OK, got done rebuilding the top end of a 2.8 due to a head gasket problem. Got everything back together and cranked it up. Within the 1st 30 seconds of it being cranked my temp guage shot up into the red. Changed out the temp sending unit and still same thing. Bought a 3rd party temp gauge and changed out the probe to the one that came with the new gauge. Temp goes up to about 240 on it. Is it possible that I could have an air bubble in the system even though I just took the truck on about a 20 mile trip. Got back home and did not hear any signs of an overheating engine. What is the easiest way to "burp" the coolant system.

 

Anybody have any ideas here?

 

If you suspect entrapped air is the problem, pulling a vacuum on the cooling system is a tried and true way to get the air out. If that is not a possibility, then you need to elevate the front end of the truck as high as you reasonably can with cap side of the radiator slightly higher so that it is the highest point in the system. Run the engine with the cap off until it reaches operating temperature. Let it cool. Add coolant as necessary, repeat. You are likely to spill some coolant out of the radiator as the engine warms up in subsequent heat cycles, but it will burp the air out.

Applying a vacuum to the system really helps to get the air out though.

 

i've never heard of a vacuum on the cooling system, can you explain how to go about this?

 

You can either do it with a purpose built cooling system vacuum filler or make your own with a hand vacuum pump. Making your own isn't all that difficult, but you do have to make up a filler cap that seals to the lower portion of the radiator fill neck.

When the system is under a vacuum, the air will be a lot easier to remove.

Attached is a picture of the purpose built cooling system vacuum filler I mentioned. It uses shop air to pull the vacuum and has the capability of pulling coolant in as well. Snap-On as well as quite a few other major tool manufacturers sell this type of tool.