Vacuum Lines = Dip/Stall and Rise on overrun

[kbvw]

Hi Again,
I'm stumped. Been reviewing various forums for a few days now.

I cant find any pictures of a '90 8v vacuum hose setup. My under hood picture shows connections to the carbon canister, throttle body, pressure governor, loop from bottom of airbox to top AND "SIL ONLY".
All is present except connection from right hand TB connection to SIL. What is this SIL? The picture looks like something just past the injector line.

I'm chasing this through as I believe I have a vacuum issue, associated with the overrun cutoff. (Overrun cutoff as described on ClubGti, but I think this may only be a 16v component, as I cant find the part they describe.)

The underlying issue is, hot or cold, the when dipping the clutch and releasing the throttle from revs/load the car will drop to 4-500, then recover itself to 12-1500, then stabilise at idle (950). However, occasionally, it will stall. I'm blipping the accel on winddown to prevent this, but would love to solve the real issue.

Any ideas on this?
Thanks muchly.
KB

[Jmac]

Couple of things to check there mate, i take it your system is Digifant????? The manual is right check the vac lines, if they look in any way perished change them. Check your throttle switches are working right, if you have a manual it should show you this. Another thing is to remove your idle stabilisation valve and clean it out with shellite/petrol/carby cleaner, remove all the carbon and retry. They are notorious for this. Check all the earths on the cylinder head, and your battery cables are clean. make sure there is no blue stuff on your battery earth pole and cable.
Good luck
Jmac

[kbvw]

Hey JMac,
thanks for the ultra fast reply. Someone else that doesnt sleep!

I'm hesitant to say that the problem could be solved.

I replaced the rubber bits of all the vac lines, but no issues there. No impact.
I then stuck a meter on the throttle microswitches (top and bottom connect to the same connector, strange!) But this showed me the lower (overrun) wasnt making. So a minor tweak to the idle stop screw/nut and I;ve now got contact.
(I'd cleaned the ISV a few months ago, checked the battery cables for blue?, and cleanliness. Earths checked, I counted 4 plus some "sensor" attached to the rocker cover just above the fuel rail.)

However...
I then took it for spin straight off. Cold.
It was still dropping significantly, but now not rising as high. so some progress.

I then let it warm up. And I thought this had made no difference. So I disconnected to ISV, to see if it would remain stable without. To do this I had to up the idle speed screw a little. Stable!
So then connected the ISV again....and still stable!
So I'll be interested to see what its like tmrw morning, cold.

I read in the Haynes Manual that the throttle switch test says these are dependant on the blue temp sender. So I can only assume the temp does have an impact.
If its still got the same issue, I'll be looking for further ideas

Will report back tmrw or later to let you know how it goes.
Apologies for essay, but after the time and effort this has taken to finally pin down, I'm hoping someone else will find this useful.

Laters... (thanks again.)

[Jmac]

Blue temp sensor is notorious for throwing everything out of whack mate, over fueling hot ( thinks its still cold thus longer fuel injector pulse). Have a read through the Mk2 section i did a write up on how to tune your Gti. If you havent changed that sensor , for the cost i would anyway.
Good luck
Jmac

[tommymac]

yer mine used to do this,

i got rid of the ISV and it was fine, forget my last comment if it was there haha stupid question

[kbvw]

Thanks for your pointers.
Now confirming after a week of driving and a good few 100 km's, all is well.

So all I did was ensure the overrun/idle microswitch made contact, then disconnected the ISV, then reconnected.

Easy.