2000 Yukon overheating..help...

[HDMonkey]

Truck began running hot, almost maxing gauge, sometimes bounces back into normal range for a while. Was pressure checked with no negative results, added coolant, no running hot for a couple days then back into hot zone. When running hot, if I let pressure out of the reservoir (by slowing opening cap) seems like it gets coolant "circulating", then truck runs in the normal range for a while.....What could it be??Thermostat?? Thanks in advance for any suggestions...

[94 Jimmy]

What year? How many Miles?
I had a similar problem with my Olds Silhouette recently. Check to see if you are getting any bubbles into the recovery tank or have any oily slug in the tank or water in the oil (looks like dirty Mayo).
As you can guess I changed the head gaskets in my Olds. I had a combustion chamber to water jacket leak which allowed combustion gases to get into the cooling system. The little bubbles were the gas escaping around the radiator cap, and the coolant in the tank smelled like gasoline. When I vented the cap (get a Stant lever-lock cap and save burned hands) the gas expanded and moved to the radiator and out of the system and the car cooled down. The combustion gases form and air block in the system or fill the water pump which will not pump them, since it's a centrifugal pump for liquids only.
Take a look at the spark plugs, if one or two are very clean(steam cleaned) or dirty there is a chance your head gasket is blown (or worse, cracked head or block).
If this is your problem you can try some alum seal or Bars Leak. Put the leak stop in then run the engine at idle until hot then shut it down. The combustion chamber pressure is around 175 PSI (actually higher) the cooling system pressure is 15 PSI, guess who wins. When you warm the engine you stir the stop leak and pressurize the cooling system, when you shut the engine down now your have a 0 to 15 PSI differential so that the stop leak can plug the gasket.
Do this repeatedly with 20 minute rest periods between runs. Keep the engine at idle to keep the combustion chamber pressures as low as possible. The combustion will blow some of the stop leak back out but with repeated heat up cool down cycles a plug might form and fix the problem or at least allow you to plan when to do the job.
This worked for me, and got me through most of the winter. Hopefully it's something simpler, but if not head gaskets aren’t really all that tough, if your engine doesn't sit sideways under the dash like mine.
See ya; let us know what you find.
94