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Bad idle

H

homesea

New member
Had an odd one today, popped out in the car at lunch. Car started fine from cold, but then wouldn't idle just bounced about between 200 and 1200rpm, gave it some revs and it settled down to its normal 850-900rpm.

Just thinking is this the start of something to come or a one off!

So what do you think? Bad ISV, TPS, air flow meter?

Would like to hear your thoughts....

Cheers,
 
My S2 had the same problem and on a couple of times it wouldn't clear with a blip of the throttle and on these occasions it got worse and worse miss firing very rich etc.
The problem was a build up of carbon/oil on the throttle plate at the top on the air intake side just where the breather pipe throws the fumes from the oil filler spout body.
After 170k miles it was causing the throttle to stick when very hot , cleaned and all ok now.

Hope this helps ,easy fix if it is the cause.

Nick [:D]
 
The TPS tells the engine management that the throttle is closed and therefore the idle stability valve must be used to control engine speed. Once you open the throttle the TPS senses this, and then the management kills power to the ISV. Idle speed is more rapidly controlled by ignition timing, ISV being a slower response that provides a basic airflow requirement upon which ignition timing then does the rapid response control (according to my memory of how Wayne Schofield described the control, Wayne reprogrammes 944 DME/KLR chips).

I suspect gummed up ISV. Take if off and soak in carburettor cleaner.

Hope this helps.
 
Thanks for the info. I was thinking maybe the ISV wasn't getting the signal when the throttle was closed. Still not done it again, but yesterday driving through start stop traffic from cold the car felt like it was bogging down or stuttering when just giving it a little accelerator to get it moving. This lasted until I could get to an empty bit of road and give the car some revs, it then cleared up and haven't had a problem again.

But can't help thinking these two are linked, I've got a spare air flow meter, so might give everything a clean and swap over the air flow meters.
 
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