1994 LT1
Mezeire HD Waterpump
Temp readings via the LCD screen

Took it on a long drive yesterday and noticed temps get up to about 240, under light load (cruising in 5th). Then they dropped quickly to 200ish.

Today, took it on another drive. Was on a small highway, 65mph, 6th gear. That's when things got VERY hot. This is a 2 lane highway, no shoulder - I could not pull over. It got up to 275 before I could exit. I let it cool some, then limped it home. The car ran completely normal before yesterday and I haven't made any changes.

When I got home I could hear coolant boiling over in the recovery tank. I used a jump start box to run my electric water pump continuously and put a fan in front of the radiator. Thing didn't want to cool. I got brave... I threw a rag over the expansion tank cap and put on my welding gloves and mask... cracked the cap free and heard a gigantic pressure release and then I could hear water pumping through the engine. Checked the gauge and the temp DRAMATICALLY dropped. I added quite a bit of water, cracked the bleeders, then let the waterpump continue to run with the engine off. The digital gauge read 190 degrees.

I put the expansion tank cap back on, fired it up and monitored. Engine idled fine, no weird noises, no white smoke out the back. Hit 201 degrees, fans kicked on and maintained that temp.

What in the world happened? It seems fine now, but this happened for a reason and I need to identify and correct. I can verify that the radiator fans DO run. When it overheated it vomited quite a bit of coolant, which I have replaced. The oil doesn't show signs of water, nor does the exhaust.

The only things I can think of:
Can the radiator cap be faulty? I only say this because when I relieved the pressure from the cap I could instantly hear water running through the block. The headgaskets seem to have survived - and I judge this by the lack of water in the oil and lack of smoke from the exhaust. I need to verify that my waterpump runs on its own (its currently jumpered to run while the engine is off). The relay is wired to the fuel pump relay, so any time the fuel pump is running so is the water pump.

Your thoughts and advice are welcome.

 

Sounds like trapped air was not purged out of the block.

 

That is an air pocket if I ever heard of one.

How old is the thermostat?

 

Interesting... the cooling system hasn't been opened up in a few months at least, so I don't know how air would get in.

I checked and my waterpump does run with the fuelpump. I used my pressure washer to clean the lower portion of the radiator (the wand is too long to get much higher) and verified no plastic bags stuck in there.

I took it for another ride. It ran great in town, 180 degrees! I drove it for a solid 15 minutes in town, it did not go above 195 once. Then I took it for 1 exit trip on the highway. The temps crawled up, and up and up... 230 before I got to the exit! Once off the highway the temps almost immediately began to drop. Within 1 minute of being off the highway I'm down to 210, and in the 190s by the time I get home ~3 minutes later.

Funny thing is, that the first time it did this was when I got on the highway. And the other day when it got up to 240, guess what, nearly highway speeds (55mph).

How can it be overheating when there is MORE air (hwy speeds) going over the radiator?

edit: and the thermostat is about 2 yrs, 3k miles old. 160*

 

Replace your thermostat....What makes me think this is where you say it got real hot and then all of a sudden it dropped real quick, sounds like the thermostat got stuck closed , then opened up letting cooler water into the block .......WW

 

If you build up large amounts of air in the cooling sytem, this could be a head gasket...

Definitely do all of the obvious checks first, thermostat, leaks, blocked radiator. etc.

Might not be a bad idea to do a Block test kit.

 

I'm beginning to lean towards headgasket... I received an informative PM (thanks Seabright) and here is why...

It overheats when I am on the highway... and when I get on the highway the kid in me comes out and I use generous amounts of throttle to get up to speed. A lot of throttle means HIGH cylinder pressure. So if there were to be a leak in the headgasket between the combustion chamber and waterjacket that is the time that it would force gasses into the water. I bet my blown gasket is between cylinder 6-8 as well. And the reason for that is my analog dash gauge skyrockets to 260 and that happens to be where the sending unit is located. The digital gauge creeps up much slower.

Can't quite explain why the temps come down once I get off the highway though. The waterpump is electric so the coolant is always pumping at 1 speed, independent of engine rpm.

