My Bronco has a pinhole leak somewhere. I've never been able to find it. I'm layed off right now, otherwise I'd replace the radiator and hoses.
Today, it was pretty hot, and I got home from a short trip with the A/C on.
I heard the coolant boiling, and popped the hood. It was steaming out of the overflow. To me, this seems to indicate the radiator cap is bad. It seemed pretty hot, but when I went to check the guage, it was at the "o" in normal, where it rides most of the summer.
I know i need to fix the leak eventually, but right now, I'm OK with putting some coolant in it every couple weeks. Am I right to think it's the radiator cap that's causing it to boil into the overflow?

 

yes that is a posibility. water underpressure (with the cap on) will not boil, or thats the way I understand it. then if you take the pressure off of it. it can boil. so if your cap is bad I guess it could cause that. the other thing that I always preach is when there is water comming out of the reservior when running to check for posible signs of a blown head gasket. such as water in the oil or vise verse. anyways GL and let us know what you do.
Scotty

 

I'm pretty sure the head gaskets are fine. I just changed the oil, and no coolant in it, and there didn't seem to be any sign of oil in the coolant either.
Also, there is no smoke out of the exhaust.

 

On my 302 van, it often does this when I stop the engine in the summer. I also notice it on my 460 in my RV. That's the reason I don't like it, that they don't have an overflow tank on them; just a rubber hose pointing down. Do you check your coolant boiling point when you add to it? Maybe you have way too much water in there and not enough antifreeze.

As far as water under pressure and it boiling, that's dependent on how much antifreeze you have in the system.

 

I try to keep the coolanf 50/50 even in the summer. The guage isn't reading hot though. That's the thing I don't understand. I topped the coolant off, and drove all over today. I smelled some coolant from the leak a couple times. It never puddles under the truck. just escapes as a vapor. The temp never got above the "N" on the guage. So I guess yesterday, when it was pointing towards the "O" maybe it was a little hot.

 

Some of those gauges are pretty touchy. When driving my RV in Florida it almost always blows out the overflow when I stop the engine. It's not much but it's enough that some people point it out and tell me I have a radiator leak.

 

50/50 boils at 227F at sea level (go higher and it drops even lower). This temp is easily exceeded in the heads, or when stopping and the coolant heat soaks. Not having the system under pressure causes spot boiling in the heads, while the rest of the coolant is fine. The "boil over protection to 265F" you read on the label has an * to the fine print: "with 15 PSI cap". The cooling system must be under pressure to function correctly.

 

It could also be a couple other things. Possibly a normal expansion of the liquid into the recovery tank, which is sucked back into the system as it cools, or could be a bad temp sending unit. To check the sending unit, you can let it come up to temperature with the cap off (don't let it heat up *then* remove the cap) and stick a good meat or candy thermometer in the coolant (an infrared heat gun would also work).

 

"don't let it heat up *then* remove the cap"
HaHaHa... Reminds me of the Blue Collar Comedy routine "Here's you sign"
I'll try the meat thermometer thing...

 

Yeah, I know....

Unfortunately, (and I'm not implying you are one of them) there are people out there who *need* to be told those things.

 

Is it a radiator without an overflow? Are the fins going up and down, or left to right?

If you fill in the center, and the fins go up and down, you're not supposed to fill it all the way to the top - Leave about 1/2" above the tubes, leaving the top tank of the radiator mostly empty - that's the expansion tank basically.