Truck temperature fluctuates

[Rev. That Guy]

bad cap is possible, but since you said that you recently changed the rad, did you fully bleed the system? most of the time it will work its way out, but some vehicles, especially GM vehicles, are prone to air bubbles causing wild fluctuations in temperature. You can pick up a new cap for a few bucks. also, you can get a cap with a pressure relief lever on it. this can be used to help bleed the system.

Check your T-stat as well. I have seen on more than one occasion that t-stats fresh out of the box can be bad. If its not too much of a pain in the ass to get to, pull it, and throw it in an old pot and stick on the burner and watch to make sure it functions as it should. It should open well before boiling point (depending on your factory temp settings for the t-stat, usually in the 190 range).

The problem with cooling systems is that all of the parts HAVE to work or none of them do.

Definitely check for air though. Some vehicles have bleeder valves in the system, usually on the t-stat housing. If not, you can simply, with a cool motor, pull the rad cap off, start the vehicle and let it run up to temp until the t-stat opens and "burp" the system. Alternatively, use the pressure relief cap i mentioned above. Another thing to check would be for any leaks in the coolant hose that runs to the res tank. If this has a hole, it may be sucking air in when it cools. If you go with the cap method (with the lever) let the engine get up to temp, shut it down, lift the lever to relieve pressure (and hopefully any air that would have risen to the cap as its the highest point in the cooling system usually) then once it cools and contracts, it will draw in new coolant from the res tank.

A water pump can be bad and not weep, seep or anything else. dont rule it out, but again, in my experience, when pumps go out, it is usually catastrophic or complete failure. there isnt really much to wear out.

Try those things first as they are the cheapest and easiest. After that, you may need to do some further investigation if that didnt fix the problem.

Any obstructions over the radiator? sounds stupid and simple enough, but you never know.

- J