Another CIS question
 

Hello,

I have a 1980 911sc. It runs a little bit rough (some bucking) in the low revs (1500-2200 RPM). I also getting pretty bad MPG, around 20 hwy, barely 14 in the city. I keep reading about people getting 26-7 mpg highway, and I wonder what going on.

I recently found a vacuum leak, and fixed it. Now, when I remove the oil cap with the engine running, the revs drop down slightly as they should. Since I fixed it, my mileage has actually been worse (I'll be lucky to get 12 MPG this time).

I thought maybe my fuel mixture was out of wack, but I see to keep reading that the mixture adjustments only control the idle, and that the Lambda system should be adjusting my mixture (when engine is warm) up to about 3000 RPM.

Does anyone have any suggestions for me? I was thinking about trying the mixture setting adjustment method explained by Motosook in a popular fuel adjustment thread, but I wonder if mixture is my problem at all. Here's the thread: http://forums.pelicanparts.com/porsche-911-technical-forum/232089-cis-idle-speed-mixture-setting-without-analyzer.html

Some other info: I'll start the car cold, it will fire after 2 seconds, immediately idle at 1000, then increase the idle to 1300-1500. After several minutes, the idle comes back down to 900m and it hangs steady there. Before I fixed the vacuum leak, the idle would surge between 950 and 1150. At the time, a friend of mine and my uncle told me they thought it was running rich.

Anyone?

replace 02 sensor.

I agree with replacing the o2 sensor, or at least unplug it and see if it makes a difference

I replaced the O2 sensor today. The car seems to be running better so far. I'll see what kind of mileage I get and update soon. Thanks guys.

Did you try running the car with the O2 sensor disconnected?

control pressures should be checked, make sure you have no more air leaks, replace plugs, cap and rotor, check timing and advance set mixture. if O2 has more than 100k on it, replace it.

I did not try to unplug the sensor first. It's a cheap part and I did not have an record of it being changed in the service history, so I just replaced it.

I'm pretty sure I don't have an other 'major' air leaks. The plugs were changed less than 10k miles ago. I have no idea how to check the timing or the advance set mixture.

The O2 sensor was extremely difficult to remove. Now that I've driven a few days, I'm noticing my foot is not nearly as deep into the gas when cruising around. I'm still waiting to get a new mileage number.

How much should

How much would it cost to have someone set the timing/advanced mixture? Is it something I can/should do myself?

Good grief! I just spent 10 minutes typing a reply and hit the wrong button and loss it all.

Short version:

with engine idling at operating temp, gently (~ 1 mm) lift the air plate and see if the engine stumbles or idle increases. Stumble = rich, idle increase = lean. Some probably compensated for the air leak, no fixed, the engine is idling rich.

Get familiar with the CIS components before you go making adjustments. Throwing things out of wack, then trying to figure out what happened to fix it is one of the hardest things about any repair or troubleshooting task.

And since there is little indication of geographic location I'd say an increase in elevation means less "stumble" and more of a crisp ever-so-slightly-hanging throttle response. Providing oil temps are in check, of course.

at a shop they'd probably charge an hour of time (it would be done in less, but they have to move cars around garage, make money, etc.). you need a CO gauge ($200 for cheap one) and timing light ($15 at harbor freight) to do yourself.

Motosook,

Is it necessary to fool with the mixture since I have an operating Lambda system? I have read a lot of info on this site, but I can't seem to find an answer to that question. Doesn't the lambda system make fuel adjustments. I've never been 100% on this.

Also, if i do need to adjust the mixture, should I unplug the O2 sensor first?

Parr, for Cis you are going to need proper tools before you go any further and screw things up. Get a gage, a dwell meter and a timing light. and any cis book, they are pretty much all the same. Moto Sook, who is also an expert, gave you advice for a quick check, but your timing and control press. needs to be set first thing. Not to hard to work on once you under stand how k jet works, and all the parts are pretty much serviceable, (you can rebuild them) Isnt there a cis primer on Pelican?

My question concerning the lambda system and adjusting the mixture still stands...

the o2 set can only compensate so much for wrong control press, any thing below 45 or over 65 , and it will not keep at stoiche.

Jim's Basement Workshop CIS Primer:

CIS Primer for the Porsche 911

the lambda system adjusts mixture using the frequency valve during closed loop operation. during open loop it does not which is why the mixture still needs to be set. this is probably an oversimplification.

This sounds similar to a problem I had but yours may be worse. I say "had" -- my WUR is currently out being recalibrated by a fellow Pelican.

I had a few issues to sort and this is (fingers crossed) the last one.

First thing is we need to know the type of WUR on your car. There is a 3 digit number like 090 (mine) to the right of the part number.

Disconnect the electrical connection on your WUR and hook up the a multimeter using alligator clips. Take a resistance reading (Ohms) and report back the results. For type 090 it should be 25-26 Ohms range, if it is less (mine was 9.2) that is a problem. When the resistance is too low, the plunger in the WUR closes too quickly making the engine lean out before it should.

Do you have a set of CIS fuel pressure gauges? Next step is to check the system and control pressure.

I'll grab the number off the WUR and give it a test tomorrow. Thanks.

Still haven't had a chance to grab the number or test the WUR. I'll get to it soon. I've been working on the house.

Another peice of information I wanted to share. When it rains, the low end performance is especially bad when the car is cold, even after changing the O2 sensor.

oh i HATE that. there is a "back" button on my phone right below the M. the keypad is small enough as it is so i hit that sucker a lot. makes me want to smash the phone.

