A leak that I cannot seem to trace...

For quite some time now Ive had a problem with a very slow coolant leak. Roughly 1 cup of fluid a month out of the overflow bottle. Its not going in the oil, its not leaking on the pavement, and the pressure is fine (it sat connected to a pressure system for almost 6hrs one day and no leak) Where o where is my coolant going. The only thing I can think of is possibly the heater core, it does occasionally smell pretty rank with the heat/def at full blast (though tthe smell doesnt happen too often) From what Ive heard if I have a heater core leak my windshield should be fogging and the floor should be wet underneat the passenger side, neither of which I have so that leaves me clueless. If anyone has some ideas or something else that I am missing the input is greatly appreciated.
TIA
Dave

A pin hole head gasket leak into a combustion chamber, but not big enough to cause over

heating in the coolant system, and good enough piston rings and well seated exh and int valves,
with both valves closed and would not allow your pressure test to leak off. The ONLY thing I can

think of. If this is the case, you will know when it makes a hole in a piston. That would put
coolant in the oil and oil in every hole an orfice it can find. Just an educated guess.

I had rather hope it's something else. It would probably be difficult to find even if you

pulled the head. I would just keep an eye out until it gets worse keeping the head gasket in mind.

What year XJ we talking about here?

Replace the radiator cap. Make sure it is a 16lb rating. This is a common problem. As the engine heats up, it fills the bottle and pushes out hot humid air. In effect, you are getting evaporation because the coolant is going into and out of the bottle too much. The radiator cap should be replaced every couple of years anyway and it is a cheap item.

Its a 2000 with 125k as of today. The rad cap was replaced a year ago (this was to replace the stock one) as I had the same problem then. Replacing it did seem to help for a while though would a cap go bad in just one year?

RoadTrippin said:

Its a 2000 with 125k as of today. The rad cap was replaced a year ago (this was to replace the stock one) as I had the same problem then. Replacing it did seem to help for a while though would a cap go bad in just one year?

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Yes it could, I went through 2 of them in less than a year. The first "new" one fell apart. They are cheaply made.
As they are cheap to replace, you would have at least eliminated that if your problem still persist.

I was going to suggest evaporation too, get hot coolant being pushed into the overflow and some of the water is going to evaporate out of the mix. Don't know if pure coolant will evaporate but it probably does too. The cap is one of those items I get at the dealer/oem, replace every two years, sometimes every year, I also flush my system yearly and use only distillted water for my mix.

I guess Ill make a run up to the dealer this morning and pick up a cap, hopefully that will cure the problem, though Ill keep my gallon of coolant and distilled water handy just in case.

Most likely its the cap as others have said but there is a freeze plug at the rear of the block that can leak but since it is covered by the dust cover that is sandwiched between the tranny and block it may evaporate before it gets to the ground. The pressure test should have found this though.

My 2001 had the same problem, and low oil pressure. Turned out to be a tiny crack in the 0331 head right under the oil filter cap. Take the filler cap off and look down at the top of the head casting with a flash light and see if you see anything out of place. If you search posts under my handle you'll see where I asked almost the same question last winter.

Best of luck!!

my wifes TJ lost just enough coolant to notice it once in a while, but not enough to trace it. until the smell started getting strong inside the cab. it was a leaking heater core, and the coolant managed to stay inside the heating ducts just enough that you couldn't see it unless you looked directly at them.

Fish'nCarz: Turned out to be a tiny crack in the 0331 head right under the oil filter cap

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Luckily I already had that problem and dealt with it and a whole boatload of issues a few years back so I dont think it would be that.

I will def take a look in the ducts just in case.