90 Coupe idle issues
Thread Starter
Melting Slicks
Joined: May 2009
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From: Decatur IL
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90 Coupe idle issues
I've been having a hesitation/miss/idle issues and it's been getting worse. I replaced the fuel filter about two months ago and helped a little and replaced the spark plugs last weekend and helped a little. I was thinking a bad plug wire, but I'm suspecting the egr valve could be sticking. The car has 45k and is not a daily driver. Any comments are appreciated.
Le Mans Master
Joined: Jul 2007
Posts: 5,356
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From: No HOA, New Hampshire
Cruise-In X Veteran
St. Jude Donor '09
If you are going to wrench on it yourself you need a FSM. What you did so far is OK but it looks like you are heading to just throw parts at it until it is fixed.
Because it is easy and cheap, I would suggest visually checking the cap and rotor. Carbon tracking or burned contacts will cause your problems.
Because it is easy and cheap, I would suggest visually checking the cap and rotor. Carbon tracking or burned contacts will cause your problems.
Burning Brakes
Joined: May 2011
Posts: 838
Likes: 2
From: Louisville Ky
I've been having a hesitation/miss/idle issues and it's been getting worse. I replaced the fuel filter about two months ago and helped a little and replaced the spark plugs last weekend and helped a little. I was thinking a bad plug wire, but I'm suspecting the egr valve could be sticking. The car has 45k and is not a daily driver. Any comments are appreciated.
some easy things are cleaning throttle body with tb cleaner, whatever, a clean rag or brush. no need to remove it. also ck TPS, throttle position sensor. yours is not adjustable, I think, but is a possibiity, since it controls fuel signals.
if it starts fine probably not tps. "sounds like" ignition problem, maybe a plug that has insulator cracked, wires leaking, etc. should have seen evidence with plug colors when you changed them.
clean inside dist cap, check for cracked cap, etc. rotor is cheap but rarely is a problem. if base timing is retarded, it can cause hesitation but not the miss, probaby not rough idle.
if MAP sensor -manifold absolute pressure- is part of the issue, it would also cause hesitatioin. you can unplug the tps, run it to see the reaction. plug in, then unplug map, same thing. this will set a code. if you have not checked for codes or don't know how, I can point you to some instructions, or others will.
joe
Thread Starter
Melting Slicks
Joined: May 2009
Posts: 2,772
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From: Decatur IL
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Thanks for the responses. I was texting a buddy today that has a low mile 90 that he's owned for two decades and he said that it could be the IAC valve not functioning. He was having similar issues as mine. He also replaced the computer at the same time and his runs a lot better. I hope this helps.
Racer
Joined: Mar 2011
Posts: 304
Likes: 0
From: Victoria Australia
I've been having a hesitation/miss/idle issues and it's been getting worse. I replaced the fuel filter about two months ago and helped a little and replaced the spark plugs last weekend and helped a little. I was thinking a bad plug wire, but I'm suspecting the egr valve could be sticking. The car has 45k and is not a daily driver. Any comments are appreciated.
Leaking injectors (albeit if leaking are not severe) will not be too noticeable under cruising and generally doesn't affect the WOT scenario. I find at idle the car feels lumpy as there is minimal air but its dumping more fuel than necessary.
I just found this out yesterday with my 1990, so a new set of injectors are on the list.
Last edited by XR_Strider_GuY; Jul 8, 2012 at 05:10 AM.
Melting Slicks
Joined: Sep 2008
Posts: 2,376
Likes: 111
From: Cherokee National Forest TN
All good suggestions.
@ 45K it is due for a cap, rotor & wires.
If still using the stock injectors they will fail. Suggest ohm testing them cold & hot.
IIRC the reading should be 14-16 ohms. Wait for someone here to verify the correct reading or a search will yeald the correct #'s. Been 3 years since the Bosch 3's were installed in my 90, it runs as new, not so much as a hic-cup.
Jon @ FIC here on the forum is the man for injectors.
GL
@ 45K it is due for a cap, rotor & wires.
If still using the stock injectors they will fail. Suggest ohm testing them cold & hot.
