but I am working on it. If you think something is missing or incorrect you can PM me.

Hope this helps!
-Tim

Last edited by tirwin; 07-20-2013 at 11:39 AM..
Old 07-01-2013, 02:25 PM
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Index

1) Recommended Reading - post #3

2) The Pearls of Wisdom - post #4

3) Overview of Potential Problem Areas - post #5

4) Test Procedure for System, Control and Residual Pressures - post #6

5) Finding Vacuum Leaks - post #8

6) Fuel Delivery Problems - post #10

7) AAR, AAV and Decel valve... What's the Difference? - post #16

Last edited by tirwin; 07-20-2013 at 11:39 AM..
Old 07-01-2013, 02:26 PM
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Recommended reading

Jim's Basement Workshop CIS primer:
CIS Primer for the Porsche 911

Bosch Fuel Injection & Engine Management by Charles O. Probst, SAE
Published by Robert Bentley
ISBN: 0-8376-0300-5

How to Tune & Modify Bosch Fuel Injection by Ben Watson
Published by MBI Publishing Company
ISBN: 0-87938-570-7

Porsche 911 SC Service Manual 1978, 1979, 1980, 1981, 1982 1983: Coupe, Targa and Cabriolet by Bentley Publishers
Published by Bentley Publishers
ISBN-13: 9780837617053

Last edited by tirwin; 07-20-2013 at 11:39 AM..
Old 07-01-2013, 02:31 PM
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The Pearls of Wisdom

Please read this before you go about trying to troubleshoot your problem!

Pearl #1: Don't just start swapping parts.

This is expensive. You might get lucky taking this approach. You also might empty your bank account before you find the REAL problem.

Pearl #2: Be methodical.

You have to start somewhere in diagnosing problems, but try to work through problems in a progressive approach rather than jumping all around.

Pearl #3: You might need some tools.

You don't have to have a lot of tools to properly troubleshoot these problems, but you will need some. A set of CIS fuel pressure gauges is really a must. There just isn't much you can effectively do without them. Yes, they may cost $100 or more, but if you are keeping the car for any length of time they are a must-have tool in the toolbox.

Pearl #4: Listen to the advice you are getting.

I didn't understand a lot of the advice I was getting at first. Listen to it. Think about it. Ask more questions if you don't understand. Just don't ignore it. Others have been there, done it and got the T-shirt. In most cases, they are telling you something for a good reason.

Pearl #5: The search function is your friend.

Chances are the question has been asked and answered before. Use the search. If you are still having problems finding the answer to your question, by all means ask the question.

Pearl #6: There is certain information we need to know in order to help you.

When you decide to start a post looking for advice, don't forget to tell us some important information such as:

- What year is your car?
- Is it a US or RoW model?
- Is it the original engine or a different year engine?
- Has it been modified from stock?
- What are your symptoms? Be as specific as possible.
- Did the problem start over time and get worse or did it start suddenly?
- Is there anything that you do that makes the problem better or worse?
- How are the cold starts?
- How are the warm starts?
- Do you have a pop-off valve installed in your airbox?
- Tell us as much history on the car as you think is relevant.

Pearl #7: Don't mess with the fuel mixture right off the bat.

A lot of people make the mistake of messing with the mixture setting hoping that will fix the problem.

A quote from Tony (boyt911sc): "But the most blatant suggestions in the CIS troubleshooting threads is adjusting the fuel mixture for starting or running problem/s. We are not dealing with a carbureted system where you could turn the mixture screw left or right until you get some result. It does not work this way for fuel injection systems."

You might improve your situation, but chances are you are probably covering up another underlying problem. Worst case you are just making it worse and adding more complexity to the problem that needs to get sorted out. And remember that you're only adjusting the idle mixture. If you have gone over everything else, THEN adjust the mixture using the 3mm Allen screw. If you have fixed other problems, you might be changing other factors that will lean or richer the mixture anyway so it's best to save that for last. See Pearls #2 and #4.

Pearl #8: You might have more than one problem.

Don't get frustrated or give up if you fix one thing and find that didn't completely solve the problem. These cars are now 30+ years old. You can get it back to the way it was supposed to run. It is entirely possible that the previous owner didn't know what to do either so something they may have done in an attempt to fix it either just covered up the underlying problem or made it worse. See Pearls #2 and #4.

