Another CIS question

[Parr]

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: CIS Idle Speed and Mixture Setting Without an Analyzer

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?

[schumicat]

replace 02 sensor.

[jrbennett]

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

[Parr]

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.

[tirwin]

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

[T77911S]

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.

[Parr]

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

[Parr]

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

[MotoSook]

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.

[manbridge 74]

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.

[schumicat]

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.

Originally Posted by Parr

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

[Parr]

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?

[don gilbert]

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?

[Parr]

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

[don gilbert]

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.

[tirwin]

Jim's Basement Workshop CIS Primer:

CIS Primer for the Porsche 911

[schumicat]

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.

Originally Posted by Parr

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?

[tirwin]

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.

[Parr]

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

[Parr]

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.