What causes pressure in radiator when engine is cold ?

 

If you get pressure immediately after startup, before the engine is even considering being warm, that's quite likely a blown head gasket.

What engine? What year is your truck? How many miles on it?

 

Hi. its a 2004 5.7 and has about 187,000 miles on it.

 

You can rent/borrow a test kit from most any of the big chain auto parts stores, to test for combustion products in the coolant. I highly recommend you do so, and soon.

 

Thanks alot.

 

Yeah, if you leave the cap off and see coolant shooting out from a dead cold start, it's VERY likely the head gasket. The coolant circulates in a closed loop in the heads, etc., till the thermostat opens. There is no heat expansion of the liquid at this point, so that would be combustion pressure. Are you overheating? How does the oil and coolant look?

 

oil looks ok and its not overheating. i let it run with the cap off for about 1/2 hour and nothing happened .i have a PO480 code can the two be related?

 

oil looks ok and its not overheating. i let it run with the cap off and nothing happened .i have a PO480 code can the two be related?

 

Do you have a cooling fan that is electric in any way, shape, form, or manner?

That shouldn't have the radiator building pressure right away. Do you see bubbles at the radiator neck while the engine is running?

 

Cooling fan only works when I hot it at the fuse box. Fuses and relay are ok. No bubbles with cap is off.