IIRC the reading should be 14-16 ohms. Wait for someone here to verify the correct reading or a search will yeald the correct #'s. Been 3 years since the Bosch 3's were installed in my 90, it runs as new, not so much as a hic-cup.
Jon @ FIC here on the forum is the man for injectors.
GL
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Melting Slicks
Joined: May 2009
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From: Decatur IL
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For some reason I'm just not feeling that it could be the injectors or the ignition. The car runs fine without load, but doesn't run under load. Yesterday I did a driveway check and noticed that when the ac is off and in gear the car holds an idle, but when I turned on the ac the car ran rough and stalled. The iac valve is supposed to keep the car from stalling under load changes and mines not. One of my projects down the road is that I'm planning on having some engine parts powdercoated. When I decide to have this done I'm gonna replace the injectors and upgrade the ignition.
Intermediate
Joined: Jun 2012
Posts: 36
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From: Iowa
According to my Chilton manual there is an easy way to check to see if your IAC valve is working properly.
Here are their instructions.
Unplug the IAC connector and check the resistance between the IAC terminals. Resistance between terminals A to B and terminals C to D should be 40 to 80 ohms. If resistance is not within specification the IAC valve must be replaced.
Check the resistance between terminls A to C, A to D, B to C and B to D. Resistance should be infinite. If not the IAC valve is faulty.
Regards
Mac
Here are their instructions.
Unplug the IAC connector and check the resistance between the IAC terminals. Resistance between terminals A to B and terminals C to D should be 40 to 80 ohms. If resistance is not within specification the IAC valve must be replaced.
Check the resistance between terminls A to C, A to D, B to C and B to D. Resistance should be infinite. If not the IAC valve is faulty.
Regards
Mac
Thread Starter
Melting Slicks
Joined: May 2009
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From: Decatur IL
2025 C4 of the Year Finalist - Unmodified
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According to my Chilton manual there is an easy way to check to see if your IAC valve is working properly.
Here are their instructions.
Unplug the IAC connector and check the resistance between the IAC terminals. Resistance between terminals A to B and terminals C to D should be 40 to 80 ohms. If resistance is not within specification the IAC valve must be replaced.
Check the resistance between terminls A to C, A to D, B to C and B to D. Resistance should be infinite. If not the IAC valve is faulty.
Regards
Mac
Here are their instructions.
Unplug the IAC connector and check the resistance between the IAC terminals. Resistance between terminals A to B and terminals C to D should be 40 to 80 ohms. If resistance is not within specification the IAC valve must be replaced.
Check the resistance between terminls A to C, A to D, B to C and B to D. Resistance should be infinite. If not the IAC valve is faulty.
Regards
Mac
Thread Starter
Melting Slicks
Joined: May 2009
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From: Decatur IL
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Today I had a buddy from the Corvette club stop by and we replaced the iac valve and cleaned the throttle body, it was carboned up where the iac valve goes. When we put it back together and started her up it was doing the same thing. It was running fine at idle without the ac on and in gear, but ran rough and stalled with ac on and in gear. A short drive said there's some hesitation with the ac off and bogging like crazy with the ac on. Does the iac valve need a learning phase before the computer recognizes it? Could there be something in the ac system that's bogging the car down? Or could the tps or map sensors not functioning?
Burning Brakes
Joined: May 2011
Posts: 838
Likes: 2
From: Louisville Ky
Today I had a buddy from the Corvette club stop by and we replaced the iac valve and cleaned the throttle body, it was carboned up where the iac valve goes. When we put it back together and started her up it was doing the same thing. It was running fine at idle without the ac on and in gear, but ran rough and stalled with ac on and in gear. A short drive said there's some hesitation with the ac off and bogging like crazy with the ac on. Does the iac valve need a learning phase before the computer recognizes it? Could there be something in the ac system that's bogging the car down? Or could the tps or map sensors not functioning?
these are clues, and may not be your problem. it should set codes for tps and map, 21 and 33. my thought is that ac on is altering fuel delivery, which may be out of balance already, from sensors, even CTS.
