CIS pressure results - Now what?

[Averitt]

New poster here, and let me start by saying the information I've received just from searching the forum's has been invaluable, so thanks for what you've already done!

Whew, now that that's over I have an issue I hope someone can help me. I ran a pressure test (actually three) on my CIS system and now that I have the pressure readings, I'm not sure what they are trying to tell me.

First, I have a 79 911SC targa US model that I purchased a year ago. I have had a lot of fun in it, but it has the infamous hard warm start issue. If I go somewhere and turn off the engine and attempt to leave within thirty minutes or so, or even five minutes, it will either not start or takes a very long time to start. It has gotten to the point where I don't go anywhere, but rather leave the house on long drives but don't turn the car off until I get back home. That's no way to drive a Porsche! It is one of those winter projects I promised myself I'd get to, but here it is spring and I'm just now working on it. Nevertheless, the results of my tests at 57 degrees F and 14 degrees C are:

System pressure (valve shut) 5.0 bar
Cold Control pressure (valve open, WUR unplugged) 2.0 bar
Warm Control Pressure (valve open, WUR plugged in) 2.8 bar
Residual pressures : 2.2 bar after 2 minutes; 1.8 bar after 5 minutes; 1.6 bar after 10 minutes; 1.5 bar after 15 minutes; 1.4 bar after 25 minutes; 1.3 after 35 minutes and 1.2 bar after 40 minutes. Also, the resistance of the WUR read 25.7.

I believe this means my WUR (045) needs to be rebuilt, but I've read some threads that suggest those readings are OK so I'm a little confused. Any help?

And here's a picture....

[Averitt]

I do have a new battery which seemingly helped the situation somewhat. I would drive around for an hour but not turn the car off until I got back home. Last night I went for some gasoline and sure enough after a five minute drive and turning the car off to fuel, it did not want to start back up. Fortunately, well I planned for it, the drive way at the dispensers slopes downward, so I was able to bump start it and get it home.

[wolds]

OK, so when you are in a cold start situation (meaning the car has sat overnight and has cooled off) the car starts and doesn't have any driveability issues while it is warming up. Is that correct? If so, the issue most likely is not with the WUR. Do you have the spec for residual pressure handy? I don't recall off hand what it is, but you could have an issue with that. Maybe a FP check valve or an accumulator issue. What's your history with the car? Have you had it awhile and this issue just started or have you been living with it for awhile? Warren

[boyt911sc]

Originally Posted by Averitt

New poster here, and let me start by saying the information I've received just from searching the forum's has been invaluable, so thanks for what you've already done!

Whew, now that that's over I have an issue I hope someone can help me. I ran a pressure test (actually three) on my CIS system and now that I have the pressure readings, I'm not sure what they are trying to tell me.

First, I have a 79 911SC targa US model that I purchased a year ago. I have had a lot of fun in it, but it has the infamous hard warm start issue. If I go somewhere and turn off the engine and attempt to leave within thirty minutes or so, or even five minutes, it will either not start or takes a very long time to start. It has gotten to the point where I don't go anywhere, but rather leave the house on long drives but don't turn the car off until I get back home. That's no way to drive a Porsche! It is one of those winter projects I promised myself I'd get to, but here it is spring and I'm just now working on it. Nevertheless, the results of my tests at 57 degrees F and 14 degrees C are:

System pressure (valve shut) 5.0 bar
Cold Control pressure (valve open, WUR unplugged) 2.0 bar
Warm Control Pressure (valve open, WUR plugged in) 2.8 bar
Residual pressures : 2.2 bar after 2 minutes; 1.8 bar after 5 minutes; 1.6 bar after 10 minutes; 1.5 bar after 15 minutes; 1.4 bar after 25 minutes; 1.3 after 35 minutes and 1.2 bar after 40 minutes. Also, the resistance of the WUR read 25.7.

I believe this means my WUR (045) needs to be rebuilt, but I've read some threads that suggest those readings are OK so I'm a little confused. Any help?

And here's a picture....

Averitt,

How long was the WUR electrical plug been connected when you took the reading for the WCP (warm control pressure)? The WCP (2.8 bar/41 psi) was too low!!!! You have only 0.8 bar differential for CCP and WCP. Are you sure the WCP had already stabilized and reached the max.?

My first reaction was to suspect the residual fuel pressure. But your numbers showed good and acceptable values.

