dresch

My car has trouble starting after sitting. Somebody said it could be the ISV. Why? What does the ISV do? Coincidentially the car has a very sparratic idle. Sometimes 2000, sometimes 1100. Is this a related problem to the starting??

QCRAZY

ISV's allow air to pass to maintain a nice smooth idle. The inards is basically just a small "flap" the moves back and forth to block the passage of air OR allow a full flow of air(and everything in between). Which position it's in depends on if your at idle or at WOT!! (A crude explination)

What "usually" happens to them is they get a little dirt built up where the "flap" travels. This doesn't allow the flap to open quick enough therefore not the proper mixture of air is allowed through. On start-up it can die (choke itself out) or cause a highly oscillating idle.

The usual symptom of a dirty ISV is: Initally (on start-up) the revs will drop and then suddenly shoot up abnormally high. Then the revs will fall back down but not as far as the first time and then again suddenly rise, again not as high as the first rise. This process will slowly continue until it settles at it's idle speed!!

They above is caused by the flap sticking. The engine will begin to choke itself out (after start-up) and then the flap will become unstuck. The flap over corrects since the car was about to choke itself out and the revs shoot high. To correct this the flap is sent to close which again it over does and they drop to low again...and so on, until it catches up!! Usually it only takes three or four big oscillations to catch up.

You can clean your ISV with carb cleaner or electric motor cleaner. It's a cheap way to rule it out. It usually doesn't go back to a perfect idle but you should get an improvement if it is in fact dirty. It's a lot cheaper than replacing it and then finding out that it wasn't the problem!!

Hope this helps!!
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Andrew
90 80Q
91 90 (New ISV)
95 90Q

mel g

I agree w fast928 about the isv explanation.
The isv problem is an annoyance only, unless the car stalls.
But I don't think isv problems will lead to inabilty to start.

Les

Exactly how do you clean the ISV with carb cleaner? Do you have to dismantle it in any way? Or do you have to open it and clean its inner parts? It looks like one large silver cylinder to me and I don't know exactly how to go about it? Thanks in advance...

mel g

there is an air tube going through it, you can usually look right thru the tube and see if the valve flap has it opened or closed. Last one I looked at you could even stick your finger in and move it yourself.
Just remove it, its no big deal.

QCRAZY

You're right it is one piece and you can't dismantle it. All you do to clean it is spray the carb cleaner in the two openings (where the lines hook up) to get the dirt off. You can also fill it with carn cleaner and shake it to remove more dirt. As mel said you can move the flap around and make sure you get that clean. Just be sure to not scrtch the flap or you will give dirt a place to "hang on" to!!

Hope this helps
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Andrew
90 80Q
91 90
95 90Q

kmnphg

engine dieseling after shutdown? As in my case, I've got a bad ISV, the flap easily get dirt build-up and sometimess when I immediately switch-off the engine if when the revs has not drop to the minimum idle, the engine diesels. I assume hot air being trapped in the throttle body/intake manifold cause this slight engine run-off(diesel)

Thanks in advance for your response. My car is Audi 100 2.0E/1991 (UK Spec)

QCRAZY

...mine has only dieseled since I replaced the ISV. Weird?? Actually it only did it once about three months ago so I've "forgot" about it!!

I'm not really the one to ask about dieseling in Audi's....no experience..knock on wood!!!

Sorry I couldn't help!!
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Andrew
90 80Q
91 90
95 90Q

kmnphg

thanks anyway, Andrew

Stephen Clark

Exactly where is it located? I have an 89 100 with a rough idle when hot.