Pearl #9: Make sure you have the correct part(s) for your year model.

I've noticed a few times that people have posted about a problem only to discover later that they had the wrong part for their year model. I've noticed this especially with the Warm-Up Regulator (WUR). Since some of the CIS components are NLA, maybe this is being more commonplace as people go to places like junkyards hunting for parts. In any case, this is something worth verifying when you begin to troubleshoot problems as it will save you a lot of headache later. Refer to the charts at Jim's Basement Workshop for the correct part numbers for your model year and the Porsche PET for more help.

Last edited by tirwin; 07-20-2013 at 11:40 AM..
Old 07-01-2013, 02:52 PM
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Overview of Potential Problem Areas

1. Vacuum Leaks (unmetered air)

2. Fuel Delivery Problems (leaking injectors, bad fuel pump, etc.)

3. Fuel Pressure (system, control, residual)

4. Air Fuel Mixture (too rich, too lean conditions)

5. Electronics Failure (failures of ECU, O2 sensor, O2 relay, etc.)

Last edited by tirwin; 07-20-2013 at 11:40 AM..
Old 07-01-2013, 02:59 PM
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Test Procedure for System, Control and Residual Pressures

This is the most "bang for your buck" test you can do. Most people who attempt to answer your questions will want to know the results found from performing these tests. If you are looking for help, it is practically impossible to diagnose CIS problems without doing this test. Yes, the gauges are not cheap. If you are keeping the car for any length of time, this is a must-have tool in the toolbox. If you are really in a pinch, post your location in your profile. Maybe you will get lucky and a helpful forum member will either let you borrow a set or better yet might help you perform the test. Note: pizza and beer have been known to lure fellow Porsche enthusiasts.

What you will need:
1) a set of CIS fuel pressure gauges
2) digital multimeter
3) fused switch (details below)
4) open-end metric wrenches
5) rags
6) eye protection
7) gloves
8) stopwatch or video camera
9) thermometer
10) fire extinguisher(s)
11) shop light
12) Bentley manual

For this test procedure, it is best to let the car be completely cool (such as sitting overnight). You will be disconnecting the fuel lines so if there is still pressure in the fuel system you get a spray of fuel. And if you get a spray of fuel, you don't want that happening around a hot engine.

A word on safety: I'm not the kind of person to belabor this stuff but you are working on the fuel system, ok? Please make sure you keep a fire extinguisher, preferably two, around just in case while you're doing this. Wear some eye protection and gloves and keep some rags handy to catch any excess fuel that spills. If you don't tighten some of the connections enough you might create a fine mist of fuel leaking at the connectors, so check, check and check again during the test and after. And it's always a good idea to keep a fire extinguisher in the car with you, especially after working on the fuel system.

Let's say you're going to do these tests the next day after letting the car sit overnight. There are some things you can do the day before to make things go faster the next day. I'll try to break things down that way to make it easier, but you don't necessarily have to do it exactly this way. The main thing is that the engine needs to cool and the fuel pressure has dropped throughout the system.

The Day Before:
Disconnect the electrical connection to the Warm-Up Regulator (WUR). Take your digital multimeter and attach it to the male spade connectors using alligator clips. Set the multimeter to read resistance (Ohms) and record the result and the ambient temperature. Example: a type -090 WUR should read 25-26 Ohms in warm ambient temps and ~9.5 Ohms when cold. If the resistance is too low, it will cause the mixture to lean out prematurely.

The next thing you'll want to do is make a switch to use to run the fuel pump without having to start the car. This will involve a trip to your FLAPS (Friendly Local Auto Parts Store) for a few simple things. You'll need to buy a switch rated for about 10A or more (my store only had a 50A rated switch at the time), about 6 inches of 16 gauge electrical wire, an in-line fuse holder and 8A fuse, a couple of ring terminals and a couple of bullet-style connectors. You can either solder or crimp your connectors and shrink tube is optional but not necessary.

Assemble the switch like this:
bullet terminal, in-line fuse holder, ring terminal, switch, ring terminal, wire, bullet terminal

I used a pair of needle nose pliers to slightly bend the end of the bullet connector to make it fit in the female end of the fuel pump relay terminals. Some people may say this isn't necessary but I did it so that the bare wire wouldn't come loose while I was trying to perform these tests.

