JoeMustang

My 302 with an auto trans has been running HOT the last few times out, and I was going to see if it was a possible head gasket by using an exhaust analyzer. I start it up with the radiator cap off and the radiator filled over the top of the fins. You can see when the thermo opens up and the water starts flowing by. I was told to wait until the thermo opens to take the reading, but within 15 minutes the water stopped flowing across and started bubbling out of the filler tube.

I thought part of the problem of a possible bad head gasket was that it PRESSURIZED the coolant system, thus making it pour out of the catch can, but since the cap was off I don't know why it poured out of the filler tube. The electric fan comes on at 180 degress and does not shut off.

Shouldn't you be able to keep the cap off and leave it run? Why does it all of a sudden stop exiting the radiator and bubble out?

2+2GT

Because water, or coolant, boils at a lower temperature if it is not pressurized, which is what'll happen if you leave the cap off. Anything below 220° is pretty normal, and of course at 220° with the cap off, it'll boil like crazy.

JoeMustang

What would make it rocket up to 220 in such a short time??

1965fastback

I would try a 13lb cap and see if that works. (13 lbs is the right one, right guys?)

JoeMustang

I have a16 lb cap when it was running ok........but my question is, why did it bubble over after only running 15 minutes? Shouldn't it just flow across as long as I want to monitor it??

JoeMustang

Anyone ever have overheating problems?????

pootypeters

Is your radiator cap pretty old or is it new? Does it open up and allow reservoir fluid into the overflow tank?

derwin66

When water boils it expands. I would suggest then that because the radiator is unpressurised it boiled at 100 degree celcius (or whatever) and then the water looks for the easiest way to expand which is the opening of the radiator. All cars will do it. When you put the cap on it lifts the temperature the water will go to before it boils,because it is now pressurised, which with a 13 pound cap is about 230 F. It too will then burst out through the overflow tube.

1965fastback

I would clean the rad. next and while you have the rad. out to clean it, flush out the water jackets of the block by using a water hose in the upper rad. hose.

JoeMustang

I am taking the radiator out this week end. I did see some small crap inside along the top. I also noticed the electric fan on it is not covering a 4 or 5 inch area along each side of the radiator, so I thing I will go to a unit that has a cowl with it. BUT...when you say clean out the water jackets, is this with the engine off??