So I have a 302 that keeps on overheating. It has a flex fan, new radiator, all new hoses, a 165 thermostat. I have tried 16 and 13 lb radiator caps.

It mainly overheats at idle; it will sometimes get a little hot when driving. It has bent every radiator cap I've put on it after about 20 minutes of idling then it will start to overheat. I have tried it connected and disconnected from the overflow bottle, it will overheat much faster with it disconnected.

I have also tried turning the idle up to pull more air and it didn't get above 170 but it still bent the radiator cap.

 

Do you have a radiator fan shroud?

And welcome to FTE!!!!!!

Guys around here love truck pics btw...

 

Hello, are you 100% sure you don't have any air in the system? You wrote that everything is new....if it's all recently been done, did you "burp" it? Maybe that's why there is air in there.....IF that's the problem.

 

Your cap should vent way before it bends. Make shure the lower hose has a spring in it to prevent collapsing. Check for exhaust bubbles in the radiator after the thermo opens.

 

Bending caps is extreme , not typical of running a little warm. is it a stock radiator or some aftermarket thing ? a poor fit on the cap could also cause it.

I would suspect a cracked head or bad head gasket.. with the cap off warm it up then have someone put a load on it and see if it blows bubbles.

 

That 165 T-stat is to small should de 180-210. At 165 it will never close.

 

Never heard of bending a radiator cap. As others said sounds like trapped air. Make sure thermostat has the hole or jiggle pin in it and oriented to the top. When filling radiator take heater hose off intake and fill system till water comes out intake connection, reconnect hose and continue filling. I don't think the 302's are known for cracking heads or blowing gaskets. But who knows after this many years. Any chance your water pump has rusted off impeller? Just throwing ideas. I think you can get testers for combustion gasses in coolant. HF has one for $25.

 

This could be a meaningful clue.

And how are the caps bending? From being put on the rad, or from the rad building pressure?

 

Sounds like he's been fighting this for a while. air will work itself out . this is why you should always drill a 1/8' hole in the thermostat so it's not an issue.

It's not trapped air it's something else. I would rule out a compression leak first .

 

Mainly overheating at idle is NOT a good sign at all......if it does this within a couple minutes of engine operation, I would suspect a head gasket worse case scenario....
Best case scenario as nothing was posted about a new water pump replacement, so a big hopefully the existing water pump is defective.


Like 440 sixpack states - I too would do a compression leak test first to rule it out . . . . .
The way I check that without any diagnostic tools of sorts is to :

1 - drain some coolant out of the radiator -
2 - remove the thermostat from the intake -
3 - fill coolant to the top of the intake thermostat inlet area -
4 - remove the water pump drive belt from the fan pulley -
5 - start engine for one to two minutes at idle and observe thru thermostat housing hole with the visible coolant present and determine if there are any air bubbles -
6 - if no bubbles are present, compression leak will be ruled out.

If #6 applies, I would check compression on each cylinder making sure there are no low readings to any adjacent cylinders or end cylinders 1-4-5-8 for good measure -

If all cylinders have adequate compression, I would replace the water pump and thermostat with quality ones (and the .125 inch hole on the thermostat housing as suggested) and check for something else that might be restrictive if running hot problems continue, perhaps install a 7 lb cap and see if the cooling system improves (which worked on my friend's 383 Plymouth after I suggested it with the same type problem) while hoping the cylinder heads are not cracked somewhere.

I never heard or ever seen a bent pressure cap either as 5851a stated, but I have experienced pressure buildup upon blow by as the upper radiator brass tank bulged out and sprung a leak at the solder connecting areas though.


I am with Christmas on this one....I was first going to comment on this very detail of a low temp thermostat which not only is way bad for an everyday driver, not to mention all the scaling deposits that will occur and chancing it by restricting the block and cylinder head water jackets which are small passages.

The minimum water temperature of sterilizing any type of surfaces is 171 * F to prevent calcium and scaling deposits, as I always run a minimum 180 * F thermostat in any of my rigs.

Best of luck on that 302 engine.

 

The other thing most folks don't realize, Hose water is taboo and will quickly screw things up.