Hi guys, I'm still trying to track down a slow coolant leak. No luck using UV dye, nothing showing up.

I might be grasping at straws here but let me pick your brains.

Seems to me that since the coolant / water pump is directly connected to the serpentine belt and will rotate faster at higher rpms. Taking this logic forward, is it fair at assume that faster engine = faster pump = faster impeller = faster flow = higher coolant pressure?

Is it possible that a small crack or something might only leak a noticeable amount of coolant at the higher rpms? If plausible, have you ever seen this?

Just trying to think of every angle,this is tough to track down.

I guess before I get too excited about a positive response I have to balance that with the fact my pressure tester produced no visible leak source. Higher pressure is higher pressure whether it's from the cars water pump or the external pressure tester right?

Yes i see my logic is going in circles here :-), but I welcome any thoughts.

BTW, can I assume dye added 1 month ago will still be visible under a UV light?

Thanks!

John

 

If you see no evidence of a coolant leak, your issue may be that your coolant reservoir cap has failed. When that happens, coolant can move from the reservoir to the larger atmospheric recovery tank in the fender well but it cannot return from the recovery tank back to the reservoir. This causes the owner to believe there is a physical leak somewhere in the system when there actually is not....

My wife's 2006 XK8 is on its second reservoir and its third reservoir cap during our ownership period (from early February 2012)….

 

Thanks Jon, I may just replace it to see.

John

 

+1 to Jon89

During the extreme hot summer in Bahrain, I had the Low Coolant dash warning and had to fill the reservoir. Hunted for the leak and, nope, couldn’t find any. So, as Jon89 suggested, I changed the reservoir cap. Seemed to have fixed the problem.

 

John,

I had a peristent small coolant loss on my 2001 XK8 which was similarly impossible to source with a pressure test. It turned out to be a hairline crack in the base of the Expansion Tank. This would only open when the engine was up to temperature and allow drops of coolant to leak and fall onto the hot exhaust manifold where they instantly vapourised.

There have been two other reports of this (or minute holes in the base) that I've read on the forum and one that I recall of a seam failure in the Expansion Tank which was also only apparent at operating temperature.

If you do need to go with a new Expansion Tank, they come complete with level sensor and pressure cap.

Graham

 

pressure stays mostly constant over the usual RPM range as there are not many flow restrictions in the system. pressure rises with temperature up to the pressure cap limit.

heat opens up cracks and leaks. this is why some very new and well sorted cars have a slight anti-freeze smell in the winter. the hose connections imperceptibly sweat with the large temperature change.

on my XK8 i had one that was not visible until it was dripping on the ground. might be true for you if you have the original radiator. the plastic hose nipples become very brittle literally held together by the hose itself as some kind of circular arch. park the car where you can leave it and work and remove the upper radiator hose. chances are the nipple end will stay inside the hose as a cracked ring when you remove it. you will be able to crush it with your hand, yet it will not fail under operating pressure.

hope that this helps.

 

You might find the usual suspects are thermostat housing, sometimes the plastic cracks, The water pump where the leak is sprayed very finely, and norma plastic connections to heater hose where it drips onto the exhaust and evaporates. These are in addition to the expansion tank and sensor, and water caps already mentioned.