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Really need advice on strange overheat problem


Well-known member
Jan 26, 2001
Asheville, NC
1973 converted to 1980 custom convertible
New problem has happened and can't figure it out. If I start car and let it idle it shows approx. 180 degrees on mechanical temp gauge and dash gauge. Then after warm up but still idle, gauges jump up to around 220-240 degrees and then quickly drop back down to 180. If I drive car over 35 mhp the temp jumps to around 220-240 degrees. As soon as I drop speed, temp drops to around 210 degrees and when I stop car it idles at around 180 degrees again. I have had the radiator flushed and rodded, new coolant, new fan clutch, two new 180 degree thermostats, new radiator hoses, had radiator cap pressure checked, installed seals around radiator, checked water pump impellers and bearings which looked great. The problem seems to be opposite what it should be. The car should idle warmer then driving 35 mhp I would think. The only thing I have not worked on is the block itself. Could it be some blockage in the block? I can see circulation in the radiator and feel flow through upper radiator hose. The strange thing is that the temp jumps up within about 1 minute or less from 180 to over 220 degrees when driving. Then it cools back down within a minute or two. If it were a real overheating problem, I would think the temp would take longer to rise and fall as coolant does not heat and cool that fast. Both the local auto repair shop and my local parts guy suggest it sounds more like air in the cooling system. If so, how do I get the air out? I even drilled a small hole in the thermostat to let air purge better. When I last filled the radiator, I jacked the front up to get the radiator higher than the engine to reduce possible air into the system. I had the parts guy use his infared temp gauge today which never showed more than 200 degrees. How can I have two temp gauges installed on the car showing an overheat while the infared temp gauge does not. Furthermore, what would cause the temp gauges to jump around like that. Any thoughts or suggestions? Anyone else have a similar story?


Whoa, this is a good one.

You sure do your work ahead of asking the question, eh?
It is a puzzler, but I'm sure that there are plenty of C3 owner's here that will be glad to help if they can. ;)

I can't help ya Jim, but I'm definitely interested as to what you find; I'll be following this post.

Good luck,

Hot C3


Are both temp gauges working off the same sending unit? If so, you need a new sending unit. Sending units can be in the block, on the cylinder head or in the radiator. The critical temperature is the cylinder head. It doesn't sound like you're running too hot though. It is strange that it changes so quickly. Keep us posted.

I have two seperate sending units. The mechanical is reading from the cylinder head while the factory gauge is reading from the top of the intake manifold at the thermostat housing. I am suppose to be in my first vette rally in three weeks but can't participate if I don't find the problem ):


Jim Check this out

J Lance Miller posted this a few months ago, and It May help.. Click Here

It sure sounds like a thermostat hanging up to me, have you tapped the thermostat housing when it starts acting wierd to see if the temp changes? I KNOW it sounds like a bubba post, but even new thermostats don't always work right.

Good Luck, you are going to be are cooling expert by the time you're done here ;)

Re: Jim Check this out

Rare81 said:
...have you tapped the thermostat housing when it starts acting wierd to see if the temp changes?

Bud, a better method, although a little more work, would be to remove the thermostat and submerge it, along with a known working thermometer, in a pan of water. Heat the water and note the temp when the thermostat opens.

This is basically the same problem I had with the big block 69. I changed the new water pump and the thermostat for the third time and problem gone. The only differance was, the 69 would get hot at idle,but would not cool down at speed either.........Good luck Jim, I feel your pain...........Steve
The next chapter in my saga involved replacing the thermostat again within a premium thermostat and new radiator cap. This did nothing for the problem.

Decided to run without any thermostat and temp stabilized dead on at 150 degrees with no fluctuation. Not sure what the means other than I have had three bad thermostats in a row. At least I am finally getting somewhere with this problem.


Hope you got the problem solved before you read this.If not here is some other things you can check.
Try checking the engine timing at idle and at full mechanical advance.This to can make your engine run hot.
As for a clogged passages in the block not probable but possible.A little trick I was taught is to drain the anti-freeze out of the car.Fill it with straight water and then dump about a half a container of baking soda in.Seal her up and run forabout 30 minutes.Repeat process about two or three times.Refill will coolant and see how she runs.
Also ,I'm sure you checked anyway,the belts and pulleys for wear.
Wish you luck let us know how you made out.
I see you are having the exact problem I am experiencing. Only difference is I have an automatic. I to have tried everything you have and found no solution. I have tried several thermostats also with no luck. New Fan clutch, antifreeze. I even ran straight water. Same result. I only have one thermostat that even works this well. I know the block isnt blocked as it has been rebuilt and cleaned. I think I am going to try running a hi flow water pump and change the radiator. I set the timming to stock and blocked oof all vacum lines to check for leaks, with no solutions. I feel we will find out the answer we are looking for is the same.

