8v Digifant - probable air leak, but where from?

[Heyho]

Hi guys, before I go ahead and pull the manifold off, I thought I'd run this by you.....

I've noticed lately that between gearchanges, particularly from cold, my engine revs have not been dropping off, but remaining at about 2000rpm.

I've had a scout around today, and I'm suspecting an air leak.  The ISV has been off and cleaned/inspected - all good.  I've also had the main bellows off, and checked that the throttle valves are closing correctly - all OK.  With the engine idling, I can clamp off the idle air supply completely, and she still ticks over (albeit a little slowly), so the air must be coming from somewhere on the engine side of the throttle valves.  I've also tried clamping off the vacuum pipe for the servo, and all of the vacuum pipes that I can see (although it was getting a little dark).  The breathers/pipes all appear to be good.

Usually I can trace air leaks using the tried and tested method of spraying brake cleaner all over the suspect area, but even this didn't yield any results!  I'm thinking maybe of manifold gaskets.  I'll have a better look tomorrow at work on a ramp, but any suggestions greatly appreciated.

[clipperjay]

The only other method for leak detecting I know is actually turn the engine off use a bike pump to 2psi on the area with pipes and listen for the leak it will either be a vacume hose being brittle or just a lose clamp on one of the pipes?

Old school but works
To be honest it could the temp sensor located near the right side plug sometimes they call it the blue temp sensor I think? Wires and plastic shrouds look for breaks on the two wires on the ISV sensor the isv might be fine the ECU signal might not be.
 

[Heyho]

I don't think I'd be able to pressurise the inlet - there would always be a valve open.  I'm pretty sure it's a physical problem, and not electrical, as it's idling with the throttle shut and the idle valve hose clamped off!

[jase555]

just had a similar problem with mine. revs and also when i pulled up it would die. after almost taking to the local grease monkey i decided to see if i could compensate the revs with the idle screw, wound it in then out then back to where it was originally and bingo!!

dont know weather it was pulling air or had cack around it but it might be worth checking.

good luck and let us know

[clipperjay]

OK re read the post and it may well could be the Oxygen sensor I had a simlar problem with a mates car that idled constant at 2500rpm until he drove off the O2 sensor was knackerd so all I did was take the sender off to check if it lowered it. Or the other test would be on the Cables from the sensors you need to see what kind of resistance your getting on each sensor to determine timing problems.
I know your saying its not electrical, but it does affect timing on a Digi in a big way. Even with the valves closed and shut the ECU might just compensate better? Sorry it is a just a guessing game at the end of the day  untill each componet is eliminated.
Good luck with it mate in daylight if might be easier to spot
 Edit sorry don't even know which digi you have I'm assuming it's a Golf not a Jetta 8v 1.8 gti 1989 digijet
 If it is there might be a Temparature control flap in the air box check if it opens when hot air is  above 30C it should close @20C

[rubjonny]

No o2 sensor on a UK MK2, unless it has a cat which is a very rare option.
1. Check the throttle cable isn't too tight, the throttle should hit up against the stop firmly.
2. Check the idle switch, it should click with the throttle closed
3. Check all the vacuum hoses around the throttle body, inluding the one round the back that everyone forgets about
4. Pull the PCV valve off the rocker cover, and check the seal hasn't split
5. Setup the engine properly as per my digifant tuneup guide

Should about do it for now

[Heyho]

Thanks for the responses guys.

If I can, I'll get out and have another look tonight.  The throttle cable definitely has plenty of slack, and I'm pretty confident that the vacuum pipes on the throttle body are OK.  Both of the throttle switches are working (well, mechanically at least).  That just leaves the breathing system.  I'll have a thorough check of all valves/seals, then I'll report back.

Paul.

[FBF]

to check for leaks run the engine and spray wd40 on each area, when the engine note changes you've found the leak

[Heyho]

I'm pretty sure now that it's not an air leak, so my suspicion now lays with the idle switch.  As the throttle is definitely closing, and the switch is definitely operating mechanically, my next step is to check continuity.

Do any of you guys know what speed the over-run cut off activates on this engine?  I'm guessing it's below 2k.

[rubjonny]

there isn't an over-run cut off valve as such, its all done by the ECU.  The way to test the idle switch wiring, is to hold the throttle open a little, then press the wide open throttle switch.  The engine revs should drop, then pick up again when you release the switch.  Then check again with the idle switch, if you suspect it may have broken up internally.

But, normally a faulty idle switch will cause the revs to hunt up & down, and make the car stall randomly.  Have you tried resetting the idle as per my FAQ?  I found my idle would like to rise every now & again, and I found it was due to a loose idle screw, the o'ring had worn away so it would unwind itself.  Dirt cheap from VW!