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  1. #1
    Member Rogue's Avatar
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    Default Coolant Leak in Cylinder Heads! Correct Diagnosis?

    I need to know what is really going on with my Corvette.

    In 2008 my '91 L98 Corvette began idling rough a day after a long drive. It felt like the car had a vacuum leak at idle. It would also surge at small throttle openings. At 35k miles, back in 2003 I had the car tuned up by the dealership which included new spark plug wires, new spark plugs, etc. In 2008 the car had 51k miles on it. I know....I don't drive the car that much! I ride a motorcycle to work that's why.

    I replaced the EGR valve thinking it was a stuck valve. The problem persisted. So I broke down and took the car to a trusted garage and took their mechanic for a test drive so that I could demonstrate to him what the issue was. After a day at the garage, the mechanic called me up and told me I had coolant in my cylinders and that's why it was idling rough and surging. I asked the mechanic how he found this out. He said he pulled two spark plugs from the left side of the engine and they were wet with coolant! The repair bill would be $1,500 to R&R the cylinder heads as it could be a cracked head or a defective cylinder head gasket.

    At first I went off the roof. Then I sat down and had coffee and thought about it. When I regained my senses, I picked up my Corvette and decided to see for myself. I began to get speptical when I remembered the coolant level had NOT gone down in the car for a while. I also checked the oil and it was clean. The car did not blow smoke out the tailpipes either. So I proceeded to replace the spark plugs just to see what would happen. What's $30 worth of plugs and a bit of time at that point? I was able to replace 7 spark plugs except for the one on the passenger's side closest to the headlight. WTF! I simply could not get to that plug without dismantling the AC and some other bracket! Every single spark plug I removed had some moisture on the threads. Although the spark plug heads themselves (the part where the electrode is located) all came out dry and had the correct discoloration on them.

    After I replaced the 7 plugs, I drove the car and holy moses!! The rough idle was gone and the so was the surging! I have since put about 3k miles on the car over the past 11 months and it is still running very good. In fact it is purring like a kitten so far.

    However, I'm not so sure.....what about the moisture on the threads of the spark plugs?

    Is my L98 truly leaking coolant into the cylinder heads?

    How on Earth do I replace the spark plug on the passenger side, closest to the headlight? What do I need to remove to get to it?

    Thanks all!
    Rogue
    1991 L98 Coupe


  2. #2
    Member gmjunkie's Avatar
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    Quote Originally Posted by Rogue View Post
    I need to know what is really going on with my Corvette.

    In 2008 my '91 L98 Corvette began idling rough a day after a long drive. It felt like the car had a vacuum leak at idle. It would also surge at small throttle openings. At 35k miles, back in 2003 I had the car tuned up by the dealership which included new spark plug wires, new spark plugs, etc. In 2008 the car had 51k miles on it. I know....I don't drive the car that much! I ride a motorcycle to work that's why.

    I replaced the EGR valve thinking it was a stuck valve. The problem persisted. So I broke down and took the car to a trusted garage and took their mechanic for a test drive so that I could demonstrate to him what the issue was. After a day at the garage, the mechanic called me up and told me I had coolant in my cylinders and that's why it was idling rough and surging. I asked the mechanic how he found this out. He said he pulled two spark plugs from the left side of the engine and they were wet with coolant! The repair bill would be $1,500 to R&R the cylinder heads as it could be a cracked head or a defective cylinder head gasket.

    At first I went off the roof. Then I sat down and had coffee and thought about it. When I regained my senses, I picked up my Corvette and decided to see for myself. I began to get speptical when I remembered the coolant level had NOT gone down in the car for a while. I also checked the oil and it was clean. The car did not blow smoke out the tailpipes either. So I proceeded to replace the spark plugs just to see what would happen. What's $30 worth of plugs and a bit of time at that point? I was able to replace 7 spark plugs except for the one on the passenger's side closest to the headlight. WTF! I simply could not get to that plug without dismantling the AC and some other bracket! Every single spark plug I removed had some moisture on the threads. Although the spark plug heads themselves (the part where the electrode is located) all came out dry and had the correct discoloration on them.

    After I replaced the 7 plugs, I drove the car and holy moses!! The rough idle was gone and the so was the surging! I have since put about 3k miles on the car over the past 11 months and it is still running very good. In fact it is purring like a kitten so far.

    However, I'm not so sure.....what about the moisture on the threads of the spark plugs?

    Is my L98 truly leaking coolant into the cylinder heads?

    How on Earth do I replace the spark plug on the passenger side, closest to the headlight? What do I need to remove to get to it?

