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A week ago my car overheated and the heat was cold

Tiny
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  • MEMBER
  • 2005 CHEVROLET UPLANDER
  • 3.5L
  • 170,000 MILES
A week ago my car overheated and the heat was cold. I replaced the thermostat, antifreeze and bled the air out of the system. Everything seemed fine and then today, a week later, it overheated again. I was able to drive it home, but the temperature kept going from 1/4 to 3/4 on the gauge and alternated back and forth every thirty seconds or so. When it went down the heat was hot and when it went up the heat was cold. What could this be another bad thermostat or water pump?
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Friday, February 3rd, 2017 AT 11:41 AM

21 Replies

Tiny
HMAC300
  • MECHANIC
  • 48,601 POSTS
Check coolant level If low have pressure checked for leaks may be a head gasket.

this guide will help you

https://www.2carpros.com/articles/engine-overheating-or-running-hot
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Sunday, May 23rd, 2021 AT 11:32 AM
Tiny
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It is the same level as it was after replacing thermostat and no leaks on the floor.
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Sunday, May 23rd, 2021 AT 11:32 AM
Tiny
STEVE W.
  • MECHANIC
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Cycling water temperature and heat sounds like a bad thermostat or an air pocket in the system.
It could be a slipping pump impeller, but I do not know of a test for that unless you have some clear hose that you could install in place of a radiator hose to watch the flow.

Pressure test or combustion gas test would help rule out a bad head gasket.
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Sunday, May 23rd, 2021 AT 11:32 AM
Tiny
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Replaced the thermostat a second time thinking I may have got a defective one. Also replaced the water pump. Refilled the system and bled it. Ran fine for 6 days with no overheating. Now it overheated again and the heat was cold. It seems to happen when sitting. Start driving it and the heat warms up and the temp goes down. What could this be? A blockage in the radiator?
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Sunday, May 23rd, 2021 AT 11:32 AM
Tiny
STEVE W.
  • MECHANIC
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Check the radiator hoses for soft areas. Then test the cooling fans for operation. Could be a simple coolant temp sensor failing and not activating the fans. Or the fan relay. If you start it and turn on the A/C do the fans turn on?
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Sunday, May 23rd, 2021 AT 11:32 AM
Tiny
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Thanks for the reply.

I checked the hoses the when it was hot to see if the bottom one was cooler and felt softer, but they both felt the same temp and pressure. Both fans were running when it got hot.

I would think if it was a fan issue the heat would be really hot when it overheats, but it is cold when the engine temp goes up.
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Sunday, May 23rd, 2021 AT 11:32 AM
Tiny
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I should mention it was only 20 deg. Outside when it overheated.
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Sunday, May 23rd, 2021 AT 11:32 AM
Tiny
HMAC300
  • MECHANIC
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Outside temp has not much to do with an overheat. Try bleeding air from system see pic there may be another screw for this so look on radiator and other cooling areas.
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Sunday, May 23rd, 2021 AT 11:32 AM
Tiny
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I bled the air out a few times. No more comes out.
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Sunday, May 23rd, 2021 AT 11:32 AM
Tiny
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Did you bleed it with the engine cold and the bleeder screw open while filling the cooling system with the engine off till the bleeder screw has a nice steady stream of coolant coming out of it?
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Sunday, May 23rd, 2021 AT 11:32 AM
Tiny
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To answer the last question, I bled it while filling it back up and then again when hot.
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Sunday, May 23rd, 2021 AT 11:32 AM
Tiny
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After the last overheating it ran fine for 5 days at normal temperature. This morning it got hot and almost overheated and then the temp went back to normal. Tonight it overheated and shut off the engine. The heat got cold when this happened. After it cooled down I drove it home. The temp gauge kept going back and forth between 1/4 and 3/4. Normal is 1/2. When the temp was rising the heat would get cold. I shifted to neutral and sped up the engine to 3000-3500 RPM. At this speed the heat would get hot again and the temp would start dropping. Then putting it back into drive and driving around 1500-2000 RPM the heat would go cold again and the temp would get hot. Any suggestions? Any tests I can do? Does this sound like a blocked radiator? If so, is there a way to test that?

Just to recap, the thermostat has been replaced 2X and the water pump was just replaced.

Thanks for your help!
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Sunday, May 23rd, 2021 AT 11:32 AM
Tiny
HMAC300
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Ya know I kind of think if it ran fine for a few days then overheated put a temp gauge like one of the cheap pocket ones that are infrared. Something should have shown up by now leak or otherwise. You may have a gauge problem or sensor that is grounding a wire out. If it's running around 210 when gauge says hot then that is the problem not the cooling system. Also pressure test the radiator cap as well
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Sunday, May 23rd, 2021 AT 11:32 AM
Tiny
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Decided to do some head gasket tests.

I don't see any antifreeze leaks.

I checked the oil for antifreeze. It looks normal, not gray.

I started it up with the radiator cap off to check for bubbling. I didn't see any. The fluid level was way down from overheating, but started rising fast. So I opened both bleeders. The fluid level went down. After about a minute antifreeze was coming out the bleeder by the water pump so I closed that one, but none from the bleeder on the other side. The fluid level camp up fast so I shut it off. It dropped down 8-9" right away. I started it back up and within a minute it came right up again and overflowed. The one bleeder was still open. I never saw any bubbling.
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Sunday, May 23rd, 2021 AT 11:32 AM
Tiny
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I don't think it is a sensor problem because when it overheats antifreeze is gushing out of the overflow. I don't think this would happen if it was just a bad reading. Also, if it was the sensor I don't think the heat would get cold. Do you agree?
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Sunday, May 23rd, 2021 AT 11:32 AM
Tiny
STEVE W.
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Go get a combustion gas test kit. From that last description it sounds like a failing gasket. Coolant rise and fall that fast has to be air/combustion gas related.
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Sunday, May 23rd, 2021 AT 11:32 AM
Tiny
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I filled up the antifreeze, bled it and started it up and I kept hearing a loud clicking sound every 30-60 seconds. I listened to various areas to figure out where it was and it was coming from the thermostat housing. Drove about 1/4 mile and overheated. The heat was cold. Could have another thermostat gone bad? I replaced it 2x in the past 3 weeks.
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Sunday, May 23rd, 2021 AT 11:32 AM
Tiny
STEVE W.
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A failed thermostat won't click. They will fail open or closed. I would still do a gas test on it. I am betting that the click is being caused by combustion gasses building up and forcing past the thermostat spring and it's then snapping shut. Because of the design of that engine with the thermostat at the bottom, you won't see bubbles in the coolant, only a combustion gas test would show it.
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Sunday, May 23rd, 2021 AT 11:32 AM
Tiny
HMAC300
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Legend you can get the kits at auto parts
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Sunday, May 23rd, 2021 AT 11:32 AM
Tiny
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It's definitely the head gasket. It keeps getting air in the engine and overheating. If I bleed it out it will run for a couple of days and then do it again.

Will drilling a hole in the thermostat or removing it buy me any time by letting the air go to the radiator instead of getting trapped in the engine?
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Sunday, May 23rd, 2021 AT 11:32 AM

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