ZJ Overheat... or is it?

Just wanted to share what happened to my 5.9 the other day.

I had just left a place and was cruising down Hurstborne when I got stopped at the light at Taylorsville road. I started smelling coolant. I looked around to see what car was overheating when I noticed steam billowing out from under my hood!!!

The temp gauge was halfway between 210 and 260, uh oh.

I pull over and pop the hood with the engine still running, my coolant recovery tank is at a rapid boil, spewing coolant and steam from the level sensor and cap.

I couldn't find any leaks, but the radiator cap was definitely not holding pressure and letting it all jet into the overflow tank causing it to violently boil. I popped the cap off the tank to help vent out the pressure so it would stop trying to push the coolant sensor out of the tank.

A previous owner had installed a radiator cap with the valve on it to release the pressure. I stopped the jeep and let it cool, all the while pressure being vented into the overflow. After a few minutes I started opening the release valve to vent the rest of the pressure. This took a while but I finally got it to the point where I could safely remove the cap. On initial inspection, the cap looked fine. I refilled the radiator and reinstalled the cap. The engine cooled down and it appeared to be holding pressure again.

I finish my drive back to work (about another 5-7 miles), and of course, I hit every red light on the way. Temp climbing up at idle, going down if I rev engine in N while stopped. Never let it get over 230.

I let it sit for several hours while I was at work. Towards the end of the shift I went out to see if I could figure out what the hell was going on.

I refilled the system without the cap and checked for flow through the hoses to make sure the pump hadn't disintegrated on this inside. That was good. No leaks anywhere. Finally, went to put the cap back on and noticed the center part was slightly tweaked. There was a plastic shaft in the center that attaches to the release lever, it has broken about 50% on one side. This was causing it not to seal properly and vent pressure.

A little "How things work" here. The pressure in the cooling system raises the boiling point of the water/coolant. Water boils at 212' F. For every 1psi, the boiling point is raised 3 degrees F. It is cruicial for the cooling system to be pressurized to keep the water from flash boiling at the hottest parts of the engine. When water flash boils, it turns to air, air won't cool your engine!!!
Antifreeze also chemically raises the boiling point even further, another reason to not run straight water, even in the summer.

I replaced the radiator cap with a standard style cap. 18psi is what is called for. Now my jeep runs right at the 200 mark, which is where it should be. Prior, it was running at 210 but never got higher until this incident.

It cost me $3.63 (I get wholesale parts costs) to solve this overheating issue and lower my normal operating temp by 10 degrees.

man i hate having overheating issues. good thing you caught it before it got worse