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90 L98 won't idle worth a @#!%&#

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dinosaurdoctor

Guest
hi guys,on the heals of our recent success with my airbag light i thought we might try another of my cars mystique items.recently after a trip the car has decided not to idle well.no codes are present,the car has been allowed to speak to another computer,the results being that everything is fine.i have checked for vacume leaks.i have cleaned and resealed the throttle body,and replaced the idle air motor.the car starts perfectly,but does start running at about 1100 rpm and then drops to a normal idle immediately.the car surges at about 1500 rpm in drive .when you decelerate the car again surges at approx 1500.i have not done a compression test as the car runs so well otherwise.i have recently replaced the cap,rotor,wires and installed bosch platinums.the tail pipes are now black where before it ran so clean that i had painted the inside of the tailpipes.when the car starts cold it idles perfectly,as temp gets higher the idle gets rougher.the idle also gets better and worse when the mood strikes it.idle is slightly better when the a/c is on,but still not perfect.i have run two tanks of fuel with injector cleaner.the air cleaner is clean.whatever this is it does not generate a fault code.the car has approx.125,000 miles.i did the throttle body work yesterday,is there something i need to do to allow the computer to adjust to the new idle motor? battery was also disconnected to repair the air bag sensor.i'am thinking it could be valves because of the rough idle and lack of fault codes.and residual compression at higher than idle speeds makes the car appear to be a good runner/rocket.i also have replaced the exhaust from the cat back. any help you can give me will be appreciated...thank you...wayne
 
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dinosaurdoctor

Guest
nyernga said:
did you say you replaced the idle air motor or did you mean the switch? I replaced mine after experiencing some of the same problems and it's been fine ever since.

my .02

Len
[/QUOTE i replaced the motor itself on the throttle body
 
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David-F

Guest
Did you say you had a scann tool hooked to it? What data did you look at? And how was it? Ar you having a problem with an idle surge? Or real low and almost stalling? Or is it a case while idling it just stalls? Here is what I would like you to do first close the hood and let it get hot, then open the hood and start rapping on the ECM not real hard just give it simm good hits with the back of your hand a watch and see if the engine starts sputtering or stalling, if it does you will need an ECM. If not then you if possibile get a hold of a scann tool and go in and see what the Block learn and Intergator are doing they should be around 128 give or take 10 points. Also as a quick check get a DVOM(digital volt ohm meter) and check the resistences of the fuel injectors while they are warm, they should all be the same or with in 0.2 of an ohm. I take it you have used some throttle body cleaner and cleaned the throttle body if not you need to. Also you have changesd the spark plugs did you also check the ignition timing? If you did, did you also remember to put the distributor into bypass by unhooking the tan and black wire connector by the wiper motor? Well check it out and let us know what you find.

David Fulcher
 
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dinosaurdoctor

Guest
David-F said:
Did you say you had a scann tool hooked to it? What data did you look at? And how was it? Ar you having a problem with an idle surge? Or real low and almost stalling? Or is it a case while idling it just stalls? Here is what I would like you to do first close the hood and let it get hot, then open the hood and start rapping on the ECM not real hard just give it simm good hits with the back of your hand a watch and see if the engine starts sputtering or stalling, if it does you will need an ECM. If not then you if possibile get a hold of a scann tool and go in and see what the Block learn and Intergator are doing they should be around 128 give or take 10 points. Also as a quick check get a DVOM(digital volt ohm meter) and check the resistences of the fuel injectors while they are warm, they should all be the same or with in 0.2 of an ohm. I take it you have used some throttle body cleaner and cleaned the throttle body if not you need to. Also you have changesd the spark plugs did you also check the ignition timing? If you did, did you also remember to put the distributor into bypass by unhooking the tan and black wire connector by the wiper motor? Well check it out and let us know what you find.I HAD AN ASE CERTIFIED FRIEND OF MINE DO THE SCAN THING.ALL IS PERFECT ACCORDING TO THE SCAN.THE CAR JUST IDLES ROUGH,BUT NEVER STALLS.BECAUSE OF TAIL PIPE COLOR I BELIEVE THE CAR TO BE RUNNING RICH WHEN AT NORMAL OPERATING TEMPS.THE TIMING HAS BEEN CHECKED,AND IS PROPER.DOES THIS CAR HAVE A COLD START SOLENOID THAT COULD BE STICKING??? I'LL DO THE RESISTANCE CHECK YOU MENTIONED...GOOOD THOUGHT...THANK YOU...WAYNE

David Fulcher
 
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David-F

Guest
No your car does not have a cold start injector they where only found on 85 to 88. If possible you could also check fuel pressure making sure the vaccumn hose is still on the fuel pressure regulator.

