So I have had a over cooling issue for awhile. First off, I am driving a 3.0 Auto. I have replaced the thermostat.

Now the issue is, I will drive the truck until it is up to temp, hit the highway, and after a few minutes of 60mph or so, the truck will slam out of O/D into 4th sometimes even 3rd. Revs shoot way up and as I keep driving, I look at the temp gauge and it starts to drop. If I continue at highway speeds the temp gauge will drop to about the 1/4 way mark and settle. Back into city driving, it comes back up and the process repeats as I hit the highway.

I am really thinking sticky fan clutch, but when the motor is warm and I pop the hood, I am able to spin the fan freely. Any input?

 

Is it possible that your new thermostat is stuck open???

 

thats what im thinking. when i had to replace mine i had to buy 6 different ones before i finally got one that worked. a simple test is to boil water and stick the thermo in.. if it opens, ok. when you take it out, if it STAYS open, then you got a problem.

 

Almost certainly the thermostat.
The thermostat controls the engine temperature.
Yours is either way too cold or simply not closing.




Fred

 

Did you replace with a Toyota OEM or after market thermostat?

 

What brand thermostat did you install? If you didn't get a Toyota one, go to the dealer, buy one, and put it in.

The genuine parts have very consistant quality, I've not had a new one fail on me yet.

Aftermarket brands are spotty, as ozziesironmanoffroad has noted.

HTH.

 

If the thermo was sticking open, the engine wouldn't get up to operating temp would it not? It was replaced by a thermo from napa.

 

I have had T-Stats stuck open that will get up to temp with city driving because the slower stop and go is a lot more taxing on your cooling system and allows it to heat up even tho it is stuck open.

However on the highway/freeway the engine is doing relatively little work and with all of that cool air running through the rad it cools off the coolant much faster than slow/stop and go driving.

Whenever i have a problem with overheating/under heating i always replace T-Stat first 90% of the time that is the problem and it is cheap and easy to replace.

 

If the thermo is stuck open, it should just take longer to warm up. It shouldn't affect most anything else. It won't kill your truck to drive it like that for a while, just take it easy till it warms up. The 15 minutes it takes to warm it up wears on ya a little after a while, though.

 

Ill second and third the OEM assesment. Has this issue, could not figure out why my tranny was acting up.. went through 3 or 4 autozone Thermostats... nothing. Broke down and bought one from the toyota dealer; hate going there; always telling me to get rid of the old runner and buy a new one.

Anyway, worked like a charm. Our trannys are so temp sensitive.

 

15 minute warm up? haha, brother, when i didnt even have a thermostat, it took 35 mins to warm up. and this was in san diego. now it takes about 15 mins to warm up... i think the thermostat failed again... friggin kragen

 

It might eventually get to OP temp, especially in start/stop conditions. I had the same problem with a NAPA thermostat, replace it with a Toyota.

 

So spent the afternoon today replacing the thermo with a toyota one, The unit is much different then the napa one I had. Took it out for a drive, still having the same over cooling issue. So i'm thinking it has to be something else, The fan is free spinning, so I don't think that is the issue.

 

Agreed.

So, my guess is it isn't a cooling system issue. Since the thermostat is the only thing that would cause over-cooling besides the fan clutch, possibly. Or, perhaps, the high rpms of the sudden downshift at highway speeds. This would cause the water pump to spin faster improving coolant flow, and combined with the higher air flow quickly lower the engine temp accordingly.

I'm thinking along the lines of a transmission control electronics foul-up. Have you checked for codes?