Erratic Idle

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May 4, 2007 | 09:38 PM

10VETTEMAN

Thread Starter

Intermediate

Joined: Oct 2006

Posts: 26

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From: ELLICOTT CITY MARYLAND

Erratic Idle

i am working on a 1990 vette that has 120,000, the customer stated to me that the car is usually only driven very short distances and last weekend he went on a cruise of about 400 miles and sometime during the trip the rear cat blew apart and the erratic idle started.we replaced the entire exhaust and still the problem existed! further diagnosis revealed the egr valve was stuck wide open, replaced egr still erratic idle, last ditch effort we replaced cap rotor and the plugs. through this whole ordeal the check engine light has not come on, we have found out that if we did disconnect the NEW o2 sensor the car will idle fine, WHATS UP WITH THAT HELP PLEASE!!!!!!!!!!

May 4, 2007 | 09:42 PM

65Z01

Team Owner

Joined: Dec 2000

Posts: 90,675

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From: SE NY

Cruise-In II Veteran

Check for an air leak.
Make sure the new EGR valve is working.
Check manifold vac.
Check that fuel pressure holds after shut down.

May 5, 2007 | 01:26 AM

Mo_Bandy

Melting Slicks

Joined: Dec 2000

Posts: 2,719

Likes: 59

From: Fremont Oh

If you are pullingthe O2, it is runnong in open loop so to speak... I would say check two thinks..

I can't remember if the 1990 had a MAF sensor but I know the 89's did... they could do it... You can eliminate it by unplugging it... it will set a code, but if the idle straightens out that should tell you...

The other thing perhaps could be the IAC valve on the throttle body... if the EGR was froze open it is possible that the IAC is also carboned upand not opening and closing to compensate for the engines demand...

Just a thought...

MO

May 5, 2007 | 12:21 PM

vinnies87

Le Mans Master

Joined: Aug 2003

Posts: 9,884

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From: Magnolia Mississippi

St. Jude '03-'04-'05-'06-'07

IAC or MAF or (2) MAF relays (behind battery), vacume leaks (major)...

May 5, 2007 | 12:58 PM

CFI-EFI

Race Director

Joined: Sep 2000

Posts: 17,298

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From: The Top of Utah

Quote:

Originally Posted by vinnies87

IAC or MAF or (2) MAF relays (behind battery), vacume leaks (major)...

Neither! The 1990 has no MAF and consequently, no MAF relays. It isn't the IAC. The solution is in the fact that it idles fine in open loop but not in closed loop. It couldn't hurt to take a look at the MAP sensor and connections, both electrical and vacuum. Have you checked to see if any codes are present?

RACE ON!!!

May 5, 2007 | 01:15 PM

SunCr

Le Mans Master

Joined: Dec 2000

Posts: 7,839

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From: San Diego, Ca

It would be easier to help if you would let us know our take on this job.

May 5, 2007 | 02:23 PM

rick lambert

Le Mans Master

Joined: Oct 2003

Posts: 6,130

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From: seattle WA

I don't mean to insult you...but you're a shop and you don't have a scanner? or FSMs for that car..year specific? And just because the light didn't come on doesn't mean you haven't got a stored code....you damn sure shoulda had one for the EGR.