irratic idle only when oil cap id off??
 

Hi everyone, my engine has been idling up and down since I bought my car. It does this all the time, hot or cold. Today I changed the oil and had it running without the oil cap on. The engine runs smooth! I can't find any vacuum leaks and the idle control value does make a sound when I shake it. I have taken the ICV off and spreader cleaner in as well.

Any help would be appreciated.

It's normal. By opening the oil cap, you're introducing a huge vacuum leak.

If the cap is off the car runs smooth, shouldit nethe opposite is there's a vacuum leak?

You need to find out why it's running so rich. Might check the connector for the cylinder head temp sensor, it might be disconnected or check the resistance. Finally, have your air flow meter adjusted.

Is the seal on you oil fill cap shot?

There is no seal on the oil cap. If there was it would still idle irratically. It'd only when the cap is right off that it runs smooth.

Hi!
Check the air flow meter . This (I'm told) has a tendency to get nearly stuck, and thereby giving off two levels at idle: too much, or too little air. Thereby a non-constant idle. But of course there can be other problems...

if the idle is going up and down, (hunting), that is a sign it is too rich. when you removed the oil cap and the idle smoothed out, you just leaned it out and confirmed it is too rich.

get the seal on the oil cap, for sure.

I'm frustrated, my son in law and his co-worker (both mechanics) can't find the problem. They haven't worked on Porsche before but even so. They checked the ICV, the O2 sensor and all the vacuum lines. Everything seems to be OK. We ran a seperate ground from the battery to the engine, just in case. Any other suggestions?

So wait, you have added the seal to the oil cap and then also figured out why the mixture is too rich and the problem still exists, or are you and the non-Porsche-mechanic son-in-law chasing problems without taking the advice you've been proffered???

Hi brimcc,
May I ask when you said "idling up and down", do you mean hunting or rough idle?

Hi 911S,
Could I ask how do you lean it out? Are you talking about the screw on the bottom of the air intake AFM (the metal block where the air box mount to), and the screw is deep inside the tunnel with a metal cap on the tunnel? Is there any tool/device to know where is correct? A friend of mine has an exhaust/O2 analyzer. Can this device use for this purpose?
Thanks.

I mean hunting and I don't know how to adjust the idle mixture.

ERRATIC idle...

You probably shouldn't just lean it out without some way to see what your emissions are.
Too lean can damage the engine.

Do you have a Bentley manual for your car?

Sounds like you're just trying different things to see if they work, any plan.

Your right, I am trying what ever I can think of. I guess I will just have to put up with it. It's kind of embassing at stop lights, everyone thinks I want to race them. The closest Porsche dealer is 200 miles away. Thanks anyway.

Get a Bentley manual. Also get Wayne's project book. (Order from Pelican). See what's involved in mixture adjustment.

There may be other Pelicans close to you.

Is there a good foreign car garage that works on Porsches? Or work on other German cars, Swedish cars, they can probably help you adjust the mixture.

Or fix what's wrong. There is a reason it's running rich, or at least acting like it is.