Overheating with no clue

Hey guys,

Newbie here; would like some help with an overheating prob I have: truck shows it's overheating with the needle reaching almost redline before I turn truck off; however, upon starting again, needle only goes 3/4 of the way and then starts coming down as I get moving; changed thermostat, water pump, radiator cap already; fan seems good with no play; no loss of coolant evident; radiator seems in working order. What could be wrong? Please help!

 

Any coolant smell in the exhaust, "milky" look in the oil, or any oil in the coolant?
Could be a blown head gasket.

Also, physically check the thermostat in a pan of water. "They don't make 'em like they used to" applies here... auto parts store thermostats and radiator caps run a very high failure rate.

Finally, could be an airlocked system.

Cool engine, park the truck on a steep hill, nose high.
Remove the radiator cap and start the engine. Allow the temp to come up and you may see air burping out with the coolant.

 

Thanks Rich.

There is no oily or milky coolant; I will try the airlocked fix you mentioned though; is there supposed to be water burping when that is done?

 

With no air in the system, you will just get a slowly rising level of coolant gently overflowing the filler neck.
If there is air in the system, it could get violent. The air pockets create hot spots, and as they move you'll get some steam flash and rapid changes in pressure.

You don't want air in the system... and with some coolants like GM's Dexcool, it can cause congealing.

 

The coolant is just slowly overflowing like you describe; no violent surges of coolant.

 

It sounds like the thermostat, but what you said about it starting at 3/4 and then dropping after you start moving could also be the fan clutch.
Is the fan shroud still in place top and bottom?

 

Are you strictly going off the needle?

 

Is it overheating at a standstill then cools as driven? I'm wondering if you need to remove the radiator and hose it down. Over years the fins can get filled up with debris. I've also seen it where the fins have all rotted off and are mostly gone. Could be a faulty temp sensor also.

 

I am going by the needle; I will replace the temp sensor first and will check again; fan seems to be good with the shroud in place. Thanks to all for your thoughts and ideas!

 

You can't tell that the fan is good just because it's spinning.

On a cold morning, does the fan seem to be loud for the first few blocks, then quiet down?
If not, the clutch may not be engaging. The fan will spin, but it won't carry any load.