David Fulcher
 

BigRed

Well-known member
Joined
May 23, 2001
Messages
948
Location
Edmonton
Corvette
1985 Bright Red Rocket
Try looking at the coolant temp sensor as well I know on mine Its runs like crap until the dash reads out a number for the oil and collant temp instead of LO.

O2 sensor is a definite possiblity as well
 

vigman

Motor head!!!!
Joined
Feb 13, 2001
Messages
3,471
Location
Valencia, CA,USA
Corvette
88 Convert ( SOLD ) /1973 coupe 4 speed/1964 Vert!
This problem ( restated)

Started a while back on a road trip ( the idle thing)

You did all the corrective measures from here correct?

Is the EGR the orig?..It might be sticking slightly open. That would mess the idle up.

Is the IAC motor a GM one or an aftermarket?

Did you do the start & immediatly shut off then restart/relearn trick?

How many miles on the car since the repair(s)? And (I assume) you have driven the car around over 45 miles an hour since the repair?

Check the tightness of the bolts ( torx heads) on the runners to the intake plenum & the intake manifold.

Also check the VAC lines going to the cruise control.. the cannonball & the T behind the distrib.

Mike
 
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dinosaurdoctor

Guest
FUEL PRESS REG.

David-F said:
No your car does not have a cold start injector they where only found on 85 to 88. If possible you could also check fuel pressure making sure the vaccumn hose is still on the fuel pressure regulator.

David Fulcher
I ALSO CHECKED THE FUEL PRESSURE VACCUUMN HOSE BOTH FOR CONNECTION AND FUEL IN THE LINE INDICATING A BLOWN DIAPHRAM...THANKS WAYNE
 
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dinosaurdoctor

Guest
Re: This problem ( restated)

vigman said:
Started a while back on a road trip ( the idle thing)

You did all the corrective measures from here correct?

Is the EGR the orig?..It might be sticking slightly open. That would mess the idle up.

Is the IAC motor a GM one or an aftermarket?

Did you do the start & immediatly shut off then restart/relearn trick?

How many miles on the car since the repair(s)? And (I assume) you have driven the car around over 45 miles an hour since the repair?

Check the tightness of the bolts ( torx heads) on the runners to the intake plenum & the intake manifold. YES I STARTED FROM ROAD TRIP,I'LL HAVE TO CHECK THE EGR....GOOD THOUGHT.THE IAC MOTOR IS NEW G.M..I DID THE RELEARN TRICK,ABOUT 6 MILES SINCE THE REPAIR,APPROX 4 OVER 45 MPH.THANK YOU FOR THE GOOD IDEAS....WAYNE

Also check the VAC lines going to the cruise control.. the cannonball & the T behind the distrib.

Mike
 
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dinosaurdoctor

Guest
BigRed said:
Try looking at the coolant temp sensor as well I know on mine Its runs like crap until the dash reads out a number for the oil and collant temp instead of LO.

O2 sensor is a definite possiblity as well
THANK YOU GUYS FOR ALL THE HELP.ATLEAST NOW I HAVE SOME NEW THINGS TO CHECK...WAYNE
 

69MyWay

Well-known member
Joined
Jul 3, 2001
Messages
4,364
Location
Auburndale, Florida
Corvette
1969 Killer Shark
Wayne,

My money would be on the EGR valve, and or a collapsed/deteriated CAT.

When the EGR fails, it will not always set a check light. That is, if it is hanging open, it is like an internal vacuum leak. The idle may go high, or low as it tries to adjust.

There are many ways to test the EGR on the car. None of them compares to yanking it off, and bench testing it. You see, the diaphram may be moving it up and down, but it could be so full of carbon underneath that it is not closing all the way all the time.

Also, you easily test the vacuum drop when it is off in your hands.

I would only replace the EGR with a G.M. part. With the miles on your car, it is time anyhow.

As far as the cat goes. You have three of them in that car. One at the bottom of each exhaust manifold, and one big one in the middle of the system. At the mile you have, it is time to start expecting trouble. What happens is they break up internally. Sometimes you can tell right away by jacking up the car, or putting it on ramps, then getting under (after it has cooled) and rapping on the cats with a good rubber hammer or your fist. If you hear any rattle in there, you have a problem. Usually though, it won't start to rattle until they have gotten really bad.

What happens is the internals break up and start to rotate inside. As this happens under exhaust pressure the broken chunck will partially block the pipe. This is like stuffing a potatoe up the pipe. It builds back pressure, and will cause all kinds of crazy performance problems, from bad idle, to poor acceleration, to hit or miss acceleration.

