Fast idle after WUR rebuild

Thanks to all the great advice on here, move rebuilt my WUR and car now starts up well. However after a short drive the idle stays high - between 1300 and 1700rpm. Seems quite random but always high.
Previous to the WUR work it was around 1100.
I could just adjust the idle down but wondered if it could be something more complex based on recent work..?


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assuming it's a DX GTI, perhaps a tune may help. also, adjust the idle valve 'screw' on the back of the throttle valve manifold 1/4 turns which lowers or highers your revs.. 

It’s an EG but I’m sure the same applies. Will try that adjustment and am also planning on getting in for a rolling road tune up so hopefully that will get it running sweet…


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Lower the idle to 1000 rpm with a 7mm spanner at the back of the throttle body, just make sure the engine is nice a warm before adjusting the idle speed.

I loosened the lock bit and used a screwdriver to adjust the threaded bolt when the engine was nice and warm. Got the revs down to about 1300 but no lower.
Took it out again tonight and it was initially still running fast at 1600/1700.
Any ideas?
Could the WUR rebuild have effected it? Even if it had, I would have thought that effect would have worn off after a few minutes when the engine was nice and warm and therefore the WUR is effectively ’redundant’ - if I understand the system correctly..?


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Not sure about early GTis, but later ones have an idle stabilisation valve that need to be disconnected when setting the idle. If you don't do this you might set the idle screw too low, but the valve will "catch" the revs from falling too low, which is not supposed to happen continuously and may cause it to idle too high.

gregpank said

I loosened the lock bit and used a screwdriver to adjust the threaded bolt when the engine was nice and warm. Got the revs down to about 1300 but no lower.
Took it out again tonight and it was initially still running fast at 1600/1700.
Any ideas?
Could the WUR rebuild have effected it? Even if it had, I would have thought that effect would have worn off after a few minutes when the engine was nice and warm and therefore the WUR is effectively ’redundant’ - if I understand the system correctly..?


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Think you have adjusted the factory set throttle stop (to stop the butterflies hitting the inside of the throttle body), the idle screw is in a recess at the back of the throttle body and doesn't have a lock nut.

Yes from reading up more i can see i’ve adjust the wrong thing!
However i had adjusted the idle screw on the back of the unit aswell as i womdered if that would help - made zero difference.
I now suspect my mixture is out so will adjust that mixture screw on the metering head with a long 3mm allen key. I also have just been given a gas analysis unit by my retired-mechanic dad so will use that too.

I did also check for vacuum leaks by spraying wd40 on the various hoses and see if the engine changed note, but nothing i could hear.

So back to basics - mixture. I really think a rolling road session is in order soon…

Have you reset the throttle stop, back it off and put a thin piece of paper in the gap, do it up until it just grabs, take the paper out and turn it in a half a turn. That's from memory.
What about the auxiliary air valve, with the engine warm squeeze the pipe and see if that lowers your idle.

Also the idle screw has seals on it which go hard and need replacing every now and then. May help to get these done.

Don’t lose the screw they are hard to get hold of.


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I have a new idle screw o ring on its way. ..

In the meantime I connected up my gas analyser and tweaked the mixture on the metering head. The mixture initially seemed very rich at 7 to 8%!
By leaning it down I reduced the tick over from 1500 to 1000 - which is good
But this only reduced the co2 down to 5 to 6%. Still far too rich.
My amateur suspicion is that the idle screw o ring is perished and screwed all the way in to make some kind of seal. I read elsewhere it should be 3 or 4 turns out.
I think that could mean I can’t increase the idle and therefore reduce the mixture - if I understand the system correctly. . ?


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The idle bypass screw is toast if you can turn it by hand.
It takes a 7mm wrench to move it is the o-ring is good.

What I always did was to wrap the threads and only the threads of the screw with Teflon tape to make sure there is no unmetered air getting in to the system.

I also would bottom out the screw then back it off about 3/4 of a turn.

I think what you need to do first is fix the idle bypass screw and reset the throttle stop, that's fundamental.

Increasing the idle won't bring your CO down, what you need to do is a fuel pressure test, I would guess there is a problem with the WUR and the pressure test will show that up.

Yep, new o ring arrived today so will try that. And I've adjusted the throttle stop already. 
I am suspicious of the WUR as this high idle only started after I rebuilt it. 
I pressure tested the system afterwards with the following results:

Cold start: 2.4 bar - this seems high as think it should be about 2.0
Warm start: 3.4 - ok i think
System pressure: 4.6 - ok i think
Residual pressure after 10 mins off: 1.0 - way too low
After 20mins: 0.5 - way too low

Any ideas on how this could be affected idle?
Btw it does have the mod on the back allowing you to adjust it if needed…

If your WUR has full throttle enrichment and the vacuum pipe isn't connected that could give you a high idle and a rich mixture.

That’s interesting. Before I rebuilt it, it wasn’t connected to the vacuum system.
But after the rebuild I bought the hose and connected it up.
If it keeps running fast, I’ll disconnect it and see if that helps.


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No, leave it connected, if you disconnect it the mixture will be too rich.
Warm it up and then set the CO at idle, I find about 2% CO is about right.

Hmm well im struggling.
1. Idle screw oxring replaced (old one was hard and flat).  Also put a couple of winds of ptfe around the thread as insurance as per [mention]Briano1234 [/mention] s suggestion. Screw out about 1 complete rotation.
2. Set throttle screw as recommended (half a screw in from touching the stop).
3. Also pulled the vacuum pipe off the WUR just to check and as [mention]cedar [/mention] said, revs picked up significantly.
4. Tried to set the mixture but struggled to get it below 5% (assuming im using the analyser correctly!).
5. End result of the above is the idle revs are still 1300-1500 when cold. After a good half an hour of running, i can get them down to 900/1000. But i suspect if i start it again tomorrow cold, it will start off at 1500 again.

When you rebuilt the WUR, did you follow instructions to set the pressures correctly? There is a doc on here that tells you how to do it, especially if you have vacuum enrichment - Rebuild manuals – K-Jet Specialists
I think it's the Basic WUR set up.
You also need to set the screw at the back of the WUR correctly on rebuild, again, see instructions on that page above for double diaphram rebuild. Having spoken to the chap that runs that business, you should not use that screw to adjust anything, it is set and forget. You can only adjust pressures by knocking the fuel cell or bimetallic post in to the WUR body. When I rebuilt mine I knocked them both out about 5mm whilst I had it all apart and then carefully knocked them in with the fuel pressure  guage attached. Used a vacuum pump to simulate vacuum.
You have to be confident that the WUR pressures are all set correct before you mess with anything else otherwise you'll just go round in circles.

Thanks. I didn’t follow that, but rather the instructions that came with the rebuild kit from missing-parts in Germany.
The difference between the two is that the latter doesn’t instruct you to remove the round fuel cell but instead shows you how to rebuild without doing so. It does state you have to very carefully ensure the screw adjustment is at exactly the same spot when reassembling it - which I did.
My post a bit further up shows the pressures I had after the rebuild which seem in tolerance. I think?


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