Car:
2000 Corvette Coupe. Manual Trans. 39,xxx mi.

Concern
Coolant over heats. 240+ degrees.

What I Have Done
First I started the car cold and monitored its warm up. Coolant temp rises fast. Cooling fans come on at the correct times, one being around 210 and the high speed near 230. Even when the fans come on temperatures continue to rise. Shut the car off and checked under the hood. Coolant is pressurized and full. No leaks that I can see. There are air bubbles going into the surge tank rapidly, seems to be boiling. Used my infrared temperature gun and measured coolant and engine temperatures. Both cylinder heads read 200 degrees. Upper radiator hose reads 160 degrees. Lower radiator hose reads 180 degrees. Heater hoses read 180 degrees. Slowly opened the surge tank cap, allowed the pressure to release. Coolant begins to boil again as boiling point decreases with lower pressures. Removed cap and measured coolant temp. Coolant in tank only measures 160 degrees. I then pressure tested the system and it holds 15psi. I check the radiator to make sure there are no restrictions in air flow, and for precautionary sake, I degrease and rinse out the radiator and condenser. After letting her cool down, I start it, and the same thing happens. So I throw parts at her.

Replaced
Thermostat, flushed and filled coolant, same problem.
Thermostat again, in-case new part was bad. Same problem.
Coolant temp. sensor, since coolant temp differed from IPC. Gauge still reads high.
Water Pump, seems to be a flow issue, no luck, still over heats.

Shoot Me
So new thermostat, new water pump, fresh coolant, radiators clean, still boils into tank. Head Gasket? I'm sure its possible but the car runs great, doesn't skip, no exhaust smell from the coolant tank, no coolant smell at the mufflers.


Help

 

First of all, you should always test a stat in boiling water to make sure it works...I test four before I got one to work correctly.. the other thing that you should do is remove the radiator completly.. the reason I say this is because although I clean my condense and radiator every spring.. this year even with the cleaning process ( In the car ) I was curious about the actual condidtion of the cooling fins.. while I always use a brush, water , and compressed air, I just felt as though the operating temps were slightly north of their usual spots this summer..
Anyway, I removed my radiator.. it takes about 30/40 minutes... Once removed I was amazed as to how many individual fins were completly blocked... but with a visual inspection while in the car everything looked good.. with the radiator completly out of the car you can remove every spec of debris.. ans with the radiaot out you can also remove everey spec of debris from the condenser..Once this was done, my coolant stayed in a much tighter zone, and my ac was much colder.. noting that I could run the fans on a very low speed without having to recirculate inside air..which is the coldest setting.. I hope this helps.. I doubt you have a bad water pump, chances of a new one being bad is very very remote.. but not the same confidence with thermostats.

 

Here's what I would do. Go buy a carbon test kit. It's a tube that you put on the tank opening and pour in this yellow fluid. If theres carbon in the system the fluid in the tube will turn blue in a heartbeat. Sounds like your coollant system is working. You might have a head gasket leak or something else I won't say. Test kit is about $30 and it will at least rule out anything related to compression.

 

If you are seeing bubbles such as pressurized air going in there and if you are sure there is no trapped air in the system, you'll need to find a kit that checks for exhaust gases in the coolant. Do that test. If you find the test positive, you've lost a head gasket. Next, do a compression test and while you are pulling the plugs, look for evidence of coolant on each plug. I hear there are problems with Cylinder #7 when things usually happen.

 

If both cylinder heads read only 200 degrees, and you're seeing air bubbles going into the surge tank, the coolant can't possibly be boiling at that low of a temperature.

Try blocking the front wheels, pull up on the parking brake, and let the clutch out part way with the engine running about 2000 RPM. Have someone observe the surge tank while you're doing this. If you see a lot of bubbles, with the engine under load, you probably have a combustion chamber to coolant leak caused by a blown head gasket.

 

Bubbles in the coolant and overheating equal failed head gasket.

 

A friends car did this, first suspicion was a head gasket. In the end it was low coolant and the air bubbles coming into the tank when he revved it were just the result of the air in the system.

 

Perhaps a cooling system pressure test is in order here.

 

From OP's first post:

"I then pressure tested the system and it holds 15psi."

 

Missed it.

 

I appreciate your thoughtful response. But having driven and worked on hundreds of C5's I know that's not it. Its only some 50* outside, even with a heat blanket on the radiator the ECT wouldn't climb rapidly and boil over.

Anyways with the car up on the lift I can see between the condenser and radiator. The radiator is clean front and back. Just for fun I did degrease and put the hose through it as while I searched here a bunch of you guys posted on everyone's over heating threads to remove the radiator and clean the fins.

Thanks for the idea. I'll call parts tomorrow and see if they got one of these kits in stock.

That's why I think there may be an internal failure. The coolant boils over, yet the coolant outside the engine reads under 200*. If there is an internal leak then its possible the boiling point is lowered when internal pressure is closer to ambient causing it to boil out.

Its much more than bubbles coming into the tank. Think of it as a deep see diver breathing. Just endless bubbles when the gauge gets up to 240+ degrees.

 

Unfortunately that does sound like an internal issue. Is the motor stock?

 

Google : Combustion-Leak-Detector

 

With an internal leak, the coolant is also being superheated by the introduction of hot exhaust gases into it.

Pull your plugs out for inspection. If you find one (or more) that are much cleaner than the others, it points to an internal leak in that cylinder.

 

The same thing happened to me when I pulled my radiator the first time

 

Even with pure water as coolant you should not see significant boiling until the system temperature approaches 212 deg F at atmospheric pressure under normal conditions.
If in fact you have a blown head gasket or compromised cooling passage in either the block or cylinder heads leaking combustion gases into the cooling system, it would seem that it should have shown in the pressure test by leaking down after pumping up the pressure.
Have the heads been removed recently? The head gaskets can be installed incorrectly and block off cooling water flow from the block to the clylinder head. Good luck with your search.

 

Had you done any cooling system repairs or servicing prior to the problem occurring?

Head gasket leaks can allow the gasses to pass between the cylinder and water passages. When this happens the temperatures climb rapidly.

We need to remember that water will boil at roughly 212 degrees but the number can vary greatly depending on atmospheric pressure etc..

That said with a function cooling system there should be expansion not boiling.

 

NAPA sold me a leak detector kit years ago, used it once and it's on the shelf. PM me an insulting offer plus $7 shipping and it's yours.

I bought the kit to diagnose an overheat, it tested positive, and I was off to the races on a head swap.

 

I did not perform any work to the car prior to this issue. It's possible some one had in the past, but not likely.

I ended up pulling plugs and Cyl 1, 3 and 5 all show white on the tips of the plugs. The passenger bank spark plugs look fine. Compression test failed due to a leak in the compression tester hose..ha.

I tried removing the thermostat and running straight water. Temps still climbed rapidly. I'm guessing the leak is right near the coolant temp. sensor because the rest of the system is less than 200 degrees.

Guess its time to pull the head, oh well.

Hey thanks! I will surely let you know if I need it.

 

You'll want to replace the forward oxygen sensor for that bank also. The silicates in the coolant poison the sensor and it reads rich permanently, causing the PCM to pull fuel and make the 4 cylinders run lean.

If you connect a scanner, I suspect you'll see Bank 1 LTFT reading negative while Bank 2 LTFT is normal % range.