I have a mystery coolant leak that has me stumped. I've done a lot of investigating to rule out a lot of things, and don't know what to check next. What would you do?

Symptom: loss of about 1-2 inches of coolant in the reservoir each week. The XJ sees between 100-400 miles during that time. 119K miles, stock 2000 XJ, 4.0 with the original 0331 header (which seems to be fine-- see below).

Possible cause 1: cooling system problem. After a full cleaning of the hoses and radiator with brake cleaner, and a couple days of driving, the system is bone dry on the outside and doesn't appear to be leaking anywhere. The water pump is also ok and not leaking. A new radiator cap did not solve it. There is never any coolant under the XJ and it seems to be lost only when driving.

Possible cause 2: cracked 0331 head. I have put over a gallon of coolant in the reservoir over the last few weeks and the oil level in the pan did not rise. I changed the oil today and it looked and smelled just like normal oil without having coolant in it. I am sending a sample to Blackstone Labs in Fort Wayne IN just to rule coolant contamination out, but I expect it will be fine. Also, I see no bubbling on top of the head between cylinders 3&4 when hot and there is no water condensation inside the valve cover.

Possible cause 3: bad head gasket. The coolant doesn't seem to be leaking into the exhaust because there is no white smoke or steam out the tailpipe and there is no exhaust gas smell inside the radiator. The coolant inside the radiator is regular greenish brown and has no oil in it.

Possible cause 4: bad heater core. No leaks either inside or outside the XJ and the heater works fine. I think I can rule this one out.

Possible cause 5: coolant getting consumed? Now I don't know how to test this, except by the process of elimination. Maybe you guys have some ideas. Is it possible for coolant to go into the combustion chambers and get burned up? The 4.0 sounds really happy and I have no trouble starting, idling, accelerating, or pushing hard up hills. Gas mileage isn't stellar but still within the normal range for XJ's (~15 mpg combined city/highway).

Possible cause 6??? What else could it be and how could I test it? Any suggestions welcome, especially by mechanics! Thanks a lot for your ideas.

 

Wow you did a very good investigation. The one thing that I would suggest is a compression test, maybe the head gasket is leaking. You can borrow a compression tester from many auto parts stores.
BTW I am not a pro mechanic (but I play one on TV).

 

I had a leak in my heater control valve. Couldn't see it leaking on the engine...just noticed a drop in ther reservor and the valve was wet. Changed the valve and it was fine after that.

 

I couldn't find a leak I had so I put coolant dye in the coolant... I found my leak it was my water pump...but the problem was it was so little it dried up before I could find it...

 

i think mine is like that too. when you hav the reservoir filled to the cold line and you run the motor for a while and when you shut the motor off, isnt the fluid supposed to be back up to that cold line? i fill mine to the full mark and then the next weekend its back on the add line. my car is the same way,but i hav a pretty good idea where its leaking from.

 

Damn good investigation ol' chap

This really helps us answer your question. With your 2000 having the original head with its casting defect, I am leaning towards one of two reasons for your coolant loss:

1) The head gasket HAS possibly sprung a small leak somewhere, yet with only 1-2 inches of coolant consumption this wouldn't exactly be "HEY HERE I AM" noticable. But, seeing that I would place a bet that you take very good care of your XJ and haven't let it overheat, I don't see the HG really being the big issue.

2) You do have a small leak somewhere (water pump [check the weep-hole], maybe a hose somewhere, and also check your T-stat housing [I had a leak in my 91, and took forever to see just a little tiny bit of coolant around it's edge]) As mentioned above, it is probably very possible that such a small leak would dry up on the engine, and prove troublesome to find. If there is a dye availible, maybe go and give it a try.

Keep us posted with any more results. Also, I would do a compression test just to confirm the HG probably isn't your problem! Hope this helps!

 

Still possible that you have a small crack in that 0331 head that you couldn't see.

Sending in oil for analysis was a good idea. That will tell the story if the head is cracked.

With a cracked 0331 head, oftentimes the oil level stays fine as the coolant is immediately burned off.

A cylinder leakdown test is helpful here if you get stuck. Compression test never a bad idea, but it is not a good test for a cracked head. It is a good test for a blown head gasket however.

Keep us posted.

 

so what is the best test for a cracked head?


i was planning on getting a compression test this weekend to see if i had a cracked head...

 

Pat, with an 00-01, I usually perform a visual test first, as the original poster did. With a cracked 0331 head, the cracks can often (not always) be seen when the valve cover is removed and inspection is performed with a GOOD light, paying special attention to the area between cylinders 3-4, which is where the casting defect is.

Other good preliminary tests include the compression test to rule out a bad headgasket, but as I mentioned, a good compression test does NOT rule out the possibility of a cracked head.

A "block test", where you sniff the coolant for the presence of hydrocarbons is also a good preliminary test. It doesn't pinpoint your problem, but if it is positive, it tells you that coolant and oil are mixing and that's not a good thing. Any shop can perform a block test, or you can even buy a do-it-yourself block test from Napa for around $30.

My personal favorite test for diagnosing many internal problems on the engine is the cylinder leakdown test. A leakdown test can pinpoint the mechanical problems that may exist inside your engine to a much greater degree of accuracy than a compression test. The leakdown test can pinpoint if the problem is an intake valve or an exhaust valve. The leakdown test can pinpoint a blown head gasket or a cracked head. A leakdown test can pinpoint piston ring or cylinder wall problems. But you do need specialized equipment for this test; SEARCH if you are so inclined.

 

ty for the info


and sorry for the thread jack

 

For possible cause #5- Burning coolant in the cylinder...
Pull your spark plugs and inspect the white porcelain area around the electrode. It should be tan to brown in color normally. If a cylinder is burning coolant, the porcelain should be stained green for the presence of coolant. Hope this helps. G/L

 

Had a similar problem in a previous vehicle, turned out it was the recovery tank had a small crack at he seam it would expand just enough for s minute amount. It seems you are onto it, I thought Id pass this along in case everything else checks out.

 

Thanks to everyone who contributed their advice! I will double check my water pump, thermostat, plastic reservoir seam, and hoses, and keep a sharp eye on things while the lab runs my oil test. I will see about getting some dye-- Auto parts store for that? If the coolant begins leaking faster than it is now, that should make it easier to find. I'm going to hold off on the compression or leakdown test until I hear from the lab if there is coolant in the oil or not. That result will definitely influence what happens next. I will keep you posted, but it may be a couple weeks before I get my results back. In the meantime, if anyone can think of anything else I should check, feel free to post it! Cheers, and thanks again. -LFH

 

You might grab a UV liquid kit and a blacklight from autozone or o'reillys or whatever parts store your near. I had a coolant leak on an old s10 and didnt find it till i put uv fluid through it and realized that it was a pinhole in the hose and it only leaked under the pressure of the truck running.

Good Luck brother

 

Quick high jack does the kit have to be for coolant? The autozones around here only carry A/C dye and they all say they have never heard of radiator dye.