Remove the red fuel pump relay from the fuse panel in the front luggage compartment. (Or the boot or the frunk or whatever you prefer to call it. )

Install your new switch across terminals 87 and 30a of the fuel pump relay and make sure the switch is in the OFF position.

Last thing. Leave your thermometer out in the garage overnight. The charts in the Bentley manual refer to certain pressures at so many degrees Celsius, so make sure you either have a thermometer that reads Celsius or use the following equation to do the conversion:

C = 5/9 * (F - 32)

The Day of the Test:

At this point, the engine should've sat overnight and be cold. Make sure the electrical connection on the WUR is disconnected before starting the test.

The first step is to install the fuel pressure gauges between the WUR and the fuel distributor (FD). Disconnect the port on the WUR going to the FD. This is where you will install the gauges. If your particular set of gauges has the on/off valve on one side or the other of the pressure gauge, make sure you install the gauges with the on/off valve on the WUR side so that the pressure can be read when closed. Your gauge set likely includes the appropriate adapters but it is a good idea to verify beforehand.

Once you have the gauges installed, the first test is the SYSTEM PRESSURE test. With the valve in the CLOSED position, turn the ignition key to the ON position (the last position before starting the car) and turn the switch you made ON to start the fuel pump. You should hear a whirring sound that indicates fuel is being pressurized and sent through the system. Check the system pressure reading and record the result. You can now turn off the fuel pump. Compare your result to the table on page XXX of the Bentley manual. (note: need to fix page reference)

The next test is the COLD CONTROL PRESSURE test. With the valve on the fuel pressure gauge in the OPEN position (and again with the ignition key in the ON position and the fuel pump running), record the fuel pressure and the ambient temp. In most cases, this should be about 1 bar. (Note: I'm not sure the Bentley manual states what the cold control pressure should be.)

Next, you will check the WARM CONTROL PRESSURE. It is called warm control pressure because the electrical connection provides voltage when the fuel pump circuit has power. This applied voltage causes the bimetallic strip in the WUR to slowly move which changes the control pressure. A lower pressure (~1 bar) means a richer mixture and a higher pressure (~3.5 bar) means a leaner mixture. It is for this reason that the WUR is sometimes called the Control Pressure Regulator and this is probably a more accurate term since it regulates pressure all the time and not just when the engine is cold. For this test you will need something to record the elapsed time. You can use something as simple as watching the second hand on a clock, a stopwatch or even the video function on a smartphone. Leave the test running exactly as before from the control control pressure test. Find the electrical connection to the WUR and plug it in. The control pressure will begin to rise. Record the time it takes for the control pressure to rise and reach it's peak value. In most cases, this should be about 3-5 minutes. Compare the peak value to the table on page YYY of the Bentley manual. (note: need to fix page reference)

The final test is the RESIDUAL PRESSURE. Turn off the fuel pump and the turn the ignition back to the off position. Record the pressure reading on the gauge at the following intervals: 5 min, 15 min, 30 min, 60 min. Compare the results to the table on page ZZZ of the Bentley manual. (note: need to fix page reference)

Now you can disconnect your fuel pressure gauges and replace the fuel pump relay. If you need help interpreting the results, post your question.

Also make sure you double-check your fuel connections afterwards. The last thing you want to do is to drive the car later and discover a fine mist of fuel the hard way! And again, make sure you keep a fire extinguisher with you!

If you need to post your test results on the forum for help in interpreting your results, here is a simple template to follow to make sure you post all the necessary information. The conversion factor for psi to bars is 1 bar = 14.5 psi, so a pressure reading of 30 psi is ~2 bars.

