CIS Idle Problems - Stalling

[Bigrojo]

Hi everybody. I am a very, very novice mechanic, but thought I would give this a shot. I have a 77 911S that will start no problem, warm up fine, but once at operating temperture will begin to rev up and down and then stall. I assume a vacuum problem?

Any thoughts would be greatly appreciated!

Thanks,

Greg

[Bob Kontak]

Welcome to the forum.

Use the search / advanced search and load in "CIS idle problem" and isolate the search to the 911 tech forum. Kick back and have a few beers and read.

You probably have a leak that is masked by the extra fuel delivered when cold and as it warms, and leans out, the engine gets grumpy with the extra air.

The fastest way to isolate leaks is to use carb cleaner (I use starting fluid) and spray at the various connection points. The rpm's will change if you find a leak. Use your head as you are spraying a flammable mist.

[T77911S]

you are running rich.

the first 2 things to do when having problems with CIS is check for air leaks and check control pressures, but i doubt you have gages.

in your case you may be lucky. the 77 has a thermo time valve (TTV). one of its jobs is to richen the mixture when cold, or another way to look at it is it leans it out when warm. it does this by being heated by 12v. if the TTV goes bad or the 12v is not there, the mixture will stay rich.
the TTV is a small round disc located on the left side of the motor, on one of the runners. it has an electrical connector and 2 vacuum lines going to it.
first test for 12v. either a MM or test light will work. check for power on one of the leads. (engine running).
if you dont have 12v, this will also make the WUR stay in a rich condition.
next, if the TTV is bad, like mine, it will block vacuum to the warm up regulator (WUR) making it run rich.
the easiest thing to do is get some vacuum line and bypass the TTV and see if it runs better, or, remove the vacuum line at the WUR and check for vacuum.

[calling911]

I agree without getting deep into it that you are rich. I did a write up on how to adjust your mixture with no gauges.. in some ways my method is better than gauges if you have a malfunctioning system.. but gauges is the way to go... you can search yourself for my post.. no idea where it is All it takes is a micro turn of the mixture screw to fix this usually.. you can make your own tool or buy one from the host.

Also, I think I did a write up on repairing a CIS fuel pressure regulator (nearly everyone incorrectly calls it a warm up regulator or WUR here) heater element.. yours is possibly bad due to age like mine was.. mine works perfect now and cost me like $10 to fix.

[ossiblue]

The hunting at idle you describe is symptomatic of an overly rich mixture, but the key to your problem is why the mixture is overly rich. Diagnosis is the most important thing and I'd suggest you follow the advice posted above and do some reading/research before you touch anything. CIS is a very reliable system but it requires that there be no vacuum leaks and that fuel pressures are within specified ranges. If either of these areas are faulty, you will have running problems of some kind.

Most problems with CIS running lean show up at cold start which demands a richer mixture. If there is a vacuum (air) leak, the cold start cycle becomes difficult. Sometimes rather than fixing the air leak, people will set the mixture screw in the fuel distributor to a richer setting to compensate. That will often solve the cold start problem, but when the engine warms up and the other components (wur, auxiliary air regulator, for example) attempt to lean out the mixture for warm running, they are unable to lean it enough and you will get a hunting idle when warm--such as you have.

The scenario above, may or may not be what you are experiencing--it is just one likely possibility. My advice is to read up on this forum about similar problems and do nothing until you have the proper tools and have a plan of attack. A general plan for trouble shooting CIS typically follows this sequence: (1) Check fuel pressures using proper pressure gauge set. (2) Check air leaks in all hoses and vacuum components. (3) Check function of air regulating components both for mechanical and electrical issues. (4) Set and adjust, if necessary, the fuel mixture via the screw in the fuel distributor--preferably with a gas analyzer.

All in all, the job looks more difficult on paper than it is. Ask lots of questions here, report back all you do and results you find, and do as much reading as you can. This issue will be solved in short order if you take a deliberate, systematic approach.

[calling911]

I agree with this... and would like to reiterate.. My documentation on how to set mixture with no gauges works well for a malfunctioning system.. the reality is, an old P car will have leaks.. and unless you want to deal with them now you have to compensate.. which is what my method would basically do..

However.. weather COULD have caused this if he is very close... one little air leak could have caused this.. Fuel could have caused this...

One "fix" thats common too is on the midies with vacum RETARD (vacumn retards the timing) is to disconnect and plug... which advanced the timing at idle.. you'll need to turn down your idle..

Originally Posted by ossiblue

The hunting at idle you describe is symptomatic of an overly rich mixture, but the key to your problem is why the mixture is overly rich. Diagnosis is the most important thing and I'd suggest you follow the advice posted above and do some reading/research before you touch anything. CIS is a very reliable system but it requires that there be no vacuum leaks and that fuel pressures are within specified ranges. If either of these areas are faulty, you will have running problems of some kind.

Most problems with CIS running lean show up at cold start which demands a richer mixture. If there is a vacuum (air) leak, the cold start cycle becomes difficult. Sometimes rather than fixing the air leak, people will set the mixture screw in the fuel distributor to a richer setting to compensate. That will often solve the cold start problem, but when the engine warms up and the other components (wur, auxiliary air regulator, for example) attempt to lean out the mixture for warm running, they are unable to lean it enough and you will get a hunting idle when warm--such as you have.

The scenario above, may or may not be what you are experiencing--it is just one likely possibility. My advice is to read up on this forum about similar problems and do nothing until you have the proper tools and have a plan of attack. A general plan for trouble shooting CIS typically follows this sequence: (1) Check fuel pressures using proper pressure gauge set. (2) Check air leaks in all hoses and vacuum components. (3) Check function of air regulating components both for mechanical and electrical issues. (4) Set and adjust, if necessary, the fuel mixture via the screw in the fuel distributor--preferably with a gas analyzer.

All in all, the job looks more difficult on paper than it is. Ask lots of questions here, report back all you do and results you find, and do as much reading as you can. This issue will be solved in short order if you take a deliberate, systematic approach.

[T77911S]

why it is rich is the most important thing and should be determined before making adjustments.

i gave my sugestion because it is a good possibility and can be checked/fixed without gauges or any special tools.
if i was in front of, well, behind the car, it would be the first thing i would look at because i could eliminate it in just a few minutes.

if this is not the problem, pressures need to be checked.


i would not make any mods or changes until the car is running as it was.

[5String43]

Here's calling911's piece on CIS setup:

How to setup CIS with no gauges

[calling911]

I think the OP is long gone.. but thanks

[boyt911sc]

You could rev your engine all day long up to red line and the fuel mixture screw would still be unchanged. It takes extra effort to change the setting for the fuel mixture screw. The main culprit why fuel mixture change occurs is fuel (pressure) or air intake. Both of these are easy to check.

What on earth would somebody change the mixture setting without knowing what causes the change in the fuel mixture in the first place?. Tinkering the mixture screw without verifying fuel pressure and vacuum is a band-aid approach to fixing the problem/s!!!! There is also a third factor which we commonly ignore too. And that's the degree of ignition for complete combustion. You could not effectively work doing a reliable troubleshooting without the use of the proper tools like gauges and analyzer. And people who are claiming to do this without the right instruments are only on denial!!!

Unless you have have a god-given ability to detect and measure the fuel pressure and gas composition would I accept your work as credible and reliable!!!!

Tony