Tony

[Eli W]

Had the exact same issue as you do now, the fuel accumulator is the first thing I would check.

If it is damaged when the car is warm the fuel turns into a gas in the fuel lines to the motor and when you go to start the car you get vapor lock. It will eventually start once the fuel pump has run long enough to cool the fuel back into liquid.

The accumulator holds a reserve of fuel in liquid form for warm restart, if the diaphragm is damaged all the fuel drains back in to the fuel tank and you get warm start problems.

[ossiblue]

I'm thinking along the same lines as Tony, your warm pressure is too low which may be causing an overly rich warm start condition. Usually, the warm pressure is ~3.2 bars (not exactly sure for a 79, but it is not 2.8, that's for certain.) Typically, symptoms like yours point to a loss of residual pressure, but your numbers appear to be good, assuming they were taken with the valve open. If your heating element is bad, the wur has to warm through the ambient heat of the engine, so you need to be certain the pressure you measured was for a fully warmed bimetallic arm and the pressure is as high as it will possibly go.

[theiceman]

Go do a search for a post made by me . Last year i did a baseline test of my perfectly functioning 78SC , which is the same wur as yours . I posted all my numbers for you to compare to.
Sounds like the typical accumulator issue . Kind of an expensive part to swap to test unless yo know someone with the same car , but if you take the vacum line off the bottom and fuel comes out .. you know the diaphragm is toast.

[Bob Kontak]

This may help the diagnosis. First graph is OP's. Suggest sending to Tony for him to tune it up if you don't want to fuss with the warm pressure tweaking.

CCP is ok at 57F/14C. Residual pressure is ok. Ohms are ok - I think, but don't bet the farm on it. My 090 for the 81 is at your number

[tirwin]

Edit: Bob, you beat me to it!

WUR 911.606.105.05 (Bosch 0 438 140 045): warm control pressure should be 3.2-3.6 bar w/ vac line attached and 2.7-3.1 with it removed.

For WUR 911.606.105.06 (Bosch 0 438 140 069) it should be the same.

Looks like there are slight differences in the cold control pressure between the two models. (The 2nd one has a slightly steeper pressure curve vs ambient temp.)

[2stroke]

The accumulator, and new filter fixed my warm start problem.

[boyt911sc]

Originally Posted by theiceman

Go do a search for a post made by me . Last year i did a baseline test of my perfectly functioning 78SC , which is the same wur as yours . I posted all my numbers for you to compare to.
Sounds like the typical accumulator issue . Kind of an expensive part to swap to test unless yo know someone with the same car , but if you take the vacum line off the bottom and fuel comes out .. you know the diaphragm is toast.

Maybe you meant fuel line instead of vacuum line. BTW, there is always fuel presence in the bottom return fuel line from the fuel accumulator because that is part of the return fuel system. So having some fuel come out from the bottom fuel line and bottom chamber of the FA is not a good test for a defective FA. This topic has been discussed and tested in the past. After you ran your engine, disconnect the bottom fuel line from the FA and you'll find fuel dripping out. And this does not signify you have a defective FA as you claimed or suggested. You have a very systematic and well organized troubleshooting style but you missed this one. HTH.

Tony

[jeffs9146]

Originally Posted by Eli W

Had the exact same issue as you do now, the fuel accumulator is the first thing I would check.

If it is damaged when the car is warm the fuel turns into a gas in the fuel lines to the motor and when you go to start the car you get vapor lock. It will eventually start once the fuel pump has run long enough to cool the fuel back into liquid.

The accumulator holds a reserve of fuel in liquid form for warm restart, if the diaphragm is damaged all the fuel drains back in to the fuel tank and you get warm start problems.

I agree the Fuel Accumulator solved my problem on my 83sc!!

[Averitt]

I have replied in detail, but for some reason my replies keep going to an administrator. Maybe this one will get through...