    Thanks all!
    I take about 10 minutes and take the wheel splash Shields out!! 2 ft extension and a wobble plug socket makes all of them but # 8 a breeze!
    junk!!
    "WARNING"
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  3. #3
    Gone but not forgotten John Robinson's Avatar
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    Default Possibility

    However, I'm not so sure.....what about the moisture on the threads of the spark plugs?


    Some thoughts I have on your question.

    1. How old was the gas in the tank

    2. How full was the tank when in storage

    3. What was the climate like while car was sitting (warm,humid,rainy)

    4. How long had the car been run before you pulled the plugs and or how long had it sat and question no 4 applies to before the plugs were pulled as well as when the problem started.

    If the gas was over 3 months old it was going bad. While it was sitting if the tank was less than full then inside the tank moisture would condensate. Particularly if the weather was humid. This moisture in the gas would then cause the car to miss and run badly.

    If you started the car and ran it only a little way and did not get it fully warmed up including the oil then when you parked it especially if it was raining or the humidity was high the cooling of the cylinders with the valves open would condense water around the thread of the plugs.

    Remove the radiator cap and start the engine while it is running and the thermostat is open watch to see if there are air bubbles in the water flowing past the opening. If so you have a leak between a water passage and the cylinder. If not then look at the oil to see if there is a milky foam in it. If so you have a leak between an oil passage and a water passage. If none of the above conditions happen keep and eye on it and drive it keeping in mind the conditions of storage the level of the gas and its age. More problems are caused by low mileage and bad gas then most people realize. Also the distributor will do the same thing inside with moisture as I mentioned above with the gas tank and cylinders.

    It is not uncommon for the distributor or the opti design to be blamed for a bad running car when they simply are the victims of age and storage conditions and not use.

    I hope this will provide you with some new ideas to pursue to diagnose what happened to your car.

  4. #4
    Member Rogue's Avatar
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    Thanks guys. I will try removing the backsplash to get to that pesky spark plug. It boggles my mind how difficult it is to change spark plugs on a longtitudinally mounted V8 engine with a clamshell hood. Theoretically, it should be a piece of cake. What were GM's engineers thinking?

    JR, answers to your questions:

    The gas tank in my car can sometimes sit in the car for weeks or a few months...I would say at most 3 months at a time. The car is NEVER in storage. It sits inside a garage in SoCal so the weather is pretty dry and warm. However, the day the car began running bad, it was August and about 95 degrees, 75% humidity, and we were driving it at highway speeds. Then I had to go through a few lights which got the motor pretty hot before going into my garage again, and that's when I noticed the idle got rough. After that, the idle roughness persisted, then later the surging. However, like I said, all that is now gone simply by replacing the spark plugs.

    The car will sometimes sit for several weeks before being driven again. Never more than a month. When I do drive it I drive it for about an hour non stop. Usually on the highway with an occassional high speed blast or a trip to redline in the first two gears.

    The last time the car got tuned up was at 35k miles. It now has 54k miles. The rough idle and surging occured at 50k miles. How often am I suppose to replace the spark plugs and re-check the timing?

    Right now, the oil is clean with no milky film on it.

    I will drive the car tomorrow and re-check the oil, coolant, and fill the tank up with fresh fuel. BTW, I always fill up with brand name 91 Octane fuel.

  5. #5
    Gone but not forgotten John Robinson's Avatar
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    Default Check The Distributor

    Get a spray bottle and put some water in it and and spray the distributor cap while the engine is warm and running. What you describe makes me think you have an issue with a cracked cap, loose wire which you fixed when you put them back on the plugs you changed or maybe you have a mouse that nibbled on one of your plugs wires and it was shorting out and again by accident you moved it away from whatever it was shorting against. On my 93 this year I had a miss when the engine was warm and idling at a light. It would idle smooth for a little while and then miss and go back to running fine and then miss. It turned out to be the new opti I had put on back in January of this year. It was shorting out because of condensation that was in it. Symptomatically your describing an electrical miss.

  6. #6
    Technical Writer for Internet & Print Media
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    The determination of whether or not the engine has a coolant leak is simple.

    Take the car to a different shop and ask them to pressure test the cooling system for leaks.

    Also, if you had a cracked head, you'd have been having to add coolant on a regular basis, you would have had bubbles in the coolant flow and, because combustion gases were escaping in to the coolant, perhaps even the smell of exhaust at the radiator cap opening.

  7. #7
    Member Rogue's Avatar
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    Hib, none of those things are occuring or have been occuring, ever.

    I'm just still worried about the moisture on the spark plug threads. All of them, at least the 7 I was able to replace had them. However, the plug electrodes and the all of the area that went into the combustion chamber were dry and had normal discoloration....just looked old. I'm thinking the damn dealership NEVER replaced the OEM plugs back at 35k miles even though they charged me for it.

    How often am I suppose to replace the plugs on a '91 L98? I suppose I can look it up.

