89 XJ 4.0 boiling over

When I shut my XJ down after even a short drive I sometimes get a boil over in the overflow bottle.

When driving down the road there is no indication of it overheating, just boils over when it is not running.

Any ideas??

Thanks,
Paul

Keep the coolant level below half full cold.
Mine would do this long ago, the radiator was mostly blocked, it had a small leak at the core and I'd over fill the surge tank (because of the small leak).
I figured the partial blockage in the radiator would slow down the convection flow (the coolant keeps on circulating even after the motor is shut down, from convection) when the motor was off causing hot spots (in the motor) to boil. The air sucked into the system from the leak when I shut the motor off and it went through a cool down cycle didn't help. And overfilling my surge bottle left little room for the coolant to expand and it would gyser on me periodically. It woud also crack the tank a lot quicker than normal.

check the overflow tank cap to make sure the plastic seal on it didnt break.. thats what happened to mine..

is yours one of the closed system ones too?

1989? Should be.

Is your radiator original, and how many miles on it? RENIX radiators (OEM) tend to be good for 150-180Kmiles, and then want replacing due to blockage. Get aftermarket (I like Modine, but I've also gotten good reports on CSF units) and you'll get rid of that problem.

Also, make sure you don't have any air behind the thermostat when you refill - search up my posts on that subject (it's not difficult to avoid, I just haven't time to go into it in detail right now...)

There's nothing inherently wrong with the RENIX cooling system - I've got four of them (and I'll probably convert my lone "open" rig back one of these days - it runs hotter than the other four...)

5-90

dont run the coolant half full..

go to the dealer, spend $9 and get a new OEM cap.

-Tim

I will look into all the mentioned items. Tonight it didn't boil over.

It has 266K miles on it. But I am not sure the rad is original.

I will check it out tomorrow when I can see better!

Thanks for the info.

Paul

jeepnuts311 said:

dont run the coolant half full..

go to the dealer, spend $9 and get a new OEM cap.

-Tim

Click to expand...

Quote: from the owners manual, Engine must be off and cool. 6 cylinder and turbo diesel engines. Fill slowly at the coolant bottle until the coolant level is at the top of the *post* located inside the bottle which indicates full.

The top of the post is actually closer to the surge tank being 1/4 to 1/3 or so full.
Half a quart of expansion isn't uncommon, if the bottle is full, where is the coolant going? Probably spewing out the pressure relief valve in the cap. It's likely going to spew as soon as the motor heats up some.
The cap has a pressure release valve in it, it lets air out under pressure (I think it's 16 PSI), holds what is has and lets air in easily. The system pressurizes from expansion.
Normally when the caps fail, it is because they have been over tightened and the threads are stripped or the pressure release valve is plugged with calcuim and the system builds too much pressure. Stuck closed and the bottle is likely to blow up like a ballon and split. Not sealing and building pressure and the coolant is subject to boil (with no pressure at around 225 F) and make steam, which will cause the coolant to spew out the cap. With 16 PSI of pressure it boils at a round 270 F. The temperature sender isn't at the hottest point in the motor in the Renix.
It could be the cap loosing pressure, but it's likely it would boil over and spew or overheat at other times than right after shutdown. But heck anything is possible.
Coolant mix is also a factor, but less so than pressure.
The next time you shut the motor down, wait a couple fo minutes and then turn the key to run, it's likely the temp will be well above normal. Right after shutdown, the motor gets hotter, before it gets cooler.
If the cap isn't sealing, you can often see a stain on the top of the bottle. Wash the top of the bottle off and check it the next time you drive. You can pick out a leak at the cap fairly easy.