I've only owned my '92 C4 since July but it has begun lately to consume a lot of coolant. I last drove it about fifteen miles and it used about a half gallon. It did similar the last couple of times I drove it. It's not leaking as I've parked it on clean, dry places at work and at home to check. The exhaust has a smell that kind of burns my eyes and nose if it's left to idle in even an open-front garage which, if I recall correctly, is even more strange with premium gasoline which I always put in it. Could that be coolant burning or just idling very rich? I've checked the dip stick to be sure the coolant is not going there and it is not. The oil on the dip stick it the right color and at the correct full mark.
Do these engines have a history of problems with intake or head gaskets? Could a bad intake or head gasket cause this problem? Are the aluminum heads prone to cracking?

 

If the coolant isn't hitting the ground and it isn't in the oil then it pretty much has to be going out the exhaust. My first guess would be a head gasket. A cracked head will act much the same as a bad gasket but you can always have them checked since you would have to pull them off anyway to do the gaskets. Try doing a leak down test then you can isolate the problem to which cylinder(s) may be leaking.

 

More than likely it is burning. The most common problem is a blown head gasket on numbers 7 or 8. Pull #7 and #8 plugs. See if they are exceptionally clean, like they are new. This indicates that coolant is getting into the cylinder. If they are black with carbon, then this is not the problem area. You can also do a compression test, but the plugs tell a great story.

There are coolant passages on #7 and #8 rear of the block that are very close to the cylinder walls and are know for the gasket to blow at these points. It is more prevalent on engines with a iron black and an aluminum head. The two metals heat and cool at different rates and the gasket between takes the beating.

 

Pull plugs

 

Before you tear into it too far, try a new pressure cap. A lot of "unexplained coolant loss" issues are caused by a weak pressure cap.

If that doesn't fix it, a leak down test is probably the next step for diagnosis.

 

Heater cores will also account for a loss of coolant. Is your carpet damp?

 

I would still drain the oil and check for coolant in it. I generally put it in a clear plastic jug and let it separate for a couple hours before checking for coolant. Also pull the plugs as suggested to see what they look like.

The suggestion of doing a "Leak Down" test is a very good/wise one, that test can pinpoint the problem fairly easily. The other to consider is to pressurize your cooling system and watch and listen for the leak.

If the heater core was bad wouldn't the interior wreak of anti-freeze? I blew a head gasket once and the smell of the exhaust had a obvious anti-freeze smell in the smoke.

The radiator cap could be the problem but the overflow tank should be full if the cap was not able to hold pressure. Your description of the temperature creeping up slowly sounds like it could be from a bad radiator cap. I try to change the radiator caps every time I change the coolant which I do every five years. A bad radiator cap would help your engine run hotter than usual as it fails to hold pressure in the coolant system which it is required to do in normal operation. An Open system will boil over even with anti-freeze somewhere in the ~240 range add the 16 lb radiator cap and the boiling point goes up to ~260. Since the stock C4's run so hot already we really need the extra protection from the 16 pound radiator cap on top of the correct mixture of A/F and Distilled Water.

A good friend in College had an old Ford Pinto that burned oil regularly and one day he was real excited as his car stopped burning oil. It was obvious that he had problems as we noticed his coolant level was dropping instead. He tried to drive the car one time too many and it locked up.

Please tell us all the cause of this ailment of your Corvette's when you figure it out!
Good Luck!!

Somebody above asked you about the air dam that attaches under the front of the car. If that air dam is missing or broken you will have climbing temperatures on the highway and the car WILL run hotter. Also be sure that the area between the radiator and the condenser is clear of leaves,branches and Chipmunk nests, it fills up quickly and does not empty itself out. Some have put screens into keep the junk from getting lodged in there....

 

Pressure test the complete cooling system.

 

I will pull #7 & 8 plug between now and the weekend. Are they numbered the same as all the old V8 Chevys?

The rad cap appears old so probably needs replacing anyway but I've had no trouble with overheating. As a matter of fact, just the opposite is noticed since the weather has been a bit cooler in Southern Middle Tennessee. It takes it several miles to heat up enough for the heater to be noticeable. Wouldn't a bad/weak rad cap cause overheating instead of slow temp rise?

I've had cars with leaking heater cores and am familiar with the smell of antifreeze inside the car, wet carpet, and continually fogged windshield. The windshield has fogged some back in the summer (I know, weird time to fog) but not like other cars I've had with know leaky heater cores. No wet carpet is noticed.

Air dam is complete. Radiator is clean and devoid of nests, leaves, and even bugs.

Isn't "leak-down test" the same as "compression test"? So this should be done if #7 & 8 plugs are really clean?

I have already scheduled a pressure test for the coolant system next Tuesday. Should that test still be performed regardless of what #7 & 8 plugs look like?

 

A weak/broken pressure cap will allow the coolant to boil at a lower temperature. It will cause coolant loss, often with the temperature gauge reading slightly lower than "normal" for the car. Overheating occurs if you drive it long enough that the coolant level gets so low there isn't enough in the radiator to keep it cool. The noticable symptom is bubbling/gurgling after a drive when it's up to operating temp, especially right after you top it off to replace the missing coolant.

 

Doing a Leak Down Test is different that doing a standard compression test. On a leak down test you raise the piston to TDC on the cylinder you are testing and then you put a specific amount of air into the cylinder through the plug hole and watch the second gauge which indicates how much pressure is being maintained in the cylinder. If you have a bad intake valve you would hear air coming out of your intake manifold. If your engine is worn badly and the air blowing by the rings can be heard coming out of the engine oil filler hole. This test is much more specific in the results if done by somebody with the right tools and training. The tool is becoming more accepted in the auto industry, I use it on any engine I own as it is very reliable.

I learned about this from my mechanic for my old Cessna 172, the aviation folks don't bother with the compression testing as much. They tested my engine every year at the annual (maintenance point) and the results showed the engines wear, if too much air is leaking past your rings the engine would show a very low maintaining number. The results are two numbers, first the set pressure and the second is the pressure maintained inside the cylinder. The numbers should all be within a certain margin to indicate the engine is okay similar to Compression test data. The less air that leaks out would indicate a fairly tight engine and be a good thing.

I may not be describing the procedure properly but there are a couple of folks here that know it far better than I. I apologize for any mistakes if I have made them describing this procedure. Come on you Piston aircraft mechanics and help me out.

Yes the engines are still numbered the same, 1,3,5 and 7 on the drivers side and 2,4,6 and 8 are on the passengers side.

I wish you the very best in solving this issue!

By the way, Welcome to the Corvette Forum!!

 

Carpet may not be wet but still have a leaky core. Stuff your hand up behind the carpet in the passenger footwell, under the dash. That's where my first 1995 heater core was leaking (against the interior firewall) and the carpet was dry.