Hello, I own a 2002 Jeep Grand Cherokee with the 4.0 straight 6. It runs well and everything, the only issue is that every 3 weeks to a month, the coolant low light will come on and the reservoir will be low. The Jeep does not overheat or anything. Also I do not see any coolant on the floor, nor, smoke out the tailpipe. I did change the sensor for the coolant level and it came without that gasket. I reused the old one. Idk if it was important. I could not find that part anyway. I've also heard of the radiator cap letting the coolant overflow when it loses pressure. Any ideas in the right direction would help.

 

I have two questions.
What is the build date of your '02?
What does the oil look like?

 

I would have the coolant tested for exhaust gasses, and the oil tested for coolant. Perform a radiator pressure test, and an engine compression test on all the cylinders. Then, go from there.

 

Mystery water loss on the 4.0 - prime suspect has to be the water pump bearing.

 

I know its a late 02 due to the paint code being PSB. The oil looks normal, nothing abnormal about it.

 

Ill have to check it out, thanks

 

Not a bad idea considering it has 258k miles on it. Thank you

 

Good things.
Late '02 means it is probably a TUPY 0331 head.
Probably not getting in the oil and trashing the engine.

 

So most likely something else like the water pump or cap you think?

 

At the moment lets go with more likely but you need to do what Noah911 said earlier.
I would start with the cooling system pressure test.

 

Look at the sticker on the side of the driver's door. It should give month and year of manufacture. Shine a strong light down the oil filer cap hole and see if it says "TUPY" on the head casting.

 

Hello, sorry for the long time of replying. I checked inside the oil cap and I do have the TUPY head.

 

Then chances are you don't have a cracked head, although anything is possible. There is a small chamber under the weep hole on the water pump that's designed to hold a small amount of leakage and allow it to evaporate without showing signs of leaking. When it gets to be too much, it leaks into the back of the water pump pulley and is thrown off on startup. This tiny bit can amount to quite a bit over time. I've owned my WJ for 8 years now and it always needs topping off. Occasionally, when I walk by the front of it while it's running and heat soaked, I can smell coolant, but can never find any leakage.

 

Yup dave, thats exactly how mine is. Top it off once in a while and I could smell the coolant. I suppose its not too bad then.