Water Boiling "IN" Radiator (not out)

06-01-2010, 02:12 PM
1999 Isuzu S 4cyl

I got home after driving the Rodeo for about 45 min trip, some w/ the A/C on, but not on the uphills. Got home and the water was boiling IN the radiator. Nothing was coming out anywhere. The reservoir was at MAX, the temp gauge in the car was fine, not showing overheating. The fan wasn't staying on as if it was hot either. Once it cooled, the reservoir was at MIN. I opened the cap and the radiator fluid was right at the top, it was normal looking.

What is going on? Bad radiator cap?

Any ideas greatly appreciated.


06-01-2010, 08:08 PM

I replaced the radiator cap. When I took the old one off, the under parts fell out. I wonder if a piece fell into the radiator...don't see anything there.

I was also low on oil (have a leak I need to figure out the origin of) and will keep on eye on the oil more frequently now.

Yet to test it out, but seeing that the radiator cap fell to pieces, I suspect that was my issue after all.

06-01-2010, 10:37 PM
think you got it.

the cap seals the system thereby increasing the boiling point of the coolant. Increase in pressure= increased boiling point. Each 1psi increase raises the coolant bp by 3 degrees. the bad cap couldn't hold pressure and the coolant boiled in the rad. the temp gauge would show fine as the coolant boiled at a low temp.

Make sure your coolant isn't more than 70% coolant. Too high of coolant concentration will also cause overheating as the dense molecular structure of the concentrated coolant won't release the absorbed heat to the rad.

06-07-2010, 07:47 PM
The new radiator cap solved the problem. I drove over 70 miles, most of it with a full load in the back, not an issue. :)

Add your comment to this topic!