Don't give up. I have solved my problem and it may work for you. I decided to run the car with no thermostat and the problem is solved. For some reasons, my car does not like having a thermostat. I tried three different ones to no avail. Today with the temp outside at around 88 degrees and no thermostat, the car ran perfectly at about 180 degrees. I spoke with three people I truly respect and all agreed that a carburated engine will no have no ill effect running without a thermostat. The only problem would be in the winter time where it would take a little longer to get heat. I was also told that the high flow water pumps are not a good idea because you need the coolant to stay in the radiator for a certain amount of time in order for it to cool as air passes by. A highflow water pump pushes the coolant through the radiator to fast and does not allow enough air to pass by thus not cooling the coolant enough. There are some water pumps with better than OEM impellers but adding 30% more flow is not always a good idea.


I think what you had was air in the system. By removing the thermostat the water fills in better, leaving less of an air bubble. After fooling with the same problem with my Moms Lumina I was able to identify and correct the problem with my car. The Lumina had vent ports throughout the system that had to be bled to remove air. Being too lazy to install ports in my car, I jacked up the right front to make the cap the highest point and filled my system. After filling I pushed the upper radiator hose down to fill the high point on that side, then finished filling. For this to work the thermostat must have a bleed hole to allow trapped air from below it to pass through. Not all thermostats have this. If you do not have a bleed hole in your thermostat you can fill the block to the base of the thermostat housing when you replace the 'stat, then install your stuff and jack as above. After this has been done initally there should be not further need to repeat the process unless you lose a lot of coolant.
I think you have found most of the problem......But you really need a thermostat even if it is a 160 degree unit. The engine needs heat for proper combustion

Two ideas!! Are your upper and lower hoses the type with springs in them? I have seen radiator hoses close up under acceleration and then re open. While looking at them with hood up and at idle, all looks well.

Finally, try "Water Wetter" made by Redline Oil. It is an anti foam agent ( eliminates bubbles and air) We use it in race cars all the time and temps. drop by 10-25 degrees.
Good luck, Jim
Sounds like I might have a similar problem. 1972 stock 350 engine. Just replaced radiator hoses, all heater hoses
and thermostat. When I drive the car the temp gauge shows a steady 180 degrees, running on the highway or cruising around town. Problem is when I return home and turn engine off, I open the hood and hear the coolant boiling in the area of the water pump?? The expansion tank is not streaming or blowing off excess coolant? I also replaced the expansion tank cap recently. Any ideas or thoughts? Thanks for listening!
Running HOT?

K Leitzel:

Welcome to the CAC Forum. Lots of friendly and helpful people here.

You may not be running hot, that sound you hear near the water pump may be just normal cooling down. Check the weep hole(s) on the water pump to see if you're losing any fluid. If so, replace the water pump. Lightly touch your radiator to feel if it boiling hot to the touch. You'll know if it's overheated by touch and smell. When it cools down remove radiator cap and check coolant level to see if you've lost any. Check the overflow tank, there is a level range depending upon whether fluid is hot or cold. Sometimes when cooling systems get older the radiator cap loses it's pressure seal, replace it. Check the simple and obvious first, use a process of elimination to determine what the problem is. Let us know what you find.
Great! Thanks for the advice. I will check it out. Another question, the radiator does not have a cap. Only the expansion tank, and I replaced that with a RC15 expansion tank cap from Corvette Central. Maybe the new cap is bad??
do you have a fan clutch if so it may be slipping,when power is put to the engine the clutch dosen't keep up with the water pump causing the block to heat up because not enough air is being pulled throuh the radiator. here is a secret to cooling a 87 to 89 caddy fleetwood with 5 litre has a seven blade fan with good pitch to it,there is a million of them in bone yards. another reason an engine will heat up could be a cracked cylinder head or leaking head gasket, when combustion occurs heat gets blown into the water jacket raising the temp.another secret paint the engine black it dissapates heat quicker

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