1. Year of engine:
2. US or RoW (Rest of World):
3. WUR model number:
4. Ambient temperature at time of test (in degrees C):
5. WUR Resistance (in Ohms):
6. System Pressure (in bars):
7. Cold Control Pressure (in bars):
8. Warm Control Pressure (in bars):
9. Time delta for Cold -> Warm Control Pressure (in minutes & seconds):
10. Residual Pressure @ 5 min (in bars):
11. Residual Pressure @ 15 min (in bars):
12. Residual Pressure @ 30 min (in bars):
13. Residual Pressure @ 60 min (in bars):

Last edited by tirwin; 07-20-2013 at 08:57 PM..
Old 07-01-2013, 03:32 PM
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Outstanding! Good man for posting this.
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Old 07-01-2013, 05:05 PM
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Finding Elusive Vacuum Leaks

Vacuum leaks are a potential problem area. In the 911 SC, there are SO many places that can be sources of vacuum leaks. It is a good idea to rule out many of the most common sources of leaks right at the beginning. The good news is you typically don't have to invest a lot of money to find them.

There are many potential leak sources. Here are a few of the more common ones:

1) Old vacuum lines

There are 43 quadrillion linear feet of vacuum hose in the 911 SC. Ok, not really that many, but it sure does feel like it. After 30 years and a few thousand heat cycles, the old lines can get brittle and crack. A good WYIT (While You're In There) project if you ever have the engine out of the car is to replace all the vacuum lines.

2) Cracked air box

A common story is "My car was running great. Then I started the car one day and it made a loud KER-PLOW! Now it's not running well..." Chances are your car didn't have a pop-off valve installed, you had a backfire and it cracked the airbox. (Or maybe it did have a pop-off valve and it blew anyway!) There is great debate over the necessity of the pop-off valve. I'm not going to wade into that debate here. But if this scenario describes what happened to you, check your airbox for a crack. It may be a hairline crack, but chances are it's there.

More reading: Blown airbox - replace, prevent and while I'm in there Pay particular attention to Peter Zimmerman's posts #10 and #11 on CIS operation theory.

3) Problem with the pop-off valve

Another common story is that the pop-off valve is installed but the car exhibits the same poor running conditions as when there is a blown airbox. Yes, isn't it ironic that the thing that is supposed to be preventing you from blowing your airbox is actually a possible cause of the same symptoms as a blown airbox? At least the pop-off valve is cheaper than a new airbox. This can be a few different issues. First, it is possible that the epoxy seal around the pop-off valve has loosened with age and is allowing false air into the system. Another possibility is that the pop-off valve may be binding or not seating properly after a backfire. It is not altogether uncommon for pop-off valves to be installed backwards either. They should be installed with the hinge side facing the front of the airbox as you face the open engine compartment.

4) Leaks around the fuel injectors

Another common leak source is the injector sleeves and the O-rings which can harden and crack with age. Solution: replace them.

Methods for finding leaks:

The easiest method to quickly determine if you have a significant vac leak is to remove the oil filler cap when the car is running and the engine is warmed up. If the idle RPMs change when you take off the cap, this means you DO NOT have a significant vac leak. You still might have one, mind you. It's just that it might not be that big. If the idle RPMs do not change, that means you DO have a significant vac leak.

Here is a DIY way to check for vac leaks:
Finding CIS vacuum leaks - the vacuum cleaner pressure test
Some people advocate using propane or other combustibles to spray around potential leak sources. This is an easy method that is safer.

The final method is to take your car to a shop that has a smoke machine. The smoke has a chemical in it that is reactive under UV light and leaks can be detected this way.
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Last edited by tirwin; 07-02-2013 at 10:16 AM..
Old 07-01-2013, 06:45 PM
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Thanks for doing this.

I had some CIS issues over the past few weeks that I finally fixed. I did as you search, search. search learned a ton but it would have been a lot less painful to have had this.
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Old 07-01-2013, 08:11 PM
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Fuel Delivery Problems

The basics of the fuel delivery system are:

- fuel tank
- in-tank filter (or screen)
- fuel pump
- fuel accumulator
- fuel filter
- fuel distributor
- fuel injectors
- cold-start injector and thermo-time switch
- frequency valve

For descriptions on each component, refer to Jim's Basement Workshop:
Components

Now is probably a good time to point out that all years models don't always have the same set of components. Here is another link to Jim's Basement Workshop that shows a table of what components are used in each year model:
Parts ID by Year for US models

General guidelines for troubleshooting fuel delivery problems:

1) Lack of Fuel Supply. Is there gas in the tank?

