byrdman454

I came out to my car like any other day (cold and wet weather) and started it up. Instead of immediately settling at the correct idle @900 rpm, it was idling at 1500 rpm. I thought possibly the carpet or a hung cable, but no dice. I revved it several times but no change. I ended up driving it for a few blocks and it started surging. The car would be a t 1500 rpm, then hiccup down to 900, then immediately back up to 1500. This happened several times. Finally the idle came back down to normal and it drove fine.

What do you guys make of this? Is there some cold weather senser which holds a fast idle till a certain temperature going out?

John Speake

Mike,
There's certainly nothing in the system that should cause the very fast idle speed you had, or the surging.

Sounds like a bad ground on senor connection (like Temp sensor 2)

Let us know if it does it again

byrdman454

Where is temp sensor 2 located? Maybe I can clean the ground and connections.

John Speake

Front of the engine, above the t'stat. has a two pin Bosch connector. The ground is through the engine. The sensor has two seperate elements in it.

Has it only mis behaved the one time ?

Marc Schwager

John,

Good tip. If the ground is through the engine then do you mean that:

a) The internal connection to ground in the sensor may be bad

or

b) Pull the sensor and clean the threads

I have also had a surging idle a couple of timeswhen warming up. Punching the throttle seemed to clear it. I just went out and de-ox'd the connector.

John Speake

Mike and Mark,

Sorry, I realise from your replies that I made a mistake (typo) in my first reply to Mike.

I meant to say "poor ground OR sensor connector" and temp sensor 2 seemed a good candidate would give a temperature dependant effect - which could affect the ECU idle stabiliser.

byrdman454

This high idle condition and surging has now happened twice.

John Speake

...........and only when the engine is cold ?

Does it consistantly disappear once the engine has warmed up a little ?

byrdman454

So far both times have happened right after it was started from being full cold. After a very short drive, a some surging, it goes away.

nc_growler

My '91 GT just demonstrated this behavior this morning.

Settled out after warm. Brydman454, did you ever figure it out?

byrdman454

Unfortunately, this was so long ago I cannot remember. I did not ever change my Temp II, so I know this was not the problem. If I were to guess, this was right around the time my LH started failing.

Sorry I did not update this post with a solution, my fault.

fonetico

This is a copy of my contribution from last week. Seems it was the idle switch as it seems to have fixed the problem.

"I've posted on this on several occasions in the past. My '90 S4 has demonstrated all the same symptoms for two years and the problem's particularly chronic in winter weather. Interestingly, if I start the car without touching the accelerator, it idles fine. As soon as the pedal is depressed the idle goes wild.
In response to the expert advice on this forum I've tested and replaced the usual suspects, to no avail. In fact, it's getting progressively worse.
Why does it only happen when the car's cold? What bits are sensitive to cold temperatures? I've checked and replaced the Temp II sensor twice with no discernible improvement. The MAF was replaced recently and there appear to be no vacuum leaks. The throttle mechanism seems well lubricated and working as expected.
The only things I haven't tested are the idle switch and the ISV. Didn't know how to without taking them apart, which would be beyond my capabilities.
Then I saw a posting from WallyP on a simple test for the idle switch. Drive the car at speed, set the dash to Instant MPG and take foot off accelerator pedal. The dash should read 90 mpg.
Mine fluctuated between 9 and 21! When the car warmed up and the idle steadied the dash read 90.
Evidently, when the switch was warm it closed normally. Once opened when cold it would fail to close until up to working temperature.
All this, however, remains as speculation until the weekend. That's when I'll collected the car from the workshop with a new idle switch and a new ISV as, I'm informed, it's worth changing both at the same time. This isn't going to be cheap but these are the actions of a desperate man.
I don't know whether this contribution of mine will be of any use to anyone but this idle problem seems to be such a frequently recurring phenomenon."

nc_growler

Thank you for posting!

I'm going to be on the phone with Roger soon...