Cold start & general CIS questions

[reedr11]

I am new to this forum. I have had an '80 911SC for about 5 years and have had no real trouble. I only put about 1,000 miles a year on it. The only work I have done to it includes oil changes, clutch adjustment, and shift linkage adjustment.

Currently I am having cold start idle problems. I have read a lot of useful information in this forum, which has lead me to a few conclusions and given me some thoughts on what to try next. The engine fires up easily, but after a second or two the idle drops to almost nothing and sometimes the engine dies. I'm pretty certain I have a lean condition, but have some work to do to find out why.

Background:
1) Prior to the cold idle problem beginning, the idle speed would hunt. Based on reviewing this forum, I assume that the mixture was rich, at least while the idle was hunting. Typically the engine would hunt after being started, not necessarily only when cold. Maybe the O2 sensor and computer were able to adjust things once the O2 sensor heated up.
2) Several threads indicate that if you pull off the oil tank cap while running and the idle doesn't drop, you have a vacuum leak. The idle has never changed while the cap is off on mine. I remeber this because a friend that helped me look over the car before I bought it made a comment. This hasn't changed. The oil tank vent is connected to the back of the snorkel between the sensor plate and the throttle.
3) The airbox does not have a pop off valve. I know I need to get that changed, I hope I'm not too late.
4) Frequency valve is working, or at least vibrating with key on.

Questions:
1) Is there a drain in the airbox (filter area) below the sensor plate? While trying to determine if this was a drain or whether a screw was missing, I put a piece of plastic trimmer(weed eater) line in the hole. Unfortunately I dropped it. I'm afraid to run the engine withou knowing where the line is sitting?
2) My airbox has an unplugged, open nipple connection on the right side, directly across from the charcoal cannister vent connection. Any idea what should be attached to this?
3) At the right, front portion of the engine bay (plug 6), there is an un attached electrical connection (green, 2 terminal). It looks similar to the one on the AAR. I couldn't find any components in that area missing a connector. Then again I cound't see much at all. Any ideas what this should be connected to?
4) I have had some minor backfiring, mostly popping. Does it take a lot to crack the air manifold? Is there a way to check w/o removing the entire system?
5) Other than the standard cold idle problem checks (vacuum leak, WUR, AAV, AAR) which are discussed at length in this forum, does anything I've said point to another problem?

Thanks in advance.

[Paulporsche]

Hello and welcome.

It sounds like you have been reading the posts. Are you aware that you can find out a lot more by using the search function, which will give you past threads. There are a lot of people on the board w/ CIS cars and after all these years, sooner or later the same problems seem to crop up w/ all the cars.

Also if you can post pictures of a particular problem area, that will help.

Regarding your questions:

1)I think there is a drain. If you can post a pic, that would help.
2)I think what you are describing is a connector for the oil breather tube.Do you have an open port across from it on the top/side of the oil tank? It is for a hose about 1" in dia.
3) There are 2 connections like this in this area. The blue one is for the cold start valve. The green is for a switch that shuts off the fuel supply in case the car tips over. Thay should each be connected.
4) You may have an air leak in your airbox, or possibly @ the rubber connectors to the intake runners. Another area is the vacuum hoses which can crack w/ age and the decel valve. You should definitely get a popoff valve in the airbox ASAP, especially w/ colder temps on the way. as a blown airbox is a definitely possibility and is expensive. It is often hard to see airbox leaks w/ the engine in the car.

I am not convinced that every car will lose idle rpm if the oil cap is removed, but that seems to be typical behavior. Some of the guys have said that their car actually stalls when they do this. Check the gasket in the cap as well as the other air leak areas.

Generally popping/backfiring means too lean.

Based on your conditions, I think your cold control pressure is too low and your mixture too rich during warmup. I am currently fooling w/ my WUR to find the correct balance between the two. I don't have a spec to go by for my engine, but if you get the Bentley manual and a fuel pressure gauge, you can verify for yourself if you are within spec.

See also the othe current WUR thread on this board.

Good luck and let us know what happens.

[reedr11]

Paul,
1) Thanks for your response. Last night I was able to confirm that the hole at the bottom of the box was a drain.
2) The large (1" or so) oil tank breather tube is connected somewhere at the back (drive side) of the engine. The connection on the airbox that I am questioning is probably about 1/2". The charcoal canister has an un-tubed connection at 1 end. Would each end of the canister have connections going to the airbox?
3) I'll look for the shut off switch.
4) Currently I believe I am running lean at warm-up as I'm getting some popping. I believe I was previously running rich at start-up whether warm or cold because I was getting surging at idle. Now I'm trying to figure out what caused the change from lean to rich. I'll need to get a pressure tester to further diagnose the problem. I found a link to a Bosch "manual" in one of the other posts. It doesn't give specs. but does an excellent job of explaining how the system works. If you or anybody else is interested http://www.phat-gti.com/downloads/boschtech-12d.pdf
Thanks again for the response. Rob

[Paulporsche]

RR,

I think I just responded to your hose question on another thread.

On my 80 engine, there are 2 hoses coming off the side of the oil sump area. The top one (from the filler) goes to the airbox side, and the lower one (from the tank) goes around behind the engine. These hoses are about 1"OD. There is a nipple off the back of the canister (about 1/2") which is open, and there is a smaller vac hose looking line from the front of it going behind the engine. Since I don't have an O2 sensor or a cat, this does not seem to be a problem. If you do, then you may have to investigate further.

The WUR on these cars go out of adjustment, but usually can be reset. This is called knocking the plug and there are lots of threads on this. If you have the pressure gauge, then you can check your control pressures and verify w/ the graph. On my car, since the engine is not stock, I don't have a spec to go by. I am currently in the process of lowering my cold cp (richer) to below the usual stock spec, in the hopes that that will help start the car better. Then I can adjust the mixture so it is right for warmup and overall running. However, as JW points out, it is hard to get these things to start/warm up perfectly in all temps. I use my car from about 35F to about 95F so I am doing it now when temps are about 60F and hoping for the best compromise.

Your change from rich to lean could be because of air leaks in hoses or components or other connections. A recent poster said his was because of a crack in that hose to the side of the airbox. Change in temps can also trigger symptoms. Do you have an O2 sensor? Faulty ones can also cause problems.

[reedr11]

I have a single wire O2 sensor. If my understanding is correct, this has no effect on operation until the O2 sensor gets hot. I'm guessing that is the reason I used to get an idle surge right after starting the car whether warm or cold. The setting was probably on the high side, but once the sensor got hot, the lamda unit corrected things. At this point it's a theory as I've only started learning about this system over the last 4 or 5 days. In any case, things have changed to what appears to be a lean condition during warmup. I don't have a P-tester yet, but it's on my list. In the meantime I'll try to track down leaks. I've ruled out the cold start injector, AAR, freq. valve, and thermal switch...anything else is still possible.

On one of the posts I looked at today, possibly one of your old ones, I saw a comment that AAV's were't really necessary. Does that apply to all model years? I know that the lambda controlled systems are set up differently than the old ones. If it isn't needed, I'd just soon pull it.

[Paulporsche]

RR,

I have also read that some posters have gotten by w/o the AAV, but I have no experience w/ this. It seems that, unless you are always using the car in hot conditions, you should retain the AAR.

[LakeCleElum]

R Reed: My opinion is that 1,000 miles per year in a humid climate is not good the the CIS or wiring. I've only put 10,000 on my '73.5 since 1991 and am paying the price as I work on sorting it out. I'm about there, but I've been thru the CIS, WUR, and several electrical issues. I'm newly retired and now vow to drive it at least twice a month....Good luck..