Boilover in '89 4.0L

[Konga Man]

I've got an '89 Cherokee that has recently started boiling over when the engine is shut off. The engine does not run hot; it will go all day right around 190*; the gauge clearly shows the thermostat opening and closing. Everything seems to be fine aside from that; the coolant reservoir and cap are new, the thermostat is new, all the hoses are new (after 20 years ).

When the engine is shut down, I can see the temperature climb on the gauge (which is completely normal). As it climbs, the coolant level in the tank also rises. If it gets high enough (like after a long drive), it boils over. This usually takes place in a matter of seconds; if it's been driven enough that that the engine is good and hot, there's no time to get out of the door (much less open the hood) before it goes off.

One thing that is very strange (to me, anyway) is that the hoses never get hard. They don't even get close to hard; even when the engine is running or the coolant level in the tank is rising and the coolant is obviously expanding after it's been shut off, it's easy to squeeze them with your hand.

My first thought was a bad cap or tank, but I've tried several with no luck. I've used a block tester to see if there was an obvious head gasket leak, but nothing showed up. I thought it might be an air pocket, but I've been draining and filling this cooling system for 20 years and I think I've got that sussed. Aside from the boil over, there's no sign of obvious coolant leakage; no green puddles on the ground, I don't need to keep adding coolant, no white smoke in the exhaust, etc..

It acts for all the world like it's a truly open system (that is, with no cap at all) that just boils over when the coolant reaches the boiling point (like a pot on the stove). In other words, it just doesn't seem to hold pressure. No, I haven't had a pressure test or a leak-down test. Even with a small leak, pressure will still build up; there's never any pressure buildup at all with this. You would think that a leak large enough to bleed off that much pressure would be accompanied by obvious signs of coolant leakage/loss, but there are none.

So, any ideas or suggestions?

Thanks.

[NIX]

Do you actually have an open system with the cap on the radiator? Then the complete lack of pressure is your problem. This makes the fluid to cook easily once hotter than the cooking point, and on some spots it gets hotter than that because the coolant isn't pumped around to cool in the radiator. With a non leaking system and a properly functioning radiator cap this is no problem. Also trapped air may cause these symptoms. How did you bleed the system? Raise the rear wheels a bit and loosen the temp sensor on the head near the firewall.

Also make sure the overflow hose between cap and tank isn't leak so it doesn't suck air in the system when cooling down. It should only draw water from the overflowtank when cooling down.

[GeeAea]

Don't shut it off until you've opened the hood. Leave it idling.

This has to be a cap issue. There's no other explanation for the coolant tank being the point of egress. Even if the cause was something else, the soft hoses and overflowing tank point to the cap. 100%.

Now it may be the RADIATOR cap flange (whatever the correct term for it is) that won't allow any cap to work correctly, but that's why it's happening every time THERE.

Again, you may have other issues that produce the problem.

My TJ's spec an 18lb cap. When it erupts, it's violent. I've only had it happen twice. Both times it was low coolant. The oddity was that there were no apparent leaks and the system held excellent pressure. Somewhere in the warm up process the coolant would push past the rad cap gasket (bypassing the 18lb threshold) ..but seal up as temps increased. You never saw a puddle ...until the low coolant boiled the tank over (remember- NOW the gasket was swelled due to heating).

Anyway, it doesn't sound like this is your particular issue, but it's definitely at that rad cap interface where this evidence is coming from.

[zzzjojozzz]

my 89 cherokee had the same problem it kept ripping the threds off the stupid cap and boiling over so i went and got a radiater water neck and all hoses off of a 92 it took care of all my problems and i had way moor heat in the winter i was amazed

[krabysniper]

zzzjojozzz, the 92 you said you got your radiator from was most likely just an early release 93 model, as they made the change over to open cooling for the 93 model year, prior to that back to 87 they were closed systems.

AS for the boil over, these guys have nailed it. You have a lack of pressure issue. You said you replaced the coolant "tank", that crappy little plastic bottle up on the fire wall on the right hand side of the engine compartment, correct? DID YOU REPLACE THE CAP? The new bottles I ever got never came with a new cap, I went through 4 of them bottles before I finally said to hell with that crap and swapped in a brand spanking new radiator for a 93 and put an end to all my cooling system headaches. But if you have not replaced that cap, and you don't want to spring to fix the problem the right way, I would start there first. Try a new cap, and get back to us.