Along with warmer weather, my old irregular idle problem has returned. Throughout winter, at temps up to 0 celsius the idle was rock solid, the needle did not move. However, yesterday the temp went from 0 to 14 celsius and now it's started again. It doesn't move much, but it's annoying and can be felt and heard.

Any idea why it's perfectly stable when cold and irregular when warm?

Tried two more intake air temp sensors. Same. :(

Leaking hose in the breather system?

I had similar symptoms when the vaccum hose for the breather system degraded and split. It's located under the inlet manifold. It's a common failure.

Here's a pic of the hose:

I looked at them in the fall and indeed the one which goes to the small one way valve (the S-like one) was broken and replaced it. All else seemed fine, though I didn't take the big lower one off completely, just looked under it using a mirror I also replaced the valve, even though it seemed ok. Didn't make any difference :(

Is there a foolproof way to determine whether they are cracked WITHOUT taking them completely off the car?

Edited 24 February, 201016 yr by jmf

boost leak testing easy to make a tester and really does make it easier to check pipe work. Loads of threads about it and a google search will turn up loads as well.

Well I ordered a tester today. It's actually just a 3" thing with a pressure gauge attached, but for $20 it was worth it. I'll let you know once I get it... Thanks for the tip

3"? not sure what ID the MAF is but you can get adapters if it is too big/small. I made mine with a BEAN can

It's the TT225 MAF, so 3"

I Have had a similar problem and managed to fix it yesterday. This is what I done.

1. Cleaned the throttle body, took about 5 Minutes.

2. Cleaned the Air Temp/pressure sensor, sits just next to the throttle body. Took about 5 minutes. Was very dirty.

3. Changed the Coolant temp sensor that are prone to fault (I only done this as a scan on VAG-COM confirmed it was knackered) Not necessarily causing you idling problem.

4. The breather hose on the L/H side top of engine was split so also changed that.

All of this work did not exactly sort my idling issue out, but research on this site had shown that if your car has a fault (Split Hose, dirty maf, dirty throttle body etc, the ECU may adapt to the fault, if you do not clear the fault from the ECU but do fix the error, the engine may will still continue operating under the adjusted values from before your fault. I was a bit sceptical but went to the garage and had the faults cleared and adaptation done on the throttle body (Basically small adjustment that VAG-COM does automatically depending on how well the engine is running).

My idling is now rock solid with no ill effects.

To sumarise, get any faults cleared after you do work.

Hope this helps.

Throttle body cleaned = check!

Air temp sensor cleaned (and replaced with two different others) = check!

Coolant temp sensor changed = check!

Y-hose changed = check!

TB Adaptation and errors cleared = check!

Perfect idle = raincheck!

Thanks anyway for the info!