After months of a mystery of why I have a slow coolant loos on my 2006 XKR Victory I figured it out this weekend.

After changing a number pf parts and hoses I still had a coolant loss issue. I had to add a cup or so of coolant every week of use. I could find no leaks but the loss started getting worse. I charged the water pump when I thought I saw a drip on it but the problem persisted.

Finally I spent a day doing pressure tests. The system was blowing the coolant from the expansion tank to into the atmospheric recovery tank. The recovery tank is hidden in the rear of the right fender. You can see the tank with the right door open and looking forward into the fender. The test results scared me because water was flowing out from under the right door sill. I thought I blew up the heater core. Instead it was just spraying from the recovery tank against the front edge of the right door.

Here is the tricky part: The expansion tank was venting pressure down the vent line so I changed the cap. It still vented with a new cap. I got a third cap and it still vented pressure. The system would not hold any pressure! I was apply pressure with a air line and regulator to the engine tip vent line and plugged of the nipple on the expansion tank.

After a couple of hours I was saying the new looking expansion tank cant be right!. I held my finger over the opening into the cap area for the vent and applied pressure by blowing in a tub on the vent nipple. Air was leaking in the expansion tank from the internal passage between the vent nipple and the cap area port. (When the cap releases pressure this port is what vents it to the recovery tank).

It was Sunday afternoon so I tried a home fix. I took the tank and reamed the vent nipple hole to 1/4" Then I took about 2 inches of 1/4" copper tubing and JB welded it into the passage I just drilled out. This sleeve stopped the leak!

Putting the tank back in the system now held pressure with no leak down. What apparently was happening was the cooling system was not pressurizing and the coolant was boiling out thru the recovery tank and being lost while driving. This occurred with no drips under the parked car.

I hope this helps someone withe the same problem

 

I too have a slow coolant leak in my recently purchased '99 XK8 that I can not find. Original plastic thermostat housing has been upgraded to the new aluminum replacement, refilled with Zerex G-05 and has no evidence of leaking. I also replaced my cap with a quality