Overheats but only sometime. No heat.

2006 Tacoma, about 6 months ago, I saw my temp guage max out while idling, would come back down while moving, and got no heat from the cabin vents. Changed the thermostat, then took it on a 16 hour road trip with zero problems and heat came back.

Like a month later heat started not coming from the vents again, but only sometimes, and only saw the temp gauge rise once, figured maybe I just got a crap thermostat. Replaced it again, no issues for awhile. Then heat started not working again on and off, slowly and progressively getting worse over a couple months. Now at best, heat will work for a couple seconds before getting cold again, if it works at all.

But this whole time never saw the temp guage move, I figured it might just be because it's been cold out and that's been enough to keep it down, however last Saturday I drove it in 75 degree weather without issues, and today it's freezing and the guage just maxed out again while idling, so I have no idea what's going on.

Did I end up with another bum thermostat? At this point I'm just gonna get rid of the thing to see if anything changes.

I don't want to think it's the pump because it wouldn't work intermittently and on hot and cold days, and I just don't really feel like doing it. And same with the radiator, if there was a blockage or anything it wouldn't work intermittently.

And I'm losing no coolant from what I can tell. Anybody who doesn't have dumb brain like me have any good ideas?

 

three bad thermostats in a row, the odds of that are slim to zero I think.

 

I would have the coolant tested for exhaust gasses. Sounds like you may have a head gasket issue.

 

Multilayer head gaskets are known for this issue.

 

But again, if it was the head gasket, wouldn't I be loosing coolant, and it should at least be consistent?

 

sounds like you are losing fluid slowly. No heat at idle and then having heat at rpms is a classic low water level indicator. Every time you change the thermostat you fill it with fluid and your heat works as it should for a while until you lose enough fluid that your heater stops working at idle and then you keep losing it until it over heats. As mentioned above a head gasket can cause you to overflow fluid...

 

Blockage could be traveling. Drain the coolent to change thermostat, blockage moves. After awhile blockage accumulates.

 

Steam pockets, or byproducts of combustion as a gas will cause the symptom you describe.
You can purchase a combustion byproduct chemical test kit from napa or Amazon.

 

Not always. You could but pushing air into the coolant system, therefore creating air pockets and causing the issues you have.

 

The heater core is the highest point in the cooling system, it's the first place to lose its supply of hot coolant if the coolant level gets low. When you replace the thermostat you're temporarily topping it off so it works again for a while until it gets low again.

You have a coolant leak somewhere, not a thermostat issue