2008 v6 KK Overheating Issue

[rl1856]

08 with 135k and electronic fan.

It started to overheat during the summer (temp gauge to about 3/4, not much higher). Service tech did a cooling system pressure test and flush, then installed a new thermostat. Declared it working fine. Continued to overheat- brough it back, and they did another pressure test (fine) block test (fine) and installled another thermostat. Declaried it was working as it should.

I could drive it around town for about 2o min then the temp would slowly go above the mid point/normal. If it ran for 10-20min above the midpoint- coolant would be bubbling in the resevoir, and overflow would come out of the hose. This happened twice.

Had another tech look at it. He did a pressure test but did not find any problems. He drove it around town for about 30min, and he said he tried to run it hard. No issues.

I drive it again, and it overheats again. I spoke with the tech who will take another look.

Yesterday I checked the oil - fine and almost clear (not milky), checked the coolant, also not milky. I let it run for about 30min in my driveway. I checked the exhaust- and there was some condensation, but the exhaust was clear (invisible) NOT white. The truck ran mostly at idle with occasional reving to about 2k rpm, then holding at 1500 rpm for 5 min or so. Temp was normal ! I checked everything again- oil and coolant were fine- and the radiator cap was cool to my touch.

I drove it around my neighborhood- about 2 miles round trip- up to about 20-25mph. At the end, the temp was just starting to move above normal. The radiator cap was hot, and there was -0- coolant in the resevoir !! All of it was gone. I could not see evidence of where the coolant leaked- no dripping, no pooling, no wet parts in the engine close to the radiator. Electric fan was running.

I don't think I have a gasket or engine block issue so what is going on ?

 

[seafish]

I’m not familiar with the KK but it sounds plausible that its cooling system finally burped out an air bubble after being low on coolant

 

[seafish]

That said, you should also check to make sure there’s good airflow through the radiator fins

And that if it’s a two speed fan, it’s working on high speed as well as low speed

 

[ikuo78]

The water pump may be operating erratically.

Is the coolant flowing properly?

I check this as follows:
Drain the coolant from the radiator until there is no coolant left in the reserve tank, then add the remaining coolant.
Air will accumulate in the coolant passages, and when you start the engine, air will be released into the reserve tank with a gurgling sound.

 

[seafish]

Air will accumulate in the coolant passages, and when you start the engine, air will be released into the reserve tank with a gurgling sound.

In my experience this is even better done with the front end of the vehicle raised up , or alternatively, with the vehicle parked on an incline. Doing this allows the air to move to the front and top of the radiator and then to the coolant overflow tank while it circulates through all the passages.

Just my .02

 

[AFT2008KK]

Happy 2026!

Had a similar issue on our 2012 JK on a road trip last summer. Had few shops look at it between Vegas and Glacier NP. All tested fine. Low speeds on trail all good. City and fwy driving spiked up at odd times. Turned out found it after got home it was water pump failing. Intermittently leaking around bearing. Only obvious once i pulled off original one. My .02 only. YMMV. But I’d start there.