Went to the trouble of pulling my radiator out because I thought I had a leak. All we found at the radiator shop was a slight leak in the petcock. Put a new one in and reinstalled the radiator. I filled the radiator and started the truck to burp the air out.....cap not on yet obviously. I watched like a hawk on the radiator and under the truck. Not a drip or drop at all. After about ten mins of running and letting the coolant circulate, I put the cap on. Instantly coolant started coming out of the overflow tube. This is a new 4# cap. The coolant wasn't even hot. I took the cap off...truck still running and coolant not even hot enough to blow out when removing the cap...and the overflow stopped completely. Cap back on, coolant flowing out overflow tube, cap off, stops again. I haven't even finished adding the coolant that came out of it back in yet. Never experienced this and I'm getting tired of this radiator. Any ideas because I have none on this. Thanks!

 

Forgot to mention that the radiator cools the truck just fine. No issues.

 

Have you looked down in the radiator to see if bubbles are coming up inside?

First I would test the cap. New doesn't always mean good.

Then I would put a pressure tester on the radiator to see how fast it's building pressure. If it's not hot but building enough pressure to blow past a 4# cap there might be more going on than just the radiator.

 

Seems like the cooling system is being pressurised some how. Head gasket?
Have you had a pressure tester on to see if it holds, then increases as you start the engine?

 

I'll check the cap. I haven't had a pressure tester on it. And it had a few bubbles when I was burping the system.
Again, I can drive the truck for how ever long I want. It doesn't get hot. Not even in a parade for an hour going from just idling every few seconds to going 2mph for a few more.

 

Measure the depth of the radiator neck and compare it with the cap
I bought a new cap and it was a bit short

 

Might be time for a leak down test.

 

Would have never thought of that! Great info.

 

Maybe drill a small hole (1/8 inch or so) in the thermostat? Allows coolant to bypass thermostat when engine not yet hot.

 

But his problem has nothing to do with overheating.

 

This may be stupid to ask and I sure don't mean to offend, but how full is the radiator tank, is there room for expansion?

 

If I read it correctly you're running a 4 lb cap. I would bump that up to a 7 lb cap since increasing pressure raises the boiling point of the coolant. Raising the pressure should theoretically keep the coolant from puking out due to boiling. I don't know what motor or radiator you're running though.

wally

 

Yeah, that's why you do it. The thermostat stays closed until engine temp is reached. That means no coolant runs from block to radiator. The hole allows the coolant to pass before the engine heats up and removes the pressure. Also helps when trying to get the air out of the system.

 

All thermostats have some means of air bleed built in. If you hold them up to the light, you can see the seat or the disc has a wavy edge, or a notch cut in it to provide air bleed/minimum flow. The only exceptions are the kinds that have a separate disc to open/close a bypass.

 

Sounds like lots of air in the system somewhere causing expansion and contraction. Usually in the heater core. Drop the end of the overflow tube into a gallon milk jug and the cycle the engine hot to cold 3-4 times. You should get less in the bottle each time if you are getting the air out.