While driving the car Friday night the SES light finally came on. I got a code 45, so I pulled the plugs on the left side to check condition. All of the plugs looked good to me. There was just a little bit of carbon on the rim and at the very base of the electrode on the 5 and 7 plugs. #7 was very hard to get out and turned out to have rusted threads. I couldn't see where any water/antifreeze might have made its way into the area where the spark plug goes into the head, though. Underneath the line where the head meets the block the whole area looks wet, not sure yet if it is oil or antifreeze. The left side is wetter than the right side, but both sides look damp like something is leaking. Now I'm thinking that I may have a head gasket problem which would outweigh the running rich problem. Does this sound like a correct diagnosis? The wires that run between the block and exhaust in this area are wet also. I know I wiped this area down with a rag a month or so ago when I changed the oil to see if it was a continuing problem or something left from the past.
Thanks for any help/replies. Another question I have is given that the engine has >75k miles on it, would you just replace the head gaskets or would you go ahead and get the whole thing reworked?

 

Well gaskets are pretty cheap, but this project will take some time. How much time and money do you want to spend?

 

Actually, I'd prefer to spend neither. It's a daily driver, so I would need to start Friday night and be done by Sunday night. Hopefully I would be able to get away with not having to shave the heads. What else would I need: intake gasket? valve cover gaskets? new head bolts for sure, anything else if I keep it strictly head gasket change?
I'm curious if there is anything else that might cause oil to run down the back of the block. I did overfill at the last oil change a little more than half a quart. I don't see any leaks around the valve covers.

 

The 45 is rich and the rear leak is probably a crumbling seal at the rear of manifold (though it also might be the distributor gasket). #7 is allmost always where the aluminum head L98 fails (though the '91 has the updated and better design) and the rust on the plug threads is a telltale sign. I'd start with a compression check though a leakdown check would be better. You're going to have to pull the manifold and I'd pull the rail with the fuel lines attached and with a paper towel underneath the injectors, turn the key to run. If any drip - or don't drip - you will have confirmed or ruled out one possible cause of the
45. Otherwise, there's a laundry list of things to check, but you can rule out the ECM by grounding the O2 harness which should then spit out something below .35 volts on a scanner.

When buying the intake gasket, make sure you get the right ones for the '91. They have retainers that hold them in place and the heads are counterbored to accept these retainers. I don't know of any aftermarket suppliers - they're dealer only and the Vette (as opposed to the F-Body) have restrictors in the rear coolant passageways. You can use the gaskets without the retainers, but the design was incorporated to stop the manifold leaks which were so commonplace on the L98 and from the postings around here, they must do a better job of it.

If the head gasket has been leaking, put a straight edge across the block at the point of the leak. If it's gone on for any length of time, it can wear a valley between the coolant passageway and the hole. Fixing that means pulling the motor (or you can slap some new gaskets on it and hope that it holds long enough for you to get rid of it).

Galvanic corrosion is sometimes a problem once this condition manifests and don't be surprised if the coolant passageways (in the heads) are full of crud. I'd chuck the radiator if it is evident and then recondition the heads or simply replace them (depending on your budget).