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Low RPM Miss

Gorgon

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Joined
Jul 15, 2002
Messages
1,040
Location
Philadelphia, PA
Corvette
None
My engine has developed an odd miss at idle. This has been going on for some time and I thought an application of Sea Foam had help out but it's not totally gone.

At idle the car has a random miss. When launching the car the engine stumbles when the RPM drops under 1000RPM. It almost seems like the clutch slips a but that's not the case. Once the car is launched and the RPMs are up the car is fine but this is getting worse.

What's been done:
Plugs and wires (AC Delco iridium tips)
Sea Foam application as well as a GM top engine clean
New air filter (K&N)
New fuel filter

There are no codes thrown and I'm looking for other options. I would think that the MAF or O2 sensors would throw codes. It seems like it might be a sensory since it normally happens in closed loop mode and not open loop mode. Any suggestions?

Leon
 

Ripp 73

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Joined
Oct 13, 2004
Messages
406
Location
Columbus, OH
Corvette
1973 L82 Blue Coupe
Well, I guess I'll be the first to throw out an idea. You would think with all the sensors on these cars that if anything was changing that a code would be thrown. You mentioned that plugs and wires have been changed. How long ago was this and did you have the miss and stumble at low RPM's before you changed them out? Also, how many miles are on your 99 FRC?
I have known a few cars to miss at low RPM because of a cracked Spark Plug. All it takes is a small hair line crack on the porcelain and you can have a miss at low RPM. Eventually the porcelain will crack even worse and if you drive a standard you won’t be able to shift into 6th because the car will shake so badly. This is also something that does not always throw a code right away. This may not be your case at all, but just an idea to play with. If you have a chance to pull the plugs you might want to look at them. Then again, wait for some other responses.
 

Gorgon

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Joined
Jul 15, 2002
Messages
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Location
Philadelphia, PA
Corvette
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This has been present for some time but just not this bad. The car has 122k miles on it and the plugs were done some time ago. The plug change was completed around 100k miles and did not affect this at all.

Leon
 

LLC5

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Joined
Sep 28, 2004
Messages
2,299
Location
Wa.
Corvette
98 black 6spd convert.
Start with the basics first if there are no codes. External vaccuum leaks should be checked, hoses, pcv valve, etc. Make sure the plug wires are clipped on tight to both the spark plug and ignition coils. Look for any burning, tears, or carbon tracks. Pulling the plugs and looking at them for any differences in burning patterns, and cracks, would be a good idea. If one plug looks different, then you know which cylinder is affected.
 

bwing

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Joined
Oct 28, 2005
Messages
517
Location
California
Corvette
03 Z06,66 coupe(sold),99 Porsche Carrera(sold)
Did you have this miss before you changed your plug wires? Check your plug wires because it sounds like you may have mixed 2 of them up. Just my humble opinion.
 

zagger

Well-known member
Joined
Apr 22, 2006
Messages
468
Location
San Diego
Corvette
99 C5 Convertible
What's been done:
Plugs and wires (AC Delco iridium tips)
Sea Foam application as well as a GM top engine clean
New air filter (K&N)
New fuel filter

I'm reaching here but when you added the K&N filter did you check if there was an excessive amount of oil on it? To much oil on these type of filters could be sucked onto MAF or the IAT sensor and that could effect the engine performance without throwing any codes. I learned that lesson long time ago on my Jeep. now whenever I recharge any K&N filter or even when buying new one I dry it by ruining a shop vac over it. That does not degrade the filter but you would be surprised how much oil they can hold. There are several different types of aerosol MAF cleaners on the market, don't remember which one I used but they all should be good. Hope this might help you, good luck.
Zig.
 

Gorgon

Well-known member
Joined
Jul 15, 2002
Messages
1,040
Location
Philadelphia, PA
Corvette
None
I'm reaching here but when you added the K&N filter did you check if there was an excessive amount of oil on it? To much oil on these type of filters could be sucked onto MAF or the IAT sensor and that could effect the engine performance without throwing any codes. I learned that lesson long time ago on my Jeep. now whenever I recharge any K&N filter or even when buying new one I dry it by ruining a shop vac over it. That does not degrade the filter but you would be surprised how much oil they can hold. There are several different types of aerosol MAF cleaners on the market, don't remember which one I used but they all should be good. Hope this might help you, good luck.
Zig.

I will try to clean the MAF but the filter I just put on was a brand new filter. I had cleaned and oiled the old one before but I honestly can't remember if the problem was there before I cleaned the filter last time.

The problem was there before and after the plug change and, in fact, there was absolutely no change in this issue when the new plugs were installed. I had hoped that would have fixed the issue. This was part of the reason I was doing the plugs to begin with.

Leon
 

Willfulone

Well-known member
Joined
Nov 12, 2006
Messages
223
Location
VA
Corvette
2004 CE Coupe
I will try to clean the MAF but the filter I just put on was a brand new filter. I had cleaned and oiled the old one before but I honestly can't remember if the problem was there before I cleaned the filter last time.

The problem was there before and after the plug change and, in fact, there was absolutely no change in this issue when the new plugs were installed. I had hoped that would have fixed the issue. This was part of the reason I was doing the plugs to begin with.

Leon


My bet is you have a coil going bad...

Good luck with your fix

Peace
Chip
 

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