A couple years ago I bought a 2000 Corvette. It is stock/un-touched, has 85K on it now and it has been very reliable. I have only had to do routine maintenance and replace a rear wheel bearing assembly- Until last Friday when it started running hot. I found the water pump weeping and low coolant even though the DIC never said low coolant. Nothing else has changed for the past couple months. I had gotten on the gas a little hard from a light earlier on the same day I noticed it running hotter than normal but nothing ridiculous.

Saturday I replaced the water pump, filled with new 50/50 Dex-Cool, and bled the cooling system as described in the GM manual. It had a normal temp for the whole time I let it run, maybe 20 to 30 minutes. As I was still purging the system, the temp hit 210 on the gauge and DIC, and I saw coolant movement past the expansion tank- no bubbling. I also verified that the bottom radiator hose was warming up. I let it run a little longer and at idle the temp stayed around 220, which it had for two years. I still saw no bubbles moving past the expansion tank and knew the t-stat was working, as are both speeds of both electric fans. I had also checked for anything blocking air flow in front of the radiator opening.

I went for a test drive and within five minutes under 25MPH stop/start load the temp went from 210 to 235 and kept climbing to 247 and then fell back down slightly, so long as I grannied it. Went home, let it cool. The coolant level might have fallen 1/2" or so and I topped of the tank and bled it again but I saw no air escape. The temp increases seemed fine at idle to 2000RPM until it hit 220 and then it started climbing any time the rev's were above idle in the driveway with no load. The gauge and DIC were both above 245 so I shut it off and had a beer(s) in disgust... Before I shut it off I could see a huge amount of bubbling moving past the expansion tank so I am about to go buy a block test kit today to see if I definitely have combustion gas in the coolant. A cranking compression test while cold today was fairly normal as I suspected it would be. 7 and 8 are 140 and 142, 3 is 147, and the rest are 149 to 153. Plugs out of the cylinders with lower PSI look like a little oil has been burning but I'm not too alarmed at that. All other plug readings look great.

At best I think I am looking at a leaking head gasket but the odd thing here is that I see no sign of oil in the coolant and no sign of coolant in the oil. There's no steam at the exhaust and no sweet smell but I am not sure if Dex-Cool has the familiar burning glycol smell or not? The exhaust smell reminds me of when I tried toluene added to pump gas in my RamAir III Pontiac, kind of like burning lacquer thinner but I can't say I ever intentionally smelled the Corvette exhaust before yesterday. The way it acts I am afraid something has a crack because the rapid heating and bubbling appear to start only at the normal operating temperature of 220 and continue to get worse. I am going out on a limb here but I assume a cracked LS1 block could let coolant in the oil but not in the exhaust and it wouldn't let oil in the coolant. A head gasket could do any/all, and a cracked head could potentially let exhaust gasses into the coolant with no other signs of failure. My bad luck prevails unless... I haven't burped the system right any of the times I did it and air pockets are causing coolant to boil. Does this sound like I may need to re-re-re-burp it?

If not my inability to bleed all the air out of the system, I do realize a teardown, head pressure test, and checking the mill of the heads are all necessary at this point but I am looking for some input from the experienced 'cause I hates the unknown, especially when it hurts the wallet : ) Notice I said wallet, not bank. Other things have come up recently : ( Any chance I am wrong and it could only have trapped air or a blown gasket? Are LS1 heads prone to crack when a dummy doesn't check his coolant level? Are head gasket failures common at high temps or does this sound more like a cracked casting? Checking around in the Corvette and LS1 forums has great info but has not been real clear in terms of what I am seeing with the temp. threshold and excessive bubbling. I'm trying to prove it's combustion gas and not boiling due to trapped air before I tear it apart. Any ideas on how to do that other that a block test kit?

Sorry for the rapid fire questions above.

By the way- The engine temp never hit nuclear on the gauge and I don't believe the coolant level ever fell so low that the heads were dry. Temp was 155 at worst last week and about 1/3 gallon topped it off when I found the low coolant condition and before I noticed the water pump shaft weeping.

Thank you in advance! Your generosity and knowledge have been a huge help over the years I have been lurking here.

 

More info- NAPA Block Check is inconclusive. Nice try but when the temp rises to the point that I can see bubbles pass the expansion tank T fitting, it's too *@$@ hot to open the tank without boil over. The coolant is good to almost -40 and 260* plus boiling point, per hygrometer. I tried to sniff any residual exhaust gasses with the block test kit that may have been in the system after it cooled a bit but there was no indication. Clear blue. Even sniffed the tail pipe to make sure the indicator dye was active. Yup... From blue to bright yellow with one sniff.

I've done everything I can think of without a teardown. Jacked up front of car a few degrees past level, bled as manual states again and again, bled crossover tube as described here: http://forums.corvetteforum.com/c5-t...erheating.html , squeezed bottom radiator hose to force any air out of the heads. Still temp climbs at idle and faster at anything off-idle.

The bubbles I can see in the T under the surge tank don't happen until just above what was the normal temp. They start at 225 and I continue to see more/larger bubbles until I shut it off at 245.

Is there anyone that has seen a head gasket or a cracked head behave this way and could help me speculate?

I was hoping I hadn't bled it well or that I would have proof of combustion gas in the coolant but what a waste of a holiday afternoon....

Jim

 

The very first thing that I would do is change the thermostat! Could be that it just gave up the ghost. Give that a try before you get too deeply involved. Are your FANS running?? Turn on the AC and see if both fans run.

Check the radiator air inlet for trash and dirt. If it were me I would back flush the radiator with a garden hose & make sure that the fan shroud is properly connected to the radiator on all four points.

On my trip to Carlisle a few weeks ago, I hit a bump in the road and after that point the car would run very hot in traffic. It NEVER runs hot!! When I got to the hotel, I opened the hood and BAM,, I saw that the fan shroud was out of the retainer brackets on the passengers side.

Once I fixed that,, all was good!

BC

Bill

 

Hats off to you, Bill!

G.D. t-stat failed at the same time the water pump started weeping... Even though I could feel the bottom hose warm up around the right time, there must have not been enough flow.

Things that make me say DUUUUUHHHH / Son of a... / and I'll be D@mn3d all at the same time! A $20 fix over pulling the heads and $170 gasket set + bolts is welcome any day and I thank you for making me revisit the t-stat. It was all too much of a coincidence for my feeble mind to grasp.

So much for the past ASE mechanic certs and a good thing I work on computers now and no longer cars. Dee-dee-dee!

Thanks a lot! I mean that! Problem solved.
Jim

 

Say Bill,

I too have a C5 with over 122k miles on the engine, showing the same symptoms Jim had but instead I installed a new thermostat, the fans are good, the radiator shroud is clear, now after about a few minutes running idle it goes hot and water bubbles back up through my reservoir. My mechanic says that it is the engine heads may be warped or cracked, any ideas?