Is it my thermostat or the main gasket that's shot?

oy! I have a 1995 Mazda Millenia. I am going crazy trying to figure out if the car is overheating due to the thermostat or the head gasket.

My car was overheating and leaking coolant. I had the radiator and hoses replaced and it still overheats. The mechanic who did the work said it’s because the overheating blew the main gasket. I can drive the car for about 30 minutes with the AC on high before it overheats. Another mechanic looked at my car and insists the thermostat needs to be replaced and it’s definitely not the main gasket. But the mechannic who replaced the radiator insists it’s the main gasket and definitely not the themostat. How can I be sure 100% what is causing the overheating on my car? The 2nd mechanic said it’s not the water pump either. If the thermostat is disconnected and it doesn’t overheat, is that a definitive way to know it is the thermostat? HELP!!

First of all, anyone who replaces a radiator and hoses and does not install a new thermostat at the same time is . . . less than competent, let’s put it that way.

Quit guessing and install a new thermostat. They cost $10.

If the problem goes away, then you know it was the thermostat.

If it continues to overheat, it may well be a blown head gasket. Hope and pray that’s not the case.

mcparadise has given you a very complete and accurate assessment, IMHO. I would suggest that you follow his advice.

Be nice to that Mazda and stop driving.

Stop driving this car until you follow the thermostat advice given. Every time this car overheats carries the risk of serious damage, well beyond just a blown head (main) gasket.

Also, to find out what’s wrong, a “leak-down test” would help identify an internal (head gasket, head, block) leak or locate an external leak (water pump, hoses, radiator cap, etc.)

I agree with McP. Get a new thermostat and put it in TODAY. I can’t imagine any mechanic checking your cooling system and not changing this simple and inexpensive part and then guessing whether or not the overheating is due to a blown head gasket or a stuck thermostat. BTW . . . what does the oil look like? How about the coolant? Are either contaminated with anything? (oil in the coolant or coolant in the oil). Change the thermostat and post back. GOod luck! Rocketman


Another idea…

Since we don’t know what your climate is like or how you drive (heavy traffic, stop and go, lots of idling?), have someone check for proper cooling fan operation. I believe most cooling fans are designed to run continuously with the air conditioning turned on. A stopped or slow moving car, especially running the A/C on a warm/hot day, with an inoperative cooling fan, would easily overheat.

I would :

  1. Get the cooling fan operation checked.
    Try this first yourself: With car running, turn on A/C and see if fan is blowing air (behind radiator). Do not touch, can cause serious injury. Just look, listen and feel air movement slightly above engine.

  2. Do the thermostat replacement next if fan works well.
    #1 and #2 can also be done together by a technician.

  3. Get a “leak down test” if the mystery remains.

Thank you so much McParadise! Ok so I followed your advice and had a mechanic take out the thermostat, he tested it and it works, so it’s not the thermostat. Then he checked the water pump and said that’s the problem, I need a new water pump and he said since the car has 92,000 miles on it he suggests getting a new timing belt. I am still not convinced it’s not the main gasket. Isn’t there a diagnostic test that can be done to know unequivocally? When they hook the car up to that diagnostic computer, couldn’t that tell you whether it’s the gasket or not? The first mechanic I took it to who said I needed a new radiator said there is no way of knowing if the main gasket is blown unless you take the motor out. Thank you so much for your expertise and time. sheckyb

PS what do you think a fair price would be to ask for this car? I will of course list it specifying all it’s problems. It is a '95 Mazda, 92,000 miles. New tires, new brakes, new radiatior and hoses, the right fender is damaged, the interior is in good shape. The rest of the body is in good shape. The AC works, it now has no thermostat. Thank you very much, sheckyB

Thank you for taking the time to reply to my post I am going to follow mcparadise’s advice. What do you think a fair price would be to ask for this car? I will of course list it specifying all it’s problems. It is a 1995 Mazda Millenia, 92,000 miles, new tires, new radiator and hoses, new brakes, new battery, new right headlight. It has some serious mechanical problems. It overheats, it needs a new water pump and timing belt and it may have a bad main gasket, it has no thermostat. What in your opinion would a fair price for the car be?
It has damage to the right fender. The interior is in good shape aside from the damaged right fender the rest of the body is in good shape. Thanks very much for your help

Hi Rocketman, thank you so much for your reply. I had another mechanic take the thermostat out and it did work so I know it’s not that. He said the water pump doesn’t work and that would mean getting a new water pump and timing belt.
The coolant looks green and the oil looks like oil. They don’t appear to be contaminated with anything. The mechanic who replaced the radiator insists he is 120% sure the main gasket is blown, he said there a different degrees a main gasket can be blown so the coolant wouldn’t necessarily mix with the oil. I tend to believe him. I think the gasket and water pump are shot, very expensive.
I am going to sell it at this point, It’s got so many big mechanical problems and it’s a 13 year old car. I don’t think it’s worth more than $400…What do you think? Thanks so much, shecky

Yes, a 13 yo car with almost 100,000 miles and maybe a blown head gasket is probably as worthless as it gets and still runs.

