Boilover in '89 4.0L
 

I'm 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 coolant level in the tank is rising and the coolant is obviously expanding, it's easy to squeeze them with your hand.

My first thought was a bad cap, 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.

cap needs to be 18psi for the four litre but i am sure you are aware of this going by your post

Mine would do the same thing. It was fine while driving but as soon as i stopped or let it idle for a while it would boil out. I tried several new caps but they would all just blow off. I went to the Junk yard and got a new reservoir that had a good cap on it and it has been fine since.

I should've added that: I also tried replacing the reservoir and cap with an aftermarket set. Not only did it not help, but after the engine would boil over, the cap would no longer thread onto the tank; it would keep spinning when you turned it, and you could pull it straight off. Furthermore, it was threaded slightly differently than the OEM tanks, so the caps were not interchangeable with OEM or other aftermarket caps.

It went back.;)

Based on all of that, there's probably still some chance that the tank that's on there now is bad.

I've thought about temporarily removing the tank from the system and connecting the input and outlet hoses together to see if the system pressurizes that way. I wouldn't want to drive very far like that, but at least it would tell me if there's a problem with the tank.

Subscribed, I am now having the same problem. How many fans do you have? My 87' has one belt driven and 1 electric, both work fine.

You could try putting some thread tape on there to keep the cap tight until you solve the problem.

any word on making yours better?

you are using the wrong cap or there is damage to the filler neck
where the cap seats

This is the solution to those plastic tanks http://www.macsradiator.com/replacme...filltanks.aspx

When you changed hoses you did re-install the spring inside the lower radiator hose. If not that could easily collapse.