2000 Cherokee XJ 4.0
I haven't topped up the coolant for a couple of weeks and know that it was just above the "Add" mark on the reservoir. I also know that the radiator was full you the brim with no air at the top (I check the coolant every time I drive due to 0331 head paranoia).
My last few journeys have been issue free in scorching summer temperatures (for the UK!).
Yesterday, I parked up after 20 minutes of driving and when I returned 10 minutes later there was a puddle of coolant under the vehicle. It was probably about half a pint.
A quick inspection suggested that the coolant had spewed out of the reservoir overflow (bizarrely positioned right next to the battery on RHD models).
Now that the vehicle is stone cold I can see that the coolant loss was only a cup full or so.
I know that cooling stops as soon as you shut the motor down but I was surprised that the residual heat would cause the coolant to actually boil over a few minutes later.

Any thoughts?

 

Not true. The temperature actually goes up for a few minutes after shut down due to the absence of circulation. You need to keep the level at full mark in the reservoir. If it drops without any obvious external leaks then you are a potential victim of the cracked 0331 cylinder head.

 

It only does it after shut down?

 

Couple of easy areas to rule out are the conditions of the radiator cap and the hose between the overfill reservoir and the radiator.

 

Mine did this during my 0331 head crack saga. I would check for the presence of exhaust gases in the coolant to rule that out, as what you could be seeing is the effect of exhaust gases boiling your coolant and spewing out through the bottle. If that's what's happening, it's probably not just happening when you stop but boils over when the system stops moving.

There's apparently a way to check this visually by running the engine up to operating temp without the radiator cap to see if exhaust gases spew from the filler neck, but I've never actually done that myself. The more reliable (and safer) way to do it is to get a combustion leak tester, which looks kind of like a turkey baster. You draw up some of your coolant into the tester's chamber and add some detection solution that changes color if exhaust gases have gotten into your cooling system. They cost about $50 USD, but a lot of mechanics have them in their shops and can perform that test for you.

 

Exactly. That's why it has me confused. When running, whether it's sitting in slow city traffic or running for an hour at 80mph the coolant temperature is completely normal - never over 200°F.

And this is the only significant coolant loss I've suffered since I've owned the Cherokee.
I've already ordered a new radiator cap and am hoping it will be that simple.
My understanding is that the later XJs with open coolant circuits self-bleed and don't have airlock problems. Is there any reason to doubt that, assuming my rad cap is working?

 

No

 

make sure that you check the level when cold and fill appropriately. Once you have the new cap installed, let us know if it still does it.

 

Thanks for the advice and reassurance.
I "installed" the new rad cap on Saturday. I haven't exactly piled the miles on since then, but so far so good.

Ironically though, having hopefully sorted one minor issue another immediately cropped up. A pre-existing slightly wobbly idle turned into a low rpm stall!
I've ordered new NGK plugs to replace the Bosch plugs that I now realise everyone hates and I whipped off the TB to give it a good clean. It was pretty grimy. As many other posters have found, a previous owner or mechanic hadn't bothered to replace the TB gasket after it's last removal. I can understand why though - even though they're cheap as chips nobody in the UK stocks them so I'm gasketless until my new one arrives from the states.

The cleaned up TB seems slightly less wobbly at idle and the engine seems to have a much clearer, raspier and slightly louder note. It sounds a bit rough - but I like it!
I'm surprised that the warmed up engine idles at such low revs though - barely 700rpm. If it wasn't electronically controlled I would be tempted to adjust it. No wonder there are so many stalling posts.

 

Just curious why you want it to idle faster?

 

To stop it stalling, although after a little research I now realise that 700rpm is normal.
Remember, I'm a Brit, so I'm used to tuned up tiny European and Japanese engines that squeeze 180bhp out of less than 2 litres of cubic capacity. They tend to idle quite a bit faster.

I'm loving my lazy, primitive, untuned American 4.0 but it's a bit of a paradigm shift for me.
Bare with me - I'm learning.

 

I'm having the exact same issue with coolant overflow after parking. Have you pinpointed the issue yet?

 

For me, it was definitely the radiator cap.
I bought a new one, speculatively, just to give it a try as a cheap fix. It worked.
Now that I understand the physics of the problem I can see why it was right.

Get your engine up to normal working temperature. Give your upper radiator hose a squeeze.
If you can really give it a good squeeze then your system is not properly pressurised. Reduced pressure will lower the boiling point of the coolant so that as soon as you turn off the engine and cut off the cooling system (water pump and fan) the residual heat in the block will make the coolant boil.
The only place where the system can boil over is through the radiator overflow pipe, into the coolant bottle, then out of the bottle's overflow onto your drive.

If the upper radiator hose is firm at normal running temperature then your system is properly pressurised, your radiator cap is fine and I cannot help you.

Good luck.

 

Ok thanks

 

lots of folks don't realize that caps are an item that wears and needs replacing from time to time. Puking coolant is a good sign that the cap is shot