I believe there are test strips that you can dip in the coolant to test for hydrocarbons. Is that test definitive? I'd like to know for sure before tearing it apart. What are the chances that my heads are warped? It hit temps of 275.

 

start with the basics. coolant will not just dissapear or go away. the radiator cap is fine, hence the ability to hold pressure. coolant loss can only be twoo things, either its on the ground or the motor is drinking it. no leaks on the ground would indicate the drinking coolant issue. you don't need the cooling fan running while driving down the road, so you can rule that out, the water pump is circulating the coolant when you added water, so you know that is working. you have the start of a headgasket or cracked cylinder head. there are dye kits that can be added to the oil or cooling systems that operate with a uv light

 

Coolant comes out the overflow, displaced by air. I really hope the heads are ok (LE2s) and that the gasket just let go.

 

I did a block test (the blue fluid that turns yellow if there are hydrocarbons present in the water)... and it came up negative (stayed blue).

The directions on the block test kit indicate to drain enough coolant so that it is a few inches below the top so that the block tester doesn't draw in coolant. I drained the coolant so it was ~1.5 inches from the top of the tank, but due to the Corvette design, there is coolant flowing about an inch beneath the expansion tank cap, its pumped in right there. You can't stop it. So I couldn't seal the block tester into the tank without drawing fluid from where it is pumped in. So I hovered as close as possible and did it that way. Fluid stayed very blue.

I went to the tail pipe, and exposed it to exhaust, it turned yellow instantly - so I know the fluid works. I cleaned out the block tester, ran the car again - at about 190* now. Then had a helper shut it. I immediately sealed the block tester in the tank and took samples of the 'air' and the fluid still stayed blue.

So either I don't have any exhaust leak into the coolant OR the test is flawed because I couldn't get it to seal. Or, I couldnt replicate the conditions in which exhaust gets into the coolant (WOT).

I'm not sure where to go from here. The only things I can think to replace are the thermostat and radiator cap. I don't think its the thermostat because the car doesn't overheat if I just let it idle indefinetly or if I drive it around town. If I go WOT, it overheats but miraculously cools down if I get off the freeway (timing coincidence?).

Could a bad radiator cap do this? I'd think not. The cap is clean and the spring, springy! I imagine a bad cap would allow coolant to escape into the overflow at a lower pressure but I'm not sure of the repercussions. And again, why does this only happen at WOT? Keep in mind I have an electric waterpump that maintains coolant flow at the same rate independent of engine rpm.

Any other tests I can do before I bite the bullet and pull the heads off?

 

Make sure the electric water pump is good. If it's intermittent...ie cycling on and off due to a poor connection (internal or external to the pump), you would see overheating.

If it's not the pump, be sure to have your heads pressure tested once they're off. The symptoms you describe are more indicative of a cracked cylinder head than of a blown head gasket.

 

How do you KNOW there are no plastic bags stuck in the radiator????

There is NO way to tell by looking and a pressure washer makes me cringe....folded over fins that block air flow.....straight fins are critical and have been the death of many engines. So much that they even make a "fin comb" to brush them out straight...its a PITA but necessary unless you just replace the damaged radiator or condensor.

To check for debris you MUST take the top radiator shroud OFF and be able to reach in between the radiator and condensor. There is no other way.

If you are building up air pressure in the radiator, that says head gasket or physical damage.. If the coolant is simply boiling out, that says lack of cooling or heat exchange or the heat is being generated faster than it can be removed due to poor air flow OR water circulation.

A stuck thermostat is the only other choice. Take the shroud off. Thats how you KNOW there is nothing blocking the air flow.

 

What temp do the fans come on at. yo could be boiling water in the block thats were its disappearing.
That c4 can be a nasty little f^%$#$ to get the air out system, I would let it idle in the driveway until hot check the hoses for heat ect.