Burning Brakes
Joined: Apr 2009
Posts: 877
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From: Broken Arrow Oklahoma
On my 90 I had a slight miss at idle, but ran great under acceleration. Turned out 2 injectors ohmed at 5.6 or less. Replaced injectors and the car ran great. I replaced the IAC for another reason.
Instructor
Joined: Jun 2003
Posts: 193
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From: Central Ill
Just asking for my own but would like any advice just to rule out different things that it might be with reason and ways to check.
1 ECM
2 Cats
3 Fuel filter
4 Head gasket
5 Vacum lines
ROD
1 ECM
2 Cats
3 Fuel filter
4 Head gasket
5 Vacum lines
ROD
Last edited by BOTY79; Jul 23, 2012 at 12:14 AM. Reason: Adding questions
Le Mans Master
Joined: May 2004
Posts: 5,866
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From: Hellinois
Finalist 2020 C4 of the Year - Modified
2. If a cat is plugged it will usually choke the engine at higher rpm than idle and exhaust temps are higher between the engine & cat.
3. They're cheap so if there is doubt, change it.
4. A bad head gasket can be detected with a compression test, the engine may have a miss.
5. Vacuum lines, check vacuum with a gauge if it is low or fluctuating a lot then it's time to start inspecting each & every one, inch by inch. I can personally attest to this one.
Thread Starter
Melting Slicks
Joined: May 2009
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From: Decatur IL
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Over the weekend I had a buddy stop by and did a number of checks. We tested the iac which was changed and the throttle body cleaned, it was ok. We checked the timing and was ok. We checked the fuel psi and the injectors and are good. Thing of it is I found out later Sunday night at a Forum buddy's house that when the spark plugs were replaced a few weeks ago the #2 cylinder plug was not in all the way and was cross-threaded. It was loosing compression there. I'm not ruling out a head gasket since I have been dealing with this hesitation for a while. Now I'm looking into somebody with experience with a threader. My Vette is undrivable and I'm not happy.
Advanced
Joined: Jul 2010
Posts: 53
Likes: 1
From: Avon CT
I have 1990 that had the same problem when i bought it . Tried the usual things plugs and wires . Still N/G then found a thread on this form to check the injectors that was it . Ohm them out cold and hot like was suggested here . I found 3 bad that I replaced ran good for a few days then started acting up again tested hot found 3 more . At that point I said what the hell and replaced them all . The car has run fine for the last two years . That 14 to 16 oms sounds about right to tell if good or bad . My bad read three to four oms .
Thread Starter
Melting Slicks
Joined: May 2009
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From: Decatur IL
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Update!!!!!
After work today I stopped by the shop expecting to get my valve covers to clean and refinish since it was in for all new gaskets since I thought I had a faulty head gasket. The shop owner fixed the #2 plug thread and said the fuel pump was running at half pressure. He also said that the MAP sensor could be an issue. While the Vettes in , it's getting a tune up and fresh coolant. I'll be cruising thks weekend!
Racer
Joined: Mar 2011
Posts: 304
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From: Victoria Australia
CRUZN C4,
How is your 1990 running now? Very ironic but I too found that plug 2 was loose and only way to tighten it was to undo the access panel at the wheel well on the right side.
I noted though once the engine is warm and coming to a stop in drive the idle can be just under 500rpm (eyeing the tacho needle) with or without aircon. Feels a touch rough but I am wondering if its just the way the car is. What does yours idle at?
Mine has been going great with new injectors from FIC. A quick drive mix of highway and busy peak hour traffic has shown an average of 20.8 MPG.
How is your 1990 running now? Very ironic but I too found that plug 2 was loose and only way to tighten it was to undo the access panel at the wheel well on the right side.
I noted though once the engine is warm and coming to a stop in drive the idle can be just under 500rpm (eyeing the tacho needle) with or without aircon. Feels a touch rough but I am wondering if its just the way the car is. What does yours idle at?
Mine has been going great with new injectors from FIC. A quick drive mix of highway and busy peak hour traffic has shown an average of 20.8 MPG.