Strange heater/coolant issue (1 Viewer)

Joined
Apr 14, 2015
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370
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Tallahassee, FL
After the engine is warmed up, while sitting idling with heater on, my heater will start to blow cold air as my temp gauge starts to rise. Just as the gauge is about to hit the red, I'll hear a weird sound under the dash, the gauge will quickly return to normal, followed by a blast of hot air from the vents (with heater turned on). The sound under the dash sounds mechanical, but also kind of like a rodent trying to escape the dash.

I've parked and seen it cycle like this about every 5 minutes. Heater running fine, cold air, temp spike, weird sound, back to normal.

This may happen while I'm driving, but I haven't seen or noticed it.
Everything else seems to function fine. Until today when it won't start (starter turns, but engine won't fire). I can't imagine how that could be related, but I'm just starting to explore this new issue.
 
Joined
Feb 18, 2020
Messages
56
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Tacoma
This sounds like low coolant level to me. Likely an air pocket getting trapped in your heater core when you get the cold air, and the noise is likely that air pocket purging from your heater core and then you get heat again. With the engine cool, check the level in the radiator itself. Probably also a good idea to check your heater T's for leaks to figure out where the coolant went.
 
Joined
Apr 14, 2015
Messages
370
Location
Tallahassee, FL
No coolant leaks that I can tell, I've never had to refill coolant in 5 years and I never smell any coolant. Heater t's were replaced a couple years ago.

Right now I'm parked nose-down on a hill with the no-start situation. When I get it started I'll turn it around and check the rad. Thanks for the input.
 
Joined
Apr 14, 2015
Messages
370
Location
Tallahassee, FL
It was definitely low on coolant. Now sure where it's leaking from, but it must be a very small leak, I couldn't find any drips on the ground, but I'll check it regularly. Heater tees look dry.

No-start issue was bad fuel pump. Managed to swap it on my lunch break, easy enough.
 
Joined
Feb 18, 2020
Messages
56
Location
Tacoma
It was definitely low on coolant. Now sure where it's leaking from, but it must be a very small leak, I couldn't find any drips on the ground, but I'll check it regularly. Heater tees look dry.

No-start issue was bad fuel pump. Managed to swap it on my lunch break, easy enough.
Glad you're back up and running!
 
Joined
Feb 18, 2020
Messages
56
Location
Tacoma
I'll start by saying I've heard this is very rare on the 2UZ, but the reason your symptoms were familiar sounding:

On my old '94 pickup with 22RE, I had a very slight leak on the head gasket into the combustion chamber. It would only leak through briefly when started cold, and once warmed up would seal back up. This would slowly, but surely burn off coolant. If I'd wait too long to check and top off the coolant, I could always tell when it was low as the first symptom was usually losing heat. It I waited even longer, I'd see the same temp gauge issue you noted.

Obviously a 2UZ isn't a 22RE, which were relatively well known to have head gasket issues, so I'd bet your coolant leak is external. Still, if it was overheated badly enough by running low? Maybe you can pressure test the cooling system to see if it holds, or to see if you can pinpoint your leak. If you don't see anything external, how's your transmission fluid level and condition, no pink milkshake in your trans? Cracks/crazing in the radiator tank?
 

AlpineAccess

Overlanding is an expensive word for car camping.
Joined
Apr 16, 2019
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Colorado
Check on the back of the motor right by the base of the heater T's. There is a steel tube which is heat sealed into an aluminum frame called the "coolant bypass" or something like that. Mine had a very small leak between the tube and the aluminum and was slowly leaking (and baking) coolant back there. I cleaned the hell out of it with a wire brush and denatured alcohol before using some JB-weld to seal the tube in better. It's been well over a year and not a drip from it. Before, it was leaking around a tablespoon of coolant every 500 miles, so not a detrimental leak. Likely the result of someone man handling the replacement of the heater T's and damaging the seal.

I have a replacement for the coolant bypass sitting in my garage as I didn't know if my seal fix would hold so well, but it has. I intend to replace the unit on the dreaded day that my starter goes out and needs to be replaced, but not before then. It requires removal of the intake manifold to get to it.
 

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