Lumpy cold idle

My 1600 GTI has always been a bit lumpy from cold and when the warm idle started to get lumpy recently I decided to give it some attention. After fiddling with the idle I now have the car purring beautifully at 975rpm once warmed up with no wandering idle. However it is still lumpy on cold start.

On a cold morning the cars idle will wander between 700 and 800 rpm. The idle smoothes out and hangs at around 800 rpm after a minute or to of driving (or allowing the car to sit for a couple of minutes) and is at 1000 rpm after about 5 minutes. I assume this means there’s must be a problem with a cold start component, but thats about it. It always starts first time and never stalls. Any ideas what component may be at fault?

Cheers

Most likely it's the Auxiliary Air Valve which allows bypass air when cold.  These can be tested by putting in the freezer and checking the valve is open when cold.  Then warm up the valve and it should close.  Low idle on cold start indicates the valve may not be opening sufficiently.

Thanks, I'll give it a go. Is the auxiliary air valve also referred to as an auxiliary air regulator or is this a different part? How much does one cost new, is it worth getting from a scrappy? Questions, questions, questions……..  

Cheers

I think the aux aur reg is basically the idle adjustment screw which controls the bypass air when hot idling.  The aux air valve is mounted on the back of the inlet manifold.

Not sure on new prices but they are not cheap (I'm thinking ?50+ although not sure).  Scrappy probably the best way as they are on many bosch models.  Anyone know of direct swap models?

Thanks, talking of parts swapping. Practical Classics are running a parts interchangabilty guide at the moment. Next issue is on Ignition components. Seems to give a full listing for the MK1.

I got the first Practical Classics that covered part swapping and thought it was pretty naff. Hardly touched VW components mainly covered English makes. I also didnt like the way they've spread out the article over 3 months with no mention of them doing this on the plastic bag that you buy the magazine in!   The first part (the one I got) was on brakes, have the other parts been any better?

Stop taking things off and putting things in the freezer right now!

Quick way to check the aux air reg.

Start engine from cold.
Squeeze rubber pipe that comes out of the aux air reg. Revs should drop.

Wait till engine warm, squeeze tube again. Revs should be unnaffected.

If there is deviation from above it's not working in which case it's the warm up regulator which you can't check without fuel pressure gauges.

There's loads of old posts on the old register about this stuff.

Peter.

Ross wrote

The first part (the one I got) was on brakes, have the other parts been any better?

The next two have more Golf parts listed in them, as you'd expect a lot of parts interchangabilty between VW & Audi. However, the first was a supplement, but the subsequent ones are printed insuide the magazine which is a bit useless for reference.