i just put a new one in to rule out the thermostat and it still over heats.the hoses arnt that old but im pretty sure the raditor cap never been replaced.i think it may be the cap because when i got back from my test trip it had alot of pessure on the system and was over heated .i gave the cap a small tap on tap and then its like the overflow started boiling all of a sudden and some water came out of the cap.

can the bad cap be causing it to build up to much pressure or something

It could, but odds of that are slim. Usually the first failure on caps is a leak. The way to check is: See if you can compress the spring by pushing down on the rubber part of the plunger (the rubber part that goes farthest into the radiator when you put it on).

Lay it on its back on a sturdy surface. your goal is to press on the rubber part of the plunger, not the center. It should compress with a smooth action if you put around ten pounds of pressure on it with your thumb. it doesn't need to go far, just a quarter inch or so.

Pretty rusty. Looks like you're due for a flush anyways. For what it'* worth, that isn't the original GM cap. It looks like a Stant possibly.

i think ive narrowed it down to the cap or the radiator.i just hope it aint the radiator because that brass radiator is almost 300 bucks lol.

if i take the thermo out and take the top radiator hose off the rad and start it for a sec would that show if my water pump is working fine

If you take the thermo out and you run the car and it does not over heat that tells you that your pump is working. You've done that when you drove the 5 miles.

Does it overheat just as much at 5AM as at 3PM?

How is your fan clutch doing?

Can it sit and idle indefinitely without overheating?

If the fan clutch is not doing its job and the weather is really hot out then you will definitely overheat, even without pulling a load. Also if it is a standard-duty fan clutch and you rev more than around 2,000RPM engine speed while driving at highway speeds, it will freewheel. A severe-duty fan clutch will bring the fan to ~1,800RPM fan speed (or lower if the engine is spinning slower) no matter the engine speed. If you're buzzing along at 2,500 or 3,000RPM engine speed and the fan clutch is freewheeling for your aural comfort then the engine will run hot.

The cap doesn't control coolant flow through the system. You should be able to keep the engine cool with no cap at all if the coolant magically stayed in somehow. If the cap is allowing too much pressure to build, your engine would still run cool until a gasket/seam/hose/etc. gives out and dumps your coolant on the ground.

It might be that the radiator is plugged up a lot. With the way the cap looked, odds are good. I'd suggest: with engine cold, open radiator cap and look inside. Hopefully you are a couple of inches low on coolant (but not much more than that). If not, siphon some out or use a turkey baster to get it a couple of inches low so you can see the tubes. Put the excess coolant in the overflow tank or an empty coolant bottle. Start the engine and monitor the upper hose temperature by occasionally holding your hand on it while also looking into the radiator. When the engine gets warm enough, the thermostat will allow coolant to flow to and through the radiator. When it does, coolant coming out of the tubes should be just dribbling. If they are squirting like miniature garden hoses with miniature thumbs on them then odds are good the radiator is plugged. If there is no flow at all, then it is plugged in the top tubes.

Agree.