Need help diagnosing SC high idle
 

A friend of mine has 80 SC that has a high idle when hot. He's not on the Web much so I'm posting for him.

Tach seems to only work when hot and after he drives for some time.

As the car gets hot, the idle rises.

He took some movies for me, you can see them below. Any ideas? I've never done anything with CIS.

Movies 1, 2 and 3

Vacuum leak.

If you do a search you will find this is not uncommon.

As mentioned, it is often a vacuum leak, and the leak is often in the decel valve, which can be adjusted. The leak creates a lean condition and a subsequent high idle speed. Check and you will see that there is a test to determine if the decel valve is operating correctly. It could also be in several other places, including the WUR and vacuum lines.

Your friend should also get a CIS fuel gauge and run through the usual tests for fuel and control pressure, so he/we can determine if his mixture is correct throughout the start and run cycle, and if the WUR is operating correctly.

I would check the operation of the decel valve first.

+1

Just plug the little hose going into the side of the decel valve with a nail. That eliminates the (designed) pathway for air to the manifold.

Maybe the diaphragm gets stuck open when warm, put another short piece of hose on the open port and plug that with a nail as well.

The decel valve is one of two goldish plated flying saucers on the right side. One has hoses you cant easily remove. The decel valve has a small dia hose that can be easily pulled off by hand.

Does not fix the decel valve but takes it out of the equation as an air leak.

+1 easy thing to check. I have three of those decel valves I'm my parts bin - they all do the same thing.

Maybe additional possibility - Years back I recall a high idle on my 81 was chalked up to sticking flyweights in the distributor. Annoying high idle when hot as well if I recall - similar to decel valve leaks. Shop disassembled and lubed / cleaned distributor and all better.

Shaun,
As others have mentioned, it could be several different things. I had a very high idle that occurred ONLY after a hot start. I had pretty much eliminated the possibility of vacuum leaks and, in my case, the deceleration value was not the culprit (I plugged and unplugged/reconnected with not differences). I replaced the distributor with a new unit and this had no affect on the problem.

What did work in my case, for reasons that I do not understand, was that I installed a switch between the Lambda unit and the oxygen sensor. If I did a hot start on the engine and when the idle went high, all I did was open the switch which caused it to go to open loop mode and the idle immediately returned to normal. It was my understanding (or misunderstanding) that the microswitch took the O2 sensor out of the loop during idle but after much experimentation, I discovered that going to open loop corrected my issue... hence my installation of the switch. Upon acceleration, I just flipped the switch back permitting proper closed loop operation. The engine behaved properly at the next 'idle-opportunity'. The problem I experienced ONLY occurred immediately after a hot start and this was on a 1981 SC.

If the O2 sensor is disconnected on his car this suggestion, of course, will have no impact. It worked WONDERFULLY on mine on every instance.

+111 on plugging the decel valve. Mine did exactly the same thing until I plugged it with a golf tee.

However, without decel valve I tend to pedal the gas to keep rpm's up while shifting at certain times.

Thank you everyone for your help, really appreciate all the good info!

Hello all and Thank You Shaun. I will work through all these posts. Anyone that could post a video or a picture on location of decel valve? I can not seem to find the little hose. I will swap out the O2 sensor that will be easy. I can not find a vacuum leak. Is there any Videos that I can watch> please feel free to post any photos also. THANK YOU ALL FOR POSTS. robrizzo@yahoo.com Rob Rizzo
508 525 0069.

http://forums.pelicanparts.com/porsche-911-technical-forum/666587-decel-valve-query.html

I'll come over to cut the crusts off your sandwich later.