Cant solve rough idle
Thread Starter
Drifting
Joined: Jul 2009
Posts: 1,258
Likes: 98
From: Ashland PA
2021 C4 of the Year - Modified Finalist
Cant solve rough idle
I have checked Ohm reading on injectors hot and cold, checked for vacuum leaks, checked EGR, checked spark plugs and wires, checked timing and ignition module, checked fuel pressure, all checked good. This rough idle started about 2 months ago after a spark plug wire came off wile driving down the highway, when at a stop in gear it idles very rough to the point I have to place it in neutral. Could the ecm been damaged by running with one spark plug wire off. Any idea what else I should look at. I have no codes on the computer. this is a 1990 auto with 185K+ on it, she starts right up hot or cold. Thanks For Any Help. 
Last edited by eutu1984; Jul 22, 2015 at 06:18 PM. Reason: add
Race Director
Joined: Aug 2000
Posts: 11,470
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so, let me see if I got this right - the idle problem started after one of the spark plug wires came off, and you did what - ??? did you replace the single wire, repair the single wire, or what? if that's the only thing that's been done, the single wire thing, seems to me, there's your problem. there shouldn't be any ECM issue. pull that plug, inspect, and take a really good look at that spark plug wire - 
Thread Starter
Drifting
Joined: Jul 2009
Posts: 1,258
Likes: 98
From: Ashland PA
2021 C4 of the Year - Modified Finalist
Instructor
Joined: Aug 2014
Posts: 144
Likes: 2
From: Ca
Maybe the distributer cap didn't like that if its older! Or after market? HEI'z can have small issues cracks not seen! I have had new caps that were crap! When that coil Sparks and it doesn't ground through a plug it finds a ground! Electricity is lazy! Goes the shortest distance! Another crossed wire? Something close to the cap? If you haven't yet! new cap n rotor! Run in dark of night with hood up! May find a small blue arch! Just saying bro!
Race Director
Joined: Mar 2015
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Maybe the distributer cap didn't like that if its older! Or after market? HEI'z can have small issues cracks not seen! I have had new caps that were crap! When that coil Sparks and it doesn't ground through a plug it finds a ground! Electricity is lazy! Goes the shortest distance! Another crossed wire? Something close to the cap? If you haven't yet! new cap n rotor! Run in dark of night with hood up! May find a small blue arch! Just saying bro!
and check the wires again. 
Thread Starter
Drifting
Joined: Jul 2009
Posts: 1,258
Likes: 98
From: Ashland PA
2021 C4 of the Year - Modified Finalist
Maybe the distributer cap didn't like that if its older! Or after market? HEI'z can have small issues cracks not seen! I have had new caps that were crap! When that coil Sparks and it doesn't ground through a plug it finds a ground! Electricity is lazy! Goes the shortest distance! Another crossed wire? Something close to the cap? If you haven't yet! new cap n rotor! Run in dark of night with hood up! May find a small blue arch! Just saying bro!
I will check the cap its not that old either so I will place the old one back on and see if it makes a difference, I checked the spark of each wire by placing the timing light on each spark plug wire and the flash from the strobe is nice and steady on each wire, also even though it idles rough the rpm is pretty steady. I did the run at night thing, it was one of the first things I checked back when the problem started.
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Thread Starter
Drifting
Joined: Jul 2009
Posts: 1,258
Likes: 98
From: Ashland PA
2021 C4 of the Year - Modified Finalist
Instructor
Joined: Aug 2014
Posts: 144
Likes: 2
From: Ca
I will check the cap its not that old either so I will place the old one back on and see if it makes a difference, I checked the spark of each wire by placing the timing light on each spark plug wire and the flash from the strobe is nice and steady on each wire, also even though it idles rough the rpm is pretty steady. I did the run at night thing, it was one of the first things I checked back when the problem started.
Racer
Joined: Dec 2010
Posts: 427
Likes: 124
Previous post suggested egr. I went through 3 or 4 in my '89. They just don't make 'em like they used to. Pull a vacuum on it and the car should stall at idle. No effect and its leaking some where or stuck partially open. That it seemed to happen when the plug wire came off might just be coincidence.
Thread Starter
Drifting
Joined: Jul 2009
Posts: 1,258
Likes: 98
From: Ashland PA
2021 C4 of the Year - Modified Finalist
Thread Starter
Drifting
Joined: Jul 2009
Posts: 1,258
Likes: 98
From: Ashland PA
2021 C4 of the Year - Modified Finalist
Previous post suggested egr. I went through 3 or 4 in my '89. They just don't make 'em like they used to. Pull a vacuum on it and the car should stall at idle. No effect and its leaking some where or stuck partially open. That it seemed to happen when the plug wire came off might just be coincidence.
I pulled a vacuum on the egr, it checks out, I also blocked off the egr tube just to play it safe and that did not change my idle problem either.
Thread Starter
Drifting
Joined: Jul 2009
Posts: 1,258
Likes: 98
From: Ashland PA
2021 C4 of the Year - Modified Finalist
tonight after work I will try swapping the cap, but from what I am seeing its not the ignition. I will also run another data log at it wile just idling.
Melting Slicks
Joined: Sep 2008
Posts: 2,376
Likes: 111
From: Cherokee National Forest TN
Stock 90 injectors are prone to fail.
Suggest ohm testing the injectors when cold then hot, 14-16 ohms.
L98's tend to swallow intake gaskets, spray WD40 through supplied straw @ all mating joints. A change in rpm = failed gasket.
If issues persist suggest a compression check, @ 185K miles the valve guides could be showing signs of wear.
Suggest ohm testing the injectors when cold then hot, 14-16 ohms.
L98's tend to swallow intake gaskets, spray WD40 through supplied straw @ all mating joints. A change in rpm = failed gasket.
If issues persist suggest a compression check, @ 185K miles the valve guides could be showing signs of wear.
Thread Starter
Drifting
Joined: Jul 2009
Posts: 1,258
Likes: 98
From: Ashland PA
2021 C4 of the Year - Modified Finalist
Stock 90 injectors are prone to fail.
Suggest ohm testing the injectors when cold then hot, 14-16 ohms.
L98's tend to swallow intake gaskets, spray WD40 through supplied straw @ all mating joints. A change in rpm = failed gasket.
If issues persist suggest a compression check, @ 185K miles the valve guides could be showing signs of wear.
Suggest ohm testing the injectors when cold then hot, 14-16 ohms.
L98's tend to swallow intake gaskets, spray WD40 through supplied straw @ all mating joints. A change in rpm = failed gasket.
If issues persist suggest a compression check, @ 185K miles the valve guides could be showing signs of wear.
Last edited by eutu1984; Jul 23, 2015 at 03:22 PM.
Thread Starter
Drifting
Joined: Jul 2009
Posts: 1,258
Likes: 98
From: Ashland PA
2021 C4 of the Year - Modified Finalist
here is a good article on checking an engine with a vacuum gauge.
http://www.gregsengine.com/using-a-vacuum-gauge.html
for anyone not familiar with it.
http://www.gregsengine.com/using-a-vacuum-gauge.html
for anyone not familiar with it.
Pro
Joined: Nov 2014
Posts: 527
Likes: 23
From: Mishawaka Indiana
here is a good article on checking an engine with a vacuum gauge.
http://www.gregsengine.com/using-a-vacuum-gauge.html
for anyone not familiar with it.
http://www.gregsengine.com/using-a-vacuum-gauge.html
for anyone not familiar with it.