Wierd Idle problem that has pattern but doesn't add up
Thread Starter
Drifting
Joined: Dec 2007
Posts: 1,672
Likes: 8
From: Sumter South Carolina
Wierd Idle problem that has pattern but doesn't add up
'89 L98 6-speed - When I start the car after sitting for at least 2 hours, its idle is textbook. Sits at 1K for a few seconds, then 700 till reaches operating temp then settles at 600-650.
However, if the car sits for anything from 30min to 1 hours I start the car and fires but then stalls, pressing the gas is only way to keep it idling, I can ease the gas down and the ecu compensates and will idle but as soon as I start driving and let off the gas to come to a stop, STALL.
I'm thinking it might have something to do with the residual heat left in the engine bay. I've visually checked for vacuum leaks and recently fixed a huge one (brake booster). but can't see any. Car runs absolutely perfect except for this. FP is perfect (new pump).
However, if the car sits for anything from 30min to 1 hours I start the car and fires but then stalls, pressing the gas is only way to keep it idling, I can ease the gas down and the ecu compensates and will idle but as soon as I start driving and let off the gas to come to a stop, STALL.
I'm thinking it might have something to do with the residual heat left in the engine bay. I've visually checked for vacuum leaks and recently fixed a huge one (brake booster). but can't see any. Car runs absolutely perfect except for this. FP is perfect (new pump).
Racer
Joined: Jul 2010
Posts: 308
Likes: 2
From: Kansas City MO
When it's having the problem:
Does it run rougher than normal at startup?
Does it clear up after driving it for a few minutes and everything seems OK again?
If so then you've either got a bad fuel pressure regulator diaphragm or a leaky fuel injector that's putting raw fuel into the intake when it's sitting and causing the issue - let it sit long enough and the excess fuel evaporates and everything seems fine...
Does it run rougher than normal at startup?
Does it clear up after driving it for a few minutes and everything seems OK again?
If so then you've either got a bad fuel pressure regulator diaphragm or a leaky fuel injector that's putting raw fuel into the intake when it's sitting and causing the issue - let it sit long enough and the excess fuel evaporates and everything seems fine...
Le Mans Master
Joined: Jan 2006
Posts: 5,621
Likes: 206
From: Orlando FL
Just as one possible suggestion.
The IAC (idle air control) motor may be defective. I have seen them do some weird things. I have had poor luck cleaning them out and having it last. I just like to change them out and get that one item out of the way. They do go bad frequently and problems can be subtle.
The IAC (idle air control) motor may be defective. I have seen them do some weird things. I have had poor luck cleaning them out and having it last. I just like to change them out and get that one item out of the way. They do go bad frequently and problems can be subtle.
Burning Brakes
Joined: Apr 2004
Posts: 762
Likes: 2
From: Huntington ma
Well.. If you are thinking it may be a temperature issue - have you checked (scan tool required) to see if the ECM is getting correct numbers for both coolant and manifold air temperatures? Manifold air temp, if faulty, Might perhaps explain this..
Actually - here's another thought.. Try disconnecting and plugging the EGR valve. Those control solenoids have a nasty habit of dying in weird ways.
Actually - here's another thought.. Try disconnecting and plugging the EGR valve. Those control solenoids have a nasty habit of dying in weird ways.
Last edited by rons85; Jan 16, 2011 at 11:10 AM.
Melting Slicks
Joined: Jun 2010
Posts: 2,178
Likes: 1
Its your IAC. I am almost positive to the nth degree. You can test the IAC with a OHM meter.
http://shbox.com/1/iac2.jpg
Also, if you are told to datalog your Car , remember this, the ECM only tells the IAC what to do, the datalogging will not show you what the IAC is doing.
http://shbox.com/1/iac2.jpg
Also, if you are told to datalog your Car , remember this, the ECM only tells the IAC what to do, the datalogging will not show you what the IAC is doing.