 

As one who has had a '94 LT1 with overheating problems, I have a possible solution that you can check out. It's the radiator. It's not running efficiently enough at highway speeds to cool the coolant. However, it's not only the crap that collects on the outside that blocks airflow through the radiator. There is also crap inside the radiator that is blocking the coolant passages. Both factors lead to reduced heat exchange and thus lower efficiency. So, what you need to do is take the radiator completely out of the car and clean it inside and out with a garden hose. It must be removed from the car because you have to turn it upside down to get all the crap out of it. Once I did this to mine, I never had any cooling problems after that!

 

The symptoms described in your first post describe an exhaust leak through the head gasket into the cooling system. No doubt.

But...... super heated coolant would not cool down as you slowed down. The coolant would remain at the hight temp, and coolant would continue being displaced at any speed. Something is not consistent.

One condition that would cause a cooler temp reading after a high reading at WOT would be that so much coolant is displaced by exhaust gases that the temp sensors have no coolant to measure.

 

you have an external leak some where. it is leaking a small amount and evaporating before dripping onto the ground. i chased the same symptoms on a 87 monte. loosing coolant and sucking air through leak when cooling down.

http://forums.corvetteforum.com/off-...erheating.html

 

An obstructed core will greatly reduce cooling Efficiency, and our radiators don't come with a large margin, of cooling efficiency to start with!

Two more things I would check, what is the boiling point of your antifreeze? to check this you need to do an antifreeze Hydrometer test. The second Item I would be concerned about is the Cylinder Head(s) steam pipe, I found my Banjo bolts almost completely blocked with rust when I had my heads off, this allows super heated coolant (steam) to trap in the heads I would think...under high engine loads.

 

Or if it was me, I would just buy a replacement radiator. They can be had for $100 to $200 dollars. The orig. radiator is 18 years old. Lots of crud in that time (18 years) , not to mention corrosion, the plastic tanks eventually leak etc.
But years ago, had a camaro doing the same thing, heat up at highway speeds but not at idle. It was the radiator.

 

It sure sounds like a airflow thru the rad problem to me.
How old is the electric waterpump??? I have seen those last a looong time and Ive seen them not make it out the driveway b4 failure..

 

Lots of responses – I appreciate them all and they are very helpful in allowing me to formulate my plan of attack. I will do my best to address them all here:

Facts:
Radiator, new 4 yrs ago (less than 5k miles). 2 coolant flush/fills since then.
When I bought the car it had Dexcool in it, I stuck with it
Thermostat 2 yrs old – 160*
Electric Water Pump 2 yrs old (Mez HD)– verified working correctly
Radiator Cap – original, clean, spring takes a good amount of effort to compress by hand
No external leaks identified anywhere, pavement is dry and nothing is wet
Verified fans kick on at 200* (where I have it programmed)
Inspected a/c and radiator as best I could from beneath car, no obstructions (didn’t pull shroud)
Electric power washer did not damage any fins on rad

Symptoms:
-Analog gauge pegged at 260 then came down with a few minutes. The digital gauge read 190 during that whole time so I assumed the analog gauge sender was failing. I now believe I may have had an air pocket or localized hot spot near the analog gauge sending unit.
-Car then overheated after a WOT pull getting on freeway
-Temps reached 275 very quickly and did not drop till I shut it off and allowed to cool
-During this time a lot of coolant pushed out the overflow
-When I released the radiator cap there was a large pressure release (steam/gas) not coolant. I then ‘heard’ coolant immediately flow through the engine.
-Motor still runs fine now, no noises, smooth , power, etc
-No smoking or mingling of water/oil

After a refill and bleed:
-Car will not overheat if I start it and let it idle indefinetly
-Car will not overheat around town
-Car overheats immediately after WOT but this time it cooled quickly when I returned to city speeds. Coolant level was about 1-2” low when I got home.

Many of you are indicating that the problem can be the radiator (internal/external), thermostat or cap – but can anyone explain why a failure of any of those items would cause an overheat ONLY at/after WOT? Doesn’t make sense to me yet and I’m trying to understand it.

What does add up for me is that cylinder pressure is obviously at its highest at WOT, so doesn’t it make sense that a failing head gasket or cracked head would leak at WOT?

Again, I appreciate each and every one of your responses and experiences. I am taking in as much info as I can.