However, I think a cooling fan check and a thermostat is a cheap try at a fix. If it works, then the car is maybe worth $2500, according to Kelley Blue Book ( Relying on the opinion of some clown that did not replace a cheap thermostat when replacing a much pricier radiator is a bad idea. Did he put on a new radiator cap, at least? And, if he still replaced the radiator, and diagnosed a BHG, he just stole money from you.

Rocketman, first of all thank you so much for taking the time to reply to my questions, I obviously no nothing about cars. The mechanic did replace my thermaostat and it did work (I had another mechanic take it out and test it that’s how I know). As far as a new radiator cap goes I don’t think I’d know the difference between the old one and a new one (unfortunately). I was trying very hard to determine when within an hour of driving my car home from the mechanic who replaced my radiator and my car overheated if he knew my head gasket was blown and he just stole my money. I called him of course and he insisted (of course) that he had no way of knowing if I had a BHG without removing my engine and that would have been a very expensive job. I did have another mechanic check to see if the fan in the engine works and it does, I am guessing that’s the cooling fan you’re referring to, it’s in the front of the engine located right behind the (grill?) does that sound right? Thank you so much for all your help, sheckyb

And that again is not true. Any true mechanic can diagnose BHGs with a few simple and inexpensive tests. There are test strips to determine if combustion gasses are entering the cooling system, simple compression test and slightly more intensive leak-down test to check the condition of seals in each cylinder, and cooling system pressure testers to determine the condition of seals within the cooling system. These should all be considered before determining a BHG. If it takes 30 minutes to an hour for the car to overheat, the problem is most likely not a BHG.

Replace the radiator cap, and check it again. It’s cheap,and you can do it yourself. If the cap is unable to hold 13-16 psi, this will cause coolant to run out into the reservior, and will lead to overheats.

thank you so much Rocketman, I am going to confront my mechanic with this information. When I asked him after my car overheated why he so sure it was the main gasket when he claimed he had no way of knowing prior he said because I told him my car overheated and all the coolant came out that was a sure sign he said that it was the main gasket but he had no way of knowing until that happened, reiterating that he’d been a mechanic for over 30 years and that’s why he was certain of his diagnoses, meanwhile I’d paid him $500 for nothing. Thanks again very much for all your help, Live and Learn…hopefully, sheckyb

PS Rocketman, R U a mechanic? Do you happen to live in the New York area?
When I get another car I would love to bring it to you when I’m having a problem.
thanks again, sheckyb

Strike up the band…

Mars ain’t the kind of place to raise your kids
In fact it’s cold as hell
And there’s no one there to raise them if you did
And all this science I don’t understand
It’s just my job five days a week
A rocket man, a rocket man

And I think it’s gonna be a long long time…

Good work Rocketman!

PS I just got off the phone with my mechanic and I asked him if he replaced the radiator cap, he said no. I asked him if performed a leak-down test or used the test strips to determine if combustion gasses were entering the cooling system, he said any mechancic will tell you you can’t do any of those tests when you have a radiator that was leaking as bad as mine and that he is 120% sure that I have small crack in the main gasket, again would you say that is B.S?
thank you very much, sheckyb

That’s why the leak-down is done after the radiator and hoses get replaced (if they were leaking) and the car still has a problem!

If they weren’t leaking it could have been done first or the cheapest easiest thing is to test and/or replace the cap. My leak-down tool tests caps, easily. Next on the list should have been the thermostat, for sure!

Again, BS. A leak down test only checks the condition of the COMBUSTION CHAMBER. It doesn’t even require coolant in the engine. And, unless the radiator is pouring coolant out like a water hose, the test strips can still be used. If you were able to drive the car to him, leak and all, he could have easily done this test, too.

Stop seeing this clown. Find another mechanic.

Busted Knuckles,
Sorry about the confusion I caused. I call it a “leak down” test when I pressurize my cooling system with my “Stant Pressuried Cooling System Tester,” as well as a combustion chamber “leak down” test done through the spark plug holes. I pump it up and wait several minutes to see if it holds pressure or “leaks down”. When pressure isn’t held then leaks can be seen in the external system, radiator, hoses, heater core, etc. Barring external leaks, pressure/coolant can be lost through the head gasket. I should have specified either combustion chamber or cooling system test. Test strips sound good, too.