[Averitt]

hmmmm

[Averitt]

OK, I'll try again. Thank you for your replies! Attempting to answer in order. Yes, when I start it up cold, the car starts great and runs perfectly. As long as I don't turn it off, it runs without a hitch. I have had the car for about a year. I noticed it was hard to start after warmed up, but it always did start. It wasn't until it left me sitting for half an hour, after a five minute stop for pizza and a six pack , that I started researching my problem. I have already changed the accumulator, tried to pick low lying fruit, and when that didn't work I ran a pressure test and have the results you see. As to the residual pressures, that's where I'm confused. Some threads have said that as long as you have 22psi, or about 1.5 bar, which I have for a full thirty minutes then you are ok. Other threads suggest you need more residual pressure than this. Confused. Also, I was asked if the WUR had attained its' highest reading after plugging back up. I wondered about this also, so ran the test again. After a minute and a half it reads 2.8 and after four minutes it still reads 2.8 and doesn't go any higher. Also, that is the reading with the vacuum line attached.

[tirwin]

Sounds like something is up with the WUR. It should be higher after 4 minutes.

After you get the WUR adjusted I would check your AFR afterward. I'd be willing to bet that some previous owner or mechanic adjusted the base mixture to compensate.

[ossiblue]

Tirwin has a good point as many times the mixture screw is adjusted to compensate for wur issues. I would suggest you take a step back and set your wur to spec for both warm and cold pressures. Since your warm pressure is low, you can do the adjustment without removing or disassembling the wur. If you choose to do this, read the following:

Set your warm pressure first. With fuel gauge hooked up, plug in the heating element, jump the fuel pump relay, and turn on the ignition to the "run" position. Monitor the control pressure until the top of wur feels hot and the pressure has risen to its highest reading. Using a socket extension (or something similar) and a small hammer, slowly and carefully tap on the top of the wur in the area between the fuel inlet/outlet. What you are doing is moving the cylinder that holds the pressure diagram, downward onto the pin that controls the pressure. Watch your gauge, work slowly and continue to tap the cylinder down until your gauge reads the correct warm control pressure for your wur.

Once the warm pressure is set, wait until the wur has cooled to ambient temp. Now, unplug the heating element and check the cold pressure. If it has changed, it will be higher than before and you can easily lower it by tapping down the plug for the bimetallic arm.

By putting your wur into spec, you've eliminated it as a problem. It would be a good time to check your A/F ratio as tirwin suggested.

[Averitt]

OK Ossie, going by the majority I'm going to give this a try either today or tomorrow when I get a chance. With a daughter getting married soon my free time isn't mine anymore! Without an O2 sensor nor exhaust analyzer, I'm not sure how to adjust the mixture. I used to use a vacuum gauge back in the day, but not sure that works on these closed systems. Still, I'll figure it out. I have two questions that seem to be sticking in the back of my head.

First, if the control pressure is too low, why does it run flawlessly? Because mixture is used to compensate?

And secondly, if the residual pressure is ok as Bob and Tony have mentioned, then why won't the danged thing start when it's warm?!

Thanks again to all who have chimed in

[tirwin]

Averitt,

The WUR is a device designed to replace the choke in prior engines that used carbs. It gives more fuel to richen the mixture on a cold start. That's when you need a rich mixture. The 12V electrical connection provides power to the WUR when the car is started. The current heats up an arm (the bimetallic strip) with a plunger at the end. When the engine is cold, that plunger sits low. The current heats up the two metals (the "bi" in "bimetallic"). Since the two metals are different they contract at different rates, just like when you flex your arm muscle. This causes the arm and the plunger to rise and close off a valve at the top of the WUR. When the valve closes off, the warm control pressure is raised to normal operating pressure (and as a result the mixture is leaned out to "normal"). The strip was designed so that the time it takes for the plunger arm to rise from the lowest point to the highest point is the amount of time needed to warm the engine. (Once the engine is warm, you don't need the richer mixture anymore, so the WUR has done it's job.) When the engine cools off, the arm in the WUR drops back to it's start position. One more thing. If the 12V connection to the WUR isn't working, then the control arm will rise but it will take longer because it has to get heated by the radiant heat coming off the engine.

What's happening in your case is that the arm isn't closing all the way which leads to an overly rich mixture, so on a warm start (when you don't need the richer mixture), you have a mixture that is too rich so you're flooding the cylinders with too much fuel, making the car hard to start. The procedure ossiblue described will adjust the starting point of the travel of the plunger arm so that your warm control pressure is correct. If you do that, I'll bet your problem will be solved.

The best way to get the AFR set is with an exhaust gas analyzer. Get the WCP sorted out first though. You definitely don't want to mess with AFR before you get that resolved.

[Averitt]

Thank you, Oh Great Tim!