I'm really striking out in the Lansing, MI area on finding someone with knowledge of this system with the appropriate equipment to set it up. Looks like I'm going to have to take my car to Munk's in Waterford, MI. Thanks for your help.

Your statement about worse performance when wet makes me think about some threads on the O2 connector failing in the engine bay.
How did the connector look when you replaced the sensor?

As others have pointed out, all you need is a gauge set, a duty cycle/dwell meter and simple tools. CIS basic or CIS K-Jet is pretty easy to understand once you educate yourself about it.

Actually, I think us DIY'ers do a better job of setting up our CIS cars than a shop can because we can adjust and tune for virtually any ambient condition we experience (winter, spring, summer, fall). Heck, Don can tune his car from the driver's seat.

Brian

Worse performance when wet can also be bad/failing plug wires.

Brian

The plug wires are like new and the plugs were changed 10,000 miles ago, so I don't think it's the plugs. The 02 sensor seems to be working properly because the hunting went away as soon as it was changed, and once warm, my idle is 900 on the dot.

When it's wet out, the idle will go way down to to 650, especially after getting off the freeway. I'm guessing it's a rich condition based on reasoning that the little air flow to fuel I am getting is even less when there is thicker/moist air.

I think what I want to do is have someone set up my CIS once, and hopefully they will let me watch them do it. Then, I'll pick up the appropriate tools and adjust it myself going forward. At this point, I'm not comfortable doing anything other than testing the WUR by myself, and I think there is great value to getting my car on an analyizer.

I checked my WUR this morning. The # on it is 072 and I hooked up my multimeter, and I was only getting 12-13 resistance. The car was not warmed up. Should I warm it up to get a more accurate reading? Is my WUR the problem?

Parr,

First, don't sell yourself short. This is not rocket science, although it sure can feel that way at times.

Secondly, let me share my personal experience. I took my car to a shop partly out of frustration, and they "fixed" my issue by unplugging the pressure plate sensor. That is very disingenuous and not a fix at all. So, if you are going to go that route, make dang sure you take it to a reputable place.

The idle dropping that low when it's wet makes no sense to me. Something else is going on. Do you have the OEM style braided plug wires with the ground strap?

You have not yet checked your fuel pressures, right? It is not that hard. If I can do it, so can you. You need a set of CIS fuel pressure gauges and a $10 worth of parts from any auto parts store.

I'll have to look up the -072 but if that resistance is too low, then it means the WUR will lean out the mix too quickly on a cold start. You do not need to warm up the WUR to take a better reading.

You're only testing part of the WUR with the resistance check. You need to know the system pressure, cold control pressure, warm control pressure and residual pressure. This requires the gauges.

Does your oil temp gauge have numbers on it or just the hash marks?

From reading the original post the behaviour of the car seems normal when cold.

The Ohm reading of the WUR is low, the 'fastest' WURs have an Ohm reading of about 18-22.
Tony has got all the testing data (i think he goes by the name of BOYTSC)
My car (a non 02 EURO car has got an Ohm reading of 32 and that is within spec for a 089).


The lower the Ohm the faster the bimetall warms the faster the fuel pressure increases the faster it leans out to normal.

You need to get pressure readings of the WUR before you go further.
You will also need to check the air sensor plate position and the timing at Idle and at 4.000 rpm.

I have struggled a long time with my car and the FP are essential. I have now made a last change to the position of the air sensor plate and that seems to have completed my very long trooubleshooting as it seems to have erradicated the last little glitch.

CIS troubleshooting........
 

Brian,

If you are adjusting and tuning your CIS every season of the year, then you are not setting the parameters correctly. A well maintained CIS engine would start any time of the year regardless of weather. If you have to tweak it seasonally, there is something inherently wrong with your system. Sorry to disagree with your post.

Tony

tirwin,

Yes, I have the braided plug wires.

Alright, I'm dumb on this point: Everybody says to be sure to check your CIS fuel pressures. OK, then what? How does this offer diagnostic help? Not trying to hijack, but I'm just ignorant and interested.

Fuel pressures in CIS.........
 

Jerome,

All fuel injection systems depend heavily on the fuel pressure parameters. If the FP is running and delivering the incorrrect amount of fuel pressure this would be a major problem. Even if the FP is running perfect, if the system pressure is off by a lot from spec, this would be a problem. Even if the FD is good and delivering the correct system fuel pressure, and the control fuel pressure is off by a lot, this could be another problem. Even if the WUR is good and working, and the residual pressure is deteriorating very fast, this would be a problem. And I could add more to these.

So without knowing your fuel pressures (system, control, and residual) from start of your troubleshooting process, you are assuming all these conditions to be on spec. Sometimes you are lucky and they are in spec. But you can not rely on luck all the time for your diagnostic tests.

That is why a fuel pressure gauge kit is MANDATORY for fuel injection troubleshooting regardless what system (FI) you have. Others would chime in about this subject.

Tony

If the pressures are wrong then you can determine if is a problem with the wur or the fuel pumping system. If these check out then you test fuel injectors. If these check out then you can begin to isolate the air side.
You cannot guess, you systematically check each component. If you spend 400 bucks on tools, you will have the tools, and eventually have a car that runs like or in my case better than new.

UPDATE.

I had Munk's of Waterford, MI take a look at my car. They checked control pressures, timing and the fuel mixture. They found the fuel misture was too lean. This is after I richened the mixture slightly after changing the O2 sensor.

So, long story short, the problem was a bad o2 sensor. At some point, someone had compensated for the bad o2 sensor. The bad sensor was causing the frequency valve to dump too much fuel. Once the o2 sensor was replaced, the mixture was way too lean. Now that it is set, I'm not having further issues.