However, my money is still on the EGR. Mainly because I had the EXACT same problem (no check light), and a bad EGR valve.
 
C

c4ever

Guest
Got to agree with Chris. My EGR was the culprit on my 85 and 82. No codes and no problem with open thrttle performance. In the case of the 82 it was a weak spring. Thank God I found a skilled tech to discover it.:s
My .02
 

vigman

Motor head!!!!
Joined
Feb 13, 2001
Messages
3,471
Location
Valencia, CA,USA
Corvette
88 Convert ( SOLD ) /1973 coupe 4 speed/1964 Vert!
When my Cat's plugged

I couldn't pass a YUGO.

It's a pretty cheap test to look at the cat's

The muffler guy drills a hole at the front & back of the main cat.

Puts a fitting on each side and measures the pressure difference between the input & the output of the main CAT if there's more then a 8 PSI difference the cat's going.. Mine had 17!

The car bearly ran over idle and had a nice Hoover vac sound ( not ideal for preformance)

If the back one is toast.. it typically means that the PRE cats have started to come apart
and that junk is now resting in the main cat.

If you have good flow.. he just tac welds a screw in the hole and off you go ( 45.00 lighter in the wallet).

Mine cost under 700 to replace all 3 ( after market)


Mike
 
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David-F

Guest
82 and 84 had a real problem with tip in stall because of EGR being just to sensitive have not found that to be a real problem on the 85 and up but it does happen from time to time, but the easest way to check this is just unhook the EGR valve and drive the car and see if it runs better, you may get some slight ping but if it doesn't surge or sag on tip in you may have found the problem I am trying to understand just what the problem is rough idle or surging under light throttle.

David Fulcher
 
D

dinosaurdoctor

Guest
69MyWay said:
Wayne,

My money would be on the EGR valve, and or a collapsed/deteriated CAT.

When the EGR fails, it will not always set a check light. That is, if it is hanging open, it is like an internal vacuum leak. The idle may go high, or low as it tries to adjust.

There are many ways to test the EGR on the car. None of them compares to yanking it off, and bench testing it. You see, the diaphram may be moving it up and down, but it could be so full of carbon underneath that it is not closing all the way all the time.

Also, you easily test the vacuum drop when it is off in your hands.

I would only replace the EGR with a G.M. part. With the miles on your car, it is time anyhow. I THINK YOU ARE PROBABLY RIGHT ON ABOUT THE EGR.I'LL ALSO CHECK THE CATS....THANK YOU ...WAYNE

As far as the cat goes. You have three of them in that car. One at the bottom of each exhaust manifold, and one big one in the middle of the system. At the mile you have, it is time to start expecting trouble. What happens is they break up internally. Sometimes you can tell right away by jacking up the car, or putting it on ramps, then getting under (after it has cooled) and rapping on the cats with a good rubber hammer or your fist. If you hear any rattle in there, you have a problem. Usually though, it won't start to rattle until they have gotten really bad.

What happens is the internals break up and start to rotate inside. As this happens under exhaust pressure the broken chunck will partially block the pipe. This is like stuffing a potatoe up the pipe. It builds back pressure, and will cause all kinds of crazy performance problems, from bad idle, to poor acceleration, to hit or miss acceleration.

However, my money is still on the EGR. Mainly because I had the EXACT same problem (no check light), and a bad EGR valve.
 

BigRed

Well-known member
Joined
May 23, 2001
Messages
948
Location
Edmonton
Corvette
1985 Bright Red Rocket
Just thought of something my mechanic told me once, "The runners going to your plenum are made of a very thin and weak metal be sure not to hurt them"

Mabe look for a crack there?
 
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dinosaurdoctor

Guest
BigRed said:
Just thought of something my mechanic told me once, "The runners going to your plenum are made of a very thin and weak metal be sure not to hurt them"

Mabe look for a crack there?
anything is worth a try...thank you...wayne
 
T

Tanzanite3

Guest
Check this out too..

The "pintle" (pointy end of the IAC) gets carbon'd up. Changing the motor will not help if this is the problem. However, what happens is that it can stick either open or closed (usually closed!) .. this will cause stalling or rough idle conditions since air will not be able to by-pass the throttle when the throttle is closed. I solved this problem on mine by carefully cleaning the pintle with a fine nail file and some 000 steel wool. Very careful as it's a delicate little beast! Re-installed and all's ok.
Just another idea.. and who knows?? It might work! Good luck.
 

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