High idle after intake gasket replacement
Thread Starter
Cruising
Joined: Feb 2006
Posts: 11
Likes: 0
High idle after intake gasket replacement
I had a independant repair shop replace the intake gaskets on my 1990 L98 6 speed and now the car has a 1250 RPM idle when warm. It used to have a 800-900 RPM idle. The car actually has its normal 1000 RPM cold idle but when warmed the idle goes up to 1250. Also, turning the AC on brings the idle down to 1000 RPM. The mechanic claims the ECM will have to relearn the idle but it seems to me like the IAC motor is screwed up, and 85 miles later plus me removing the 2 ECM fuses to reset the ECM the car still has the same high idle. Note the car has a 52MM BBK throttle body, an open air box with K&N filter, a smooth air duct, and some kind of polished aftermarket intake runners which have no name or markings. I would appreciate any ideas.
Safety Car
Joined: Oct 2002
Posts: 3,991
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From: Phoenix AZ
Its an air leak somewhere in the intake track, but it could be anywhere after the mass air meter, its also making the motor run lean as well, so be careful. I would take it back to the shop immediately.
Thread Starter
Cruising
Joined: Feb 2006
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Interesting..I called the shop to tell them it's coming back, and the owner thinks maybe it's the air duct, but he's got that on better now than before. I had to originally stretch the air duct over the larger throttle body and the fit was so off I couldn't even get clamps on it. Now that it's been stretched out for 2 years the mechanic was able to get clamps on the duct and the fit is actually better than ever. But wouldn't a vacuum leak also affect the cold idle and the idle with ac on? The thing that throws me is that the idle is only high when it should be at its lowest i.e. when engine is warm and AC is off.
Safety Car
Joined: Oct 2002
Posts: 3,991
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From: Phoenix AZ
My bellows actually had small cracks on the inside of the accordian creases and my car did exactly what you are discribing. It also had small backfires every once in awhile, but I don't have cats so it is easier to hear that on my car.
Cold idle is in open loop mode so it shouldn't be as effected. When the engine is warm it's reading how much air is flowing by the MAF, or closed loop mode, but any leak in the intake after the MAF will result in more air going into the motor which will cause a higher idle. The air is not measured so the car will start to go lean. The O2 sensor will read more O2 and increse fuel, but it's delayed so it will run rich for a second when you come off throttle and pop from unburt fuel in the exhaust ingniting.
Cold idle is in open loop mode so it shouldn't be as effected. When the engine is warm it's reading how much air is flowing by the MAF, or closed loop mode, but any leak in the intake after the MAF will result in more air going into the motor which will cause a higher idle. The air is not measured so the car will start to go lean. The O2 sensor will read more O2 and increse fuel, but it's delayed so it will run rich for a second when you come off throttle and pop from unburt fuel in the exhaust ingniting.
Last edited by stubbs; Feb 23, 2006 at 11:56 AM.
Race Director
Joined: Sep 2005
Posts: 13,269
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From: Sharpsburg Georgia
GA Events Coordinator
Originally Posted by Muffin
I did a cam swap two weeks ago and ended up with the exact same thing. Am almost done with new ,and more carefully installed gaskets.
Pro
Joined: Nov 2005
Posts: 658
Likes: 12
From: Clearwater Fl
Originally Posted by blown87
It is amazing how carefull you can be the second time.
Old saying:
There is never enough time to do a job right the first time,,,,,
BUT,,, there is always enough time to do it again.
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Thread Starter
Cruising
Joined: Feb 2006
Posts: 11
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Thanks for the replies. Ironically this job took the mechanic 10 hours to do, so I was hoping he at least took the time to get all bolts and seals in place. I've used him for years and this is the first time I've had a problem with his work. I paid him for 6 hours labor as that's what his estimate was and that's what a service repair manual said it would take.
Race Director
Joined: Sep 2000
Posts: 17,298
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From: The Top of Utah
For $200.00, a factory service manual and the gaskets, you could have done it yourself. The bigger part of the $200.00 (the manual) is retained for the next and all subsequent jobs.
RACE ON!!!
RACE ON!!!
Thread Starter
Cruising
Joined: Feb 2006
Posts: 11
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No thanks I don't mess with later model computer controlled cars, and the few guys I've run into that do work on their own late-model cars screw up as much or more than they fix. I undoubtedly would have done worse than the guy doing it for a living. Also on site car repairs are forbidden in my condo by-laws. Actually they're forbidden by any association by-laws I've ever seen. They all fear the car up on blocks look.