2) Defective fuel pump. Get underneath the car and locate the fuel pump. Have someone try to start the car. Can you feel the fuel pump hum or vibrate? If not, check the fuel pump fuse and relay. You can alternatively use the method outlined above in the section on the fuel pressures test (post #6) to run the fuel pump without the car running. If the fuel pump runs with the switch but not when the relay is installed, the relay is bad (you do keep an extra fuel pump relay in the glove box don't you? If not, you should to keep from getting stranded someplace). You can use a test light connected to the positive side of the fuel pump connector and the ground wire. If the light turns on, then it is receiving power and is grounded. If not, check the ground wire to the fuel pump to ensure it is grounded properly.

3) Restrictions in the fuel lines. Common places are the screen in the tank filter, the fuel filter in the engine compartment and the return line to the tank. If you suspect there is varnish or other blockage, try running some fuel injector cleaner through the system such as Techron or BG44K. If this doesn't help, replace the fuel filter (do you know when the fuel filter was replaced last?). If the car has been sitting for a long time, there could be water and/or sludge in the fuel tank. If this is the case, the tank will need to be emptied, removed and cleaned.

4) Leaking or restricted fuel injectors. The best way to determine if the injectors are functioning properly is to perform a simple test like this: excellent way to test injectors on 78 911 You want to see a fine mist pattern with no drips. Another possible way to see if you have any injector issues is by looking at your spark plugs. If you have any doubts about how to "read" your plugs, pull the plugs one at a time and lay them down on a piece of paper being careful to write the cylinder number down to each plug. Take a picture of all the plugs together on the paper (in good light) and post it on the forum. Another thing to note is if the injectors are spraying an equal amount. Using the method listed above, let the injectors spray into the container for a few seconds. The volume of fuel in the containers should be fairly equal.

5) Problems with the fuel distributor (FD). The fuel distributer (as the name implies) is responsible for distributing fuel to the injectors. The FD is made up of two chambers - upper and lower - that are separated by a diaphragm. The pair of chambers is sometimes called the differential pressure regulator because there is a pressure differential between them. If the FD develops leaks, it won't be able to pressurize fuel properly. Further reading: fuel distributor leak? The FDs are no longer available for sale new but they can be sourced elsewhere and there are several places that will rebuild them as well.

6) Problems with the cold-start injector (or the cold-start valve). The cold-start injector is also sometimes called the cold-start valve. Two of the most common problems with the cold-start injector is that it either doesn't work (making it hard to start the engine when cold) or that it leaks (causing overly-rich running conditions). The cold-start injector receives voltage whenever the starter is engaged and is grounded by the thermo-time switch when the engine is cold (below 95 F). If the engine is cold, the TTS grounds the injector allowing it to run (and only for a few seconds - 10s or less - to avoid flooding the engine).

7) Problems with the fuel accumulator. The accumulator does two things: it evens out the flow of fuel coming from the fuel pump and it helps maintain the fuel pressure. To do this, it has a diaphragm inside. If the diaphragm fails, the system pressure will drop. If the car is running when this happens, it might shut off and not restart.
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Last edited by tirwin; 07-03-2013 at 08:54 PM..
Old 07-01-2013, 08:36 PM
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Many Thanks...

As soon as the temps drop a bit around here (107 yesterday), I'll put some of this info to good use...
Old 07-02-2013, 07:49 AM
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Thanks.
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Old 07-02-2013, 09:31 AM
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Beautiful... Thanks!
Old 07-02-2013, 09:45 AM
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Oxygen relay for CIS.......

Tim,

Why do people keep calling it OXS relay? Go check the wiring diagram and you'll never find one!!!! But there is the FV relay in the system. Just my two cents.

Tony
Old 07-02-2013, 10:52 AM
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Quote:
Tim,



Why do people keep calling it OXS relay? Go check the wiring diagram and you'll never find one!!!! But there is the FV relay in the system. Just my two cents.



Tony
Good question. I don't know but in the Probst book it calls the frequency valve the lambda valve. Some sources call the warm-up regulator the control pressure regulator. This was one of the most confusing things for me early on -- simply understanding terminology and whether or not X was really the same thing as Y. I will try to note where things have more than one possible name to help avoid confusion.

