Overheating Issue — Coolant Not Circulating Even After Replacing Multiple Parts (Resolved… Sorta)

Hey y’all, I’ve got a 1998 Tacoma V6 TRD Racing Edition with about 299k miles on it. Recently I’ve been chasing an overheating issue and today I replaced the following parts:

•Heater control valve

•ECT sensor

•Thermostat



After replacing everything, I topped off the coolant and started the engine. But the coolant still wasn’t circulating, there was no heat in the cab, and the upper radiator hose was hot while the lower hose stayed cold.



So I pulled the thermostat back out and reinstalled just the thermostat gasket. Once I started the engine again and began topping off the coolant, it was immediately obvious that coolant was finally flowing properly. I was also able to add over a gallon more coolant—something that wasn’t happening before—indicating the system was now circulating from the radiator as it should.



My question is: what was causing this issue?

Is it likely I just bought a bad thermostat that wasn’t opening at all? As soon as it was out, circulation returned.



Anyone else had similar issues with faulty new thermostats? Or is there something else I could’ve missed?



Appreciate any input—trying to get this truck dialed in and running reliably.

 

Possible bubble in cooling system

drop tht Tstat in pot of hot water, see if opens
Then reinstall

 

I just went through this with a friend's 5VZ 4Runner last weekend. They installed a new Toyota thermostat and it wouldn't take coolant.

The solution for us was to heat-cycle the engine several times, adding coolant each time. With the radiator cap off, run it until the temp gauge starts climbing (give it some healthy RPMs along the way), then let it sit for an hour or so to cool off. Once cool, top off the radiator and overflow tank and repeat.

In our case, the engine eventually took enough coolant to run normally.

Test the thermostat as @ControlCar described. If it opens in boiling water, it's good, and you're likely just dealing with stubborn air bubbles.

 

You didn't write the brand of the thermostat that you installed.

If I were you , I would just replace whatever's in the engine with one from your "favorite Toyota dealer".

 

It was a cheap MotoRad from Oreillys haha. Yeah I think thats a smart move I plan on buying one and installing this week.

 

Could have thrown the old thermostat back in and/or put both in hot water to test that they open. Could you have installed the thermostat wrong?

 

I still have the old one and I may test it but it's the thermostat that was in it when it began overheating and than triggered code p0115. Im not sure if the thermostat triggered this although I replaced the ECT sensor on the front end of the engine that connects to the ECU.

It's highly possible it was installed wrong because I had someone helping me and I feel they weren't as thorough as I may have been on the install. The end with the spring was installed into the engine but it's possible the Jiggle valve was not oriented right or that there was an air pocket.

No check engine light since this but I need to get another thermostat installed so It can operate properly.

 

Yea just get a new oem thermostat. Make sure you install it right. Fill the cooling system, follow bleeding procedure and you should be all set. This is why I really try to stay away from buying aftermarket parts. Especially for cooling system related parts. Good luck