go ahead and change those plugs out. AC Delco standard 41-629 or Rapidfire 5 and see if any change.
Will the wrong plugs cause rough idle?
Thread Starter
Intermediate
Joined: Jan 2005
Posts: 46
Likes: 0
From: Woodstock Georgia
Will the wrong plugs cause rough idle?
I replaced the plugs on my 87 a couple of weeks ago. I had a rough idle but I thought I had narrowed it down to a bad plug. I put the new plugs in but still had a rough idle but not the same as it was. It is not constant , sounds more like a cam lope. Anyway after doing a lot of troubleshooting, it turns out I got plugs for iron heads instead of AL.
Could that be the source of the problem? I have removed the TB and cleaned it and the IAC pintle. Ohmed the wires, Checked fuel pressure, ohmed the injecters. I have no codes and the engine miss is not detectable above idle. It just sounds rough sitting at a traffic light.
Any ideas?
Could that be the source of the problem? I have removed the TB and cleaned it and the IAC pintle. Ohmed the wires, Checked fuel pressure, ohmed the injecters. I have no codes and the engine miss is not detectable above idle. It just sounds rough sitting at a traffic light.
Any ideas?
Thread Starter
Intermediate
Joined: Jan 2005
Posts: 46
Likes: 0
From: Woodstock Georgia
I have not replaced the cap or rotor. I am not one to throw parts at problems. I have cleaned the cap and rotor and they looked ok. I don't know how long they have been on the car. I thought i had a vacuum leak as well but I can't detect any. I have tried the propane trick and cannot find anything. I have used a steth to find hissing and cant find anything. How do I check the EGR?
Burning Brakes
Joined: Jan 2006
Posts: 847
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From: Colorado Springs CO
To check, loosen the union and disconnect the EGR line to the exhaust manifold at the bottom of the EGR valve. Then plug the open bottom port of the EGR valve. If the idle smooths out, it is because the valve was leaking and should be replaced. I hope this makes sense.
I don't like throwing parts at a problem either.
I don't like throwing parts at a problem either.
Le Mans Master
Joined: Sep 2006
Posts: 6,050
Likes: 147
From: Phoenix AZ
2021 C4 of the Year - Modified Finalist
plugs
be sure to get the correct plugs installed. using the incorrect plugs will fill up the spark plug threads with soot and carbon, and make it difficult to actually install the correct ones.
The other benefit from using the correct plugs is the compression ratio will return to normal, (higher) and the end of the spark plug will be back into the combustion chamber as it should be.
Crisper throttle response should be evident also.
I would do a compression check and determine cylinder sealing is intact.
unleaded gas doesn't protect the exhaust valves like leaded used to, and the exhaust vave seats and the valve faces get pits in them, and helps to erode the compression in various cylinders.
after that, check the EGR valve, and look for multiple vacuum leaks.
The other benefit from using the correct plugs is the compression ratio will return to normal, (higher) and the end of the spark plug will be back into the combustion chamber as it should be.
Crisper throttle response should be evident also.
I would do a compression check and determine cylinder sealing is intact.
unleaded gas doesn't protect the exhaust valves like leaded used to, and the exhaust vave seats and the valve faces get pits in them, and helps to erode the compression in various cylinders.
after that, check the EGR valve, and look for multiple vacuum leaks.
Last edited by coupeguy2001; Jan 23, 2007 at 12:24 AM.
Team Owner
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Also, make sure the plugs are gapped correctly. Just because they say they're pre-gapped on the box doesn't mean the pucks didn't get bent during shipping.
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code5coupe
Thread Starter
Intermediate
Joined: Jan 2005
Posts: 46
Likes: 0
From: Woodstock Georgia
I replaced the plugs as well as the wires, cap and rotor. It runs better but still has a telltale miss. It turns out the miss is not just at idle but occurs at all RPMs. I Checked the EGR and it seems to be OK but not the source of the problem. I suspect I have a vacuum leak somewhere still. I'll check compression. Thanks for all the input. I'll keep you posted.
Safety Car
Joined: Nov 2004
Posts: 4,991
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From: The Pines South Jersey
I replaced the plugs as well as the wires, cap and rotor. It runs better but still has a telltale miss. It turns out the miss is not just at idle but occurs at all RPMs. I Checked the EGR and it seems to be OK but not the source of the problem. I suspect I have a vacuum leak somewhere still. I'll check compression. Thanks for all the input. I'll keep you posted.
Le Mans Master
Joined: Aug 2005
Posts: 5,002
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From: No Opti Here Tn
St. Jude Donor '06
When my 86 did this it was bad injectors (3)
Yours could be just one
I would try a fuel pressure test
If your fuel pressure drops quickly after shut down that is your problem
Could also cause hard starting
Yours could be just one
I would try a fuel pressure test
If your fuel pressure drops quickly after shut down that is your problem
Could also cause hard starting
Thread Starter
Intermediate
Joined: Jan 2005
Posts: 46
Likes: 0
From: Woodstock Georgia
I've checked the fuel pressure and it stays steady at 40# and doesn't leak down. It's possible one or more of the injectors are dirty. I've Seafoamed it as well with no change. The car has 110K on it so things are starting to wear out and and don'thave a complete maintenance history on the car. I don't know when the fuel filter has been changed or the O2 sensor. I don't suspect I have a filter problem but I could unplug the O2 sensor and see if that changes anything.
Team Owner
Joined: Dec 2000
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From: SE NY
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Clip a timing light in turn onto each plug wire and see if missed flashes coincide with engine miss; if so it's a wire or cap issue.
If electrical is ok, do a resistance check on each injector.
If all are consistent pull the set and send to Rich at Cruzinperformance to be reconditioned.
If electrical is ok, do a resistance check on each injector.
If all are consistent pull the set and send to Rich at Cruzinperformance to be reconditioned.
Melting Slicks
Joined: Apr 2002
Posts: 2,118
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From: St. Charles, MO Route 66 Corvette Club
Lilred87: When you changed out the spark plugs.....did you examine each plug to see if one or more plugs had a different appearance from the other plugs? Sometimes, this will indicate a potential problem with one or more cylinders. If you know anyone close to you that has a scan tool, this could help pinpoint your problems. Otherwise it is a gamble on changing out parts.
Drifting
Joined: Oct 2006
Posts: 1,694
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From: Beaverton Oregon
I replaced the plugs on my 87 a couple of weeks ago. I had a rough idle but I thought I had narrowed it down to a bad plug. I put the new plugs in but still had a rough idle but not the same as it was. It is not constant , sounds more like a cam lope. Anyway after doing a lot of troubleshooting, it turns out I got plugs for iron heads instead of AL.
Could that be the source of the problem? I have removed the TB and cleaned it and the IAC pintle. Ohmed the wires, Checked fuel pressure, ohmed the injecters. I have no codes and the engine miss is not detectable above idle. It just sounds rough sitting at a traffic light.
Any ideas?
Could that be the source of the problem? I have removed the TB and cleaned it and the IAC pintle. Ohmed the wires, Checked fuel pressure, ohmed the injecters. I have no codes and the engine miss is not detectable above idle. It just sounds rough sitting at a traffic light.
Any ideas?
