The truck is an 04 GMC Sierra ECSB 5.3L 4x4. I am having some intermittent overheating issues. Back in February it overheated on the interstate and blew a lot of coolant out of the expansion tank. I replaced the thermostat, refilled the coolant, and everything went back to normal. Last week it started to run hot again. Here is what is odd. The truck overheats when moving such as on the highway or interstate. When I come to a stop the truck will cool back down. As soon as I get going again it will start to overheat again. I logged with HPTuners and it is definitely running hot (ie not a gauge issue). It hovers around 237-241 while going down the road and goes back to normal when I stop.

The truck has LS1 electric fans on it. I thought the issue might be related to those so I took them off yesterday and the problem still exists. Nothing really changed by swapping back to the stock clutch fan. Only thing I noticed last week was that some hot air was blowing out of the vents when it wa running hot. This happened with he HVAC system completely turned off. Could it be an issue with the heater core?

This has really got me stumped and any help would be appreciated. What should I check next?

-Josh

 

I was going to say maybe you have your fans turning the wrong way but since you took those off that doesnt sound like it...

...maybe the pulley on the waterpump is loose so when you are cruising on the highway at a higher rpm than idle it just slips instead of actually turning the pump...

 

I would check the block pressure the water pump is creating. It sounds like either the belt is slipping or the impeller broke off inside the pump.

 

Blown head gasket.

 

What would lead you to believe I have a blown head gasket? Why would the truck cool off like it frequently does? It spends half the time running hot and the other half at normal operating temperature.

How do I check "block pressure ay the water pump"?

It is almost as if something triggers in the pcm when I get below a certain rpm that gets the coolant flowing, so to speak. Feels like a blockage that comes and goes. I would suspect thermostat if I hadn't just replaced it at the end of February.

Thanks for the help, keep it comin'

Josh

 

It sounds to me like you have a air pocket in your cooling system. Take the cap off of reservoir and start it and let it get to normal temp and see if the water fluctuates any or just stays steady.

 

Flush the system out and see if there's any particles such as to be a waterpump problem if not I would change the tstat again with a good on and try..

It won't be a problem with the fans they should shut off once you get above 30 or so mph anyways

 

Air in the cooling system could cause it but I haven't seen it be a problem on these engine getting the air purged.

To test block pressure you just tee in a gauge on the pressure side of the cooling system.

 

Have a chemical block test done. It will test for exhaust gasses in your cooling system. The harder and longer you drive it, it will overheat. Idling and not pushing it as hard, it will cool back down. It's a test that just takes a few minutes. Most auto parts stores sell the kits if you want to buy one and do it yourself.

 

+1, this is an easy test and a head gasket can very well be the problem. Also, what I've seen through the years when a vehicle overheats on the highway but is OK at idle, there is either a blockage internally or in front of the radiator (clogged or debris in between radiator and condenser), and a few times I've seen the impellers broken/worn off of the inside of the water pump.