Edit: Tony, I did go and look at the diagram for an '82 and it is called the OXS relay! See here: http://www.pelicanparts.com/911/911_parts/Electrical/911_electrical_82SC_Part6-2.jpg. Or did I miss something?

Last edited by tirwin; 07-05-2013 at 07:31 PM..
Old 07-02-2013, 12:59 PM
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AAR, AAV and Decel valve... What's the difference?

These terms thoroughly confused me when I first started looking into CIS. To make matters worse, they can go by different names and people also tend to get them mixed up (such as by saying AAR when they mean AAV) and the AAV and the Decel Valve kinda even look the same. Heck, I've mixed them up just reading the letters.

I'll try my best to help un-confuse things.

For more info, see boyt911sc answer in post #3 here: What's the difference between AAR and AAV

AAR on the left and AAV on the right. (picture courtesy of boyt911sc)



Auxiliary Air Valve (AAV)

The AAV provides extra air on startup. When the vacuum reaches a certain point, the AAV closes off.

Test procedure: How to test Auxiliary Air Valve

Auxiliary Air Regulator (AAR)

The AAR provides extra air when starting a cold engine. Like the WUR, it has an internal bimetallic strip that is heated via an electrical connection. When the engine is first started, the valve is open. This also causes the higher initial idle at startup. The AAR gets power from the same fuel pump relay circuit as the the WUR. As the strip heats it slowly closes off the valve, reducing the airflow into the throttle and reducing the idle speed.

Test procedure: Is This AAR Good or Bad?

Adjustment: AAR Revisited and Auxiliary Air Regulatory Adjustment

Deceleration Valve

The decel valve (called the "vacuum limiter" in the PET and also sometimes referred to as the "auxiliary air device") is the third component that can bypass air around the throttle. The purpose of the decel valve is to prevent an overly-rich condition when the throttle suddenly closes. The result is a slower throttle response. Some people do not like the slower throttle response so it is not uncommon to find the vac line connection disconnected and plugged, as well as the connection on the decel valve itself. The decel valve looks deceptively similar to the AAV. If the diaphragm in the decel valve leaks, it can cause a high idle.

Last edited by tirwin; 07-20-2013 at 11:41 AM..
Old 07-03-2013, 08:59 AM
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One thing I would like to mention regarding the deceleration valve - Porsche actually calls this part the vacuum limiter (according to the PET).
Old 07-09-2013, 08:15 AM
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Quote:
Originally Posted by Ronnie's.930 View Post
One thing I would like to mention regarding the deceleration valve - Porsche actually calls this part the vacuum limiter (according to the PET).
Noted and editing post above to reflect that. Thanks!
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Old 07-09-2013, 06:52 PM
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Hello
I have a stock '83 engine and was thinking that the WUR was wacky. I recorded the resistance in the WUR both when the engine was cold and when it was warm and it i always 9.3 ohms. Your note says: "Example: a type -090 WUR should read 25-26 Ohms in warm ambient temps and ~9.5 Ohms when cold. If the resistance is too low, it will cause the mixture to lean out prematurely." Our temperature today when I measured this was about 20 degrees C. In the literature referenced here I did not find a reference to this resistance but did find lots of references to pressures. I am a little confused at this point. Could I still have normal pressures and yet have this apparently low resistance?

Thank you.

Dave
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Old 08-19-2013, 08:37 PM
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Dave,

Yes, the pressures can be normal and still have those resistance numbers. The WUR has a bimetallic strip inside. On the end of the strip is a plunger. When the car is started, the bimetallic strip gets 12v. There are two different metals - one on each side of the strip (plunger arm). Hence the term bimetallic. The current causes each metal to contract at a different rate which makes the arm bend sort of like flexing a muscle. When the car is first started, that is the cold control pressure. When the strip heats up and the arm bends it closes off the valve in the WUR, the fuel pressure increases (the warm control pressure). When the resistance is low, it just means that the strip heats too quickly making the control pressure rise too fast. The WUR is essentially part of a system designed to replicate a choke. It's job is to richen the mixture when the car is first started until everything is at normal operating temp. If it closes off too fast, it is leaning the mixture before the rest of the engine is ready.

Last edited by tirwin; 08-20-2013 at 05:23 AM..
Old 08-20-2013, 05:21 AM
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