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Rough Idle/Surging - FIXED!


Well-known member
May 18, 2003
Orange County, CA
1991 Turquoise Metallic Coupe
This story is a bit long and winded but it does have a happy ending--at least so far. Time will tell.

My 1991 L98 has about 53k miles on it. The car is fairly stock with an automatic. It does have a Corsa cat-back exhaust and K&N open box and filter.

Back July, on a very hot summer evening, I took my 'Vette out for a drive. It was probably around 85 degrees still and I got stuck in some stop and go traffic on my home, so the engine was at 220 degrees for a bit. When I got home, I noticed the engine was running a bit rough but thought nothing of it. Well, the problem got a bit worse. At idle, the engine most definitely had a miss and would stumble now and again. At cruise, there was a definite surging around 1,400 RPM under a low load. Now this problem afflicts many older C4's and that's obvious with the many posts asking for help in the Technical Section. Although my car was not throwing a code, I suspected the EGR Valve and PCV Valves were at fault. After replacing both, the problem persisted. I figured well it could be a vacuum leak. The car's acceleration under heavy throttle began to suffer and it no longer responded smoothly. It felt as if the power curve had a hole coming out of 2nd gear, and the rough idle and surging persisted. I finally threw in the towel and took the car to a reputable service shop. The mechanic and I went on a test drive so I could demonstrate to him the problem. The Mechanic was convinced the engine was misfiring and it was most likely a bad spark plug or plug wire. The next series of events are amazing!

Before all this happened, and about 20k miles ago, I had my local Chevy dealership give my car a tune up, which included new spark plugs, new ignition wires, cap and rotors, and timing adjustment. Per the book, the car didn't need another tune up until another 10k miles. I let the mechanic know about this but he was still convinced it was a bad spark plug or ignition wire due to how the engine was behaving.

After a day of tinkering with it, I got a phone call from the shop and they told me the bad news. The mechanic pulled the spark plugs on the driver's side of the car and he claimed to have seen coolant in the plugs! The coolant was due to a blown head gasket and/or possible crack in the cylinder heads. This caused coolant to seep through into the combustion chambers, which in turn made the car idle rough and surge at small throttle openings. Repairs: replace cylinder head gasket - $1700 parts and labor! :eek:hnoes

I was freakin' shocked! From a misfire to this? How can this be possible? After a bit of thought, I asked the shop if all the spark plugs on the driver's side were "wet" and the answer was yes. My reply was, if this was the case, shouldn't the engine be running worse, and shouldn't there be coolant in the oil, and shouldn't I be loosing coolant? None of these corroborating evidence existed. The shop owner couldn't really give me a straight answer after that, just a justification for their findings. I then asked if they could simply replace all 8 spark plugs and see what happens. The shop owner questioned that route and asked what the point of that would be if the cylinder had gasket was leaking. The hairs in the back of my neck were fluttering with doubt but what do I know? I'm not a mechanic. I took the car back home to contemplate my situation.

Due the absence of any other evidence to suggest coolant leaking into the combustion chambers, I decided to pull the plugs and see for myself what the mechanic was telling me. I pulled all 4 plugs on the driver's side and they all looked identical: perfectly normal color and wear at their tips, with a shiny threaded area (looked oily/wet). I wiped the threads and came up with just thin oily streaks. I figured, screw it! Since I had the plugs, out, may as well replace the plugs with new ones just to test my theory. I went ahead and changed the rest of the plugs on the other side (what a freakin' ***** that was!). After a lot of cussing, bruised knuckles and a couple of cuts, I got the spark plugs replaced.

The result: low and behold! The rough idle was gone. The surging at low RPM and small loads was gone. The acceleration is once again smooth and linear. I drove the car about 75 miles and it ran great! :upthumbs

Questions: If indeed the cylinder head gaskets were leaking, would fresh plugs cure the problem only for a bit until they get "wet" again? Is this fix just temporary?

I sure hope not! Right now I'm just happy to get my 'Vette running great again. Still, I can't help but wonder: How can my trusted repair shop mis-diagnose my car's problem so badly? They were ready to pull the cylinder heads out and charge me a whopping $1700! I can't help but wonder what if I trusted them? These thoughts scare me! It scares me because I'm not a professional mechanic and yet, I was able to (temporarily anyway) fix the problem simply by looking at the evidence and/or lack thereof before making a diagnosis! I sure hope I'm right. So far, it appears I am.
If you have a cracked or leaking cooling system,
Have pressure test of the system done.
if it holds pressure without losing pressure,then you do not have a problem.
you may be able to rent one at your favorite local parts house and they can show you how to use it.
"bill1vette" is right.
Speculation is pointless.

A simple cooling system pressure test will identify leaking head gaskets.

It is possible for a head gasket to fail and not have oil in the coolant. Typically when that happens, when the engine runs, exhaust gases go into the coolant and, then, when you shut the engine off, pressure in the cooling system causes the "flow" to reverse and coolant ends up in the cylinder surrounded by the leaking head gasket.
I'll have the pressure test done as soon as possible.

However, at this point the car is purring like a kitten. The stumbling at idle and the surging at low speed and small throttle openings are completely gone. Both of these were what got me to take the car to get checked anyway.

I'll keep an eye out. In the meantime, I'm loving the car all over again. If the pressure test comes out okay, I'm moving forward with my bolt on engine mods! :thumb

Oh yeah! I found a new shop and mechanic and who works on Corvettes all the time from simple tune ups to complete build ups. The plus side is, they're a mere 5 minutes away.
GRC Performance: http://http://www.grcperformance.com/default.cfm

They're located in south Orange County, CA. The owner/operator also does the work. They have a dyno and do full service work including high performance upgrades, crate motor swaps, you name it.
When I went there, they had equal numbers of Vettes and Stangs. They also do Chrysler. Hell! They were doing work on a Saturn and Jeep (tune up). They do everything and they have to in order to survive in that industry. Tuning only Mustangs won't last very long.

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