My 91 XJ recently started leaking overheating and leaking coolant everywhere. I thought I was a bad water pump so I replaced that and the upper and lower radiator hoses, but it still kept overheating. There was no pressure in any of the hoses so I thought it was a bad thermostat so I took it out all together to see if that would help. It didn't. I don't have a clue. Any ideas?

 

T-stat has nothing to do with the system building pressure.....that's the rad cap's job, presuming no more leaks anywhere. Put a new stat back in and replace the radiator, rad cap and fan clutch. Fan shrouds intact/in place? Efan working?

 

I was let to believe that the thermostat acted as a dam of sorts. When the engine runs hot, the thermostat's spring system engaged allowing coolant to flow freely, and visa vera when the engine is cooler. All fans are running properly, coolant is getting to the radiator, no codes are thrown. Its just not going to the engine.

 

When the thermostat is open, coolant flows through the radiator. When closed, coolant bypasses right back to the engine. It's always flowing. Unless you have a clog somewhere. If you force water into the upper hose with the t'stat removed, does it flow back out the bottom?

Air bubbles/pockets are also a pretty big deal. The water pump can't move air very well. Did you bleed the system?

 

The t-stat does act like a dam of sorts, it changes the direction of coolant flow at a given temp. The rad cap seals up the system creating pressure as coolant temp rises which in turn raises the coolant boiling point. When the t-stat is closed coolant flows one direction (in a loop thru the block and head). When the stat opens, coolant flows in another direction and that's thru the radiator. when the stat is open, coolant flows from the block, to the head, to the rad and back to the block. When the stat is closed, coolant flows from the block, thru the head, back thru the block/head via the bypass loop and in a continuous loop.

The radiator cap is what causes pressure inside the cooling system to build (coolant/air expands when heated) and when the caps rated pressure is reached, pressure is released to the recovery bottle and later drawn back from the recovery bottle to the rad as the motor cools (overnight for example). Pressure is what raises the coolant boiling point. Pressure
can be lost thru a leak somewhere or lost thru a bad rad cap.

 

My xj was having a hard time getting air pockets out. I'd let it run long enough to burp, but when I'd turn it off to place the cap on, the hot coolant would flow out. So I had to let it burp then open the throttle so it sucked the coolant down more, top off with the throttle open, place the cap on, and close the throttle...any excess just fills the overflow and the air pockets were gone.

 

It takes several heat/cool cycles for air in the system to migrate to the bottle and be eliminated from the cooling system. That's presuming the system has zero leaks and the 2 way valve in the rad cap is functioning correctly.