    I'm about to take the car for a nice drive today and I will check the oil, coolant, etc.

  8. #8
    Member gmjunkie's Avatar
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    Quote Originally Posted by Rogue View Post
    Thanks guys. I will try removing the backsplash to get to that pesky spark plug. It boggles my mind how difficult it is to change spark plugs on a longtitudinally mounted V8 engine with a clamshell hood. Theoretically, it should be a piece of cake. What were GM's engineers thinking?
    If your just going after #2 plug,just the middle section of the splash shield would need removed!

    If your not adding any coolant,I don't think you have a problem!!But as others have said,Pressure test the system!!
    PS,You can probly buy a tester and test it yourself cheaper or about the same $$,and still have the tool!!
    junk!!
    "WARNING"
    The views and opinions of this forum user might not be suitable for others, this user may appear to be heavily over-medicated, under medicated, intoxicated, listening to the voices in his head or just plain being an ass, so some viewers may be offended!~!!!
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  9. #9
    Member Rogue's Avatar
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    Okay just got back from driving the 'Vette.

    I checked the oil, twice. It is clean as can be.

    Unfortunately, I checked the reservoir and the coolant level was low. Crap! I had to add about 6 cups! Not a good sign. I guess I now move on to pressure checking it.

    The car was running nice still.

    Around 50k miles, I had the garage (same one that diagnosed the leaking coolant into the cylinders) swap out my old coolant with fresh coolant. I remember back then I got the car back and drove it around for about 100 miles. Then I checked the coolant reservoir and it was nearly empty. So I refilled but I don't remember how much of it I did. I figure this was the case since the garage didn't properly fill it or "burp" the cooling system....not sure and my memory is a bit fuzzy.

    I need to put aside an afternoon to pressure check the system and change out that one spark plug. Other than this, the car is still purring like a kitten and running strong!

  10. #10
    Gone but not forgotten John Robinson's Avatar
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    Default Pressurer test the radiator cap

    On my 93 I was replacing fluid in my over flow tank frequently and thought I had a leak some were in the system. I went to Advance Auto Parts and used their pressure tester on the system only to not find out I did not have any leaks then out of curiosity I tested the pressure cap and it would not hold pressure. I tested it again and it held pressure so I went for the two out of three times to and sure enough not holding pressure. Replaced the cap put a 50/50 mix in the overflow tank and topped up the header tank and all has been good since. As others have said that is a lot cheaper then doing a head job if you don't need to. It is a lot more rewarding to work up to a problem and find out what you were afraid of was not what the problem was and the fix was simple.

  11. #11
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    Member G Winter's Avatar
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    If you had a cracked head or head gasket leak it would be unlikely that you would have it in all cylinders. Now if by some freak your throttle body was cracked and leaking water to the intake that would be a diff story,also very unlikely.


    Glenn

  12. #12
    Member Rogue's Avatar
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    That's what I'm thinking too. How can all 8 be moist at the threads?

    In any case, my reservoir cap and the reservoir itself is pretty worn and the cap fits somewhat loose. It is afterall 18 years old! It feels like I can easily pull the reservoir cap without twisting it.

    I'm driving the car daily this week and see if I need to refill the coolant again.

    I took the 'Vette to work today in the pouring rain (yeah we have this major rainstorm passing through SoCal). On my lunch break, I took it on this quiet canyon road near my workplace and put about 50 miles on her. She was purring like a kitten! Smooth, responsive, powerful, the cozy interior gave me the impression of being strapped to a jet, and the responsive handling and torque felt like it was a jet. The rain was pouring and the road was very wet with the surrounding hills looking very forboding. Meanwhile I was inside listening to my tunes and nice and warm with the heater blowing warm air. Sweet! I think I will do that again tomorrow!

  13. #13
    Member Rogue's Avatar
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    Okay so drove my C4 for 7 days straight. Back and forth to work, running errands, then drove it all over the place over the weekend, even in the rain! Then on Sunday I went to check the coolant level after topping it up 7 days before.

    The coolant level was normal.

    Whew!

    All is good then. No signs of any coolant leakage so far and the car is running beautiful. I still need to track down several minor things. I may take it to a shop to get the cooling system pressure checked. But after my last trip to a previously trusted shop, I am extremely cautious at giving someone else access to my Corvette so they can give me a false diagnosis. The last shop was just about ready to remove the cylinder heads on my L98! Wow! I get scared just thinking about the kind of crap they would have done to my car!

  14. #14
    Member JAKE's Avatar
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    Just borrow a coolant pressure tester from AutoZone, Advance or even O'Reilly's and do it yourself. It's a simple and quick test to do and you won't have to rely on someone else's work.

    Jake

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  15. #15
    Member Rogue's Avatar
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    Thanks guys.

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