I have, what was until today a minor coolant leak near the front of my 1999 xk8. Sometimes there are a few drops or even a silver dollar sized leak of coolant under the car, at the front, near centre... On my way home two days ago, the coolant warning came on. I got the car home fine, when the engine cooled I checked the reserve. It was 3/4 empty. I left the hood up and the cap off to remind me to fill the reservoir before driving. I went down to the garage today and there was about 1 litre of coolant under the car( that much water to fill the reservoir to overflowing). I could see drips coming off the front of the pan, with a light I checked the water pump , it looked like a dry trail of old coolant but nothing that left it wet. With the "rad" cap on there is no noticeable leak, with the cap off it starts to drain/drip slowly. Anyone heard of this? there are drips off the front of the pan, rad is dry and seems fine. I still suspect a water pump but in all my years Ive not seen a rad cap stop a coolant leak, a low pressure effect of some sort I guess. Anyone experience anything like this?

 

Do a coolant pressure test. You can rent a kit at autozone or any of the of other big parts chain stores.

 

I had a very minor coolant leak from one of the O-Rings on the radiator. Could inspect the O-Rings because mine was cracked.

 

Mine seems to have negative pressure after cooling as the upper hose sucks in. Could explain why no leaks with system closed.

 

How old is the reserve tank cap? I had to replace my tank due to age and leakage last year but foolishly thought the old cap was still serviceable. It is not, and the reserve tank would allow coolant to blow out. A new cap fixed it instantly!

 

Ironically the only tool not in my box, I was hoping someone here might have some sage insider information Ive ordered a water pump, but will buy a pressure tester, not a bad tool to have around with my cars, and luck with cooling systems as of late.

 

Well I changed the water pump today, as I did finally catch it leaking. A thankfully easy job...when I removed it I found signs of past leaks, and did a clean up and minor repairs where there were old tracers. I also found a leak , following a trail from the top of the water pump upward to a long very brittle PVC/plastic line from the expansion pipe to the top of a sort of coolant transfer station for lack of knowing the name of the part with about 200 outlets/inlets lol. Now just a minor oil leak to deal with. I think cam button.