Why is this doing this???

Had a simple radiator leak and replaced the radiator, thermostat, and radiator cap. Flushed the block completely and filled cooling system per spec. Noticed the following:

When driving the jeep with air and/or heat on, the jeep runs fine and never overheats (stays around 195). When shutting off the engine, you can watch the coolant resivour level start to rise, then begin spilling over. Just as normal, coolant level in the resivour is between 'Add' and 'Full' at normal operating temp; however, it boils over every time. When you shut the motor off, you can hear air and coolant passing through the radiator cap into the resivour. Within a minute, coolant is coming out of the resivour fill cap then you see air start pumping into the resivour from the radiator (boiling). If you stop driving and let the motor idle for a few minutes, the effect is not as bad, but it essentially does the same thing.

If you run the jeep without AC or heat, it does the same thing, but not to the same extreme. Took it to a jeep dealer and they told me that there was probably a block in the heads and/or block, and they would have to tear apart the motor($$$) - pass. Flushed cooling system again, and eliminated all possible air from the cooling system by the way I filled it and thought everything would be fine - and it seemed to be.

2 months later I hear "very bad" noise coming from the motor and I have coolant steam coming out of the spark plugs (scrap one motor).

Had the motor replaced, with different thermostat, water pump, etc. and had the heater core flushed. Mechanic tested everything (even used actual monitor for coolant temp that goes on the radiator - assuming the radiator cap) and said "all is well".

IT'S DOING IT AGAIN!!!! Exact same problem as before I blew the first motor. The only commonality between the two is the radiator, the radiator hoses and cap. I'm seeing the exact same thing - coolant resivour boiling over when shutting off the engine. Also, if you start it up again after a minute or two, when the motor starts, the upper radiator hose collapses, then slowly expands back to normal.

WTF over? Can someone tell me what is wrong?!? The local jeep dealer nor my mechanic know what this is - as neither of them have seen this behavior before.

Help Thagan, Please. I have the same problem!

Let me start my reply with some background so you don't think I pulled my answer out of my butt. I've been an ASE master certified tech for over 15 years in Phoenix Arizona. I've seen a few cooling system problems :(

I read your post shortly after you wrote it and have been thinking about it since. I keep coming back to the inescapable thought that your cap is not holding pressure. Here is why...
When you shut off the engine, the coolant in the block superheats...this is normal since it is no longer circulating. The problem is that if the superheated coolant is not held under pressure, it will vaporize. Obviously, when liquid changes state to vapor, it takes up a lot more volume and thus expands into the overflow. Also, vaporized coolant, doesn't cool very well, so the process feeds on itself, eventually boiling over.

It also makes sense that your hose collapses...once the coolant is moving again, it will condense back into a liquid. Since the system isn't under pressure, this is putting the system into a vacuum until the volume can be replaced with coolant from the bottle.

That being said, I do want to point out that you have more common cooling system related parts than you think. Ignition timing, fuel mixture and exhaust backpressure can all have a major effect on cooling system performance.

I hate to say it...but I hope I am right. I think your radiator cap isn't doing its job. Get a new one and make sure it is rated 18 PSI.

THAGAN- there is a list of possible problems and I agree strongly with what STEVEBO5 says.

but here are some other things to look at....

- Overfilled Cooling System (make sure coolant is at the ADD mark on the bottle when completely cold)
- Quick shutdown after hard (hot) run.
- Air in system, resulting in occasional 'burping' of coolant.
- Insufficient antifreeze, allowing coolant boiling point to be too low.
- Antifreeze deteriorated because of age of contamination.
- Leaks due to loose hose clamps, loose nuts, bolts, drain plug, faulty hoses, or defective radiator.
- Faulty head gasket.
- cracked head, manifold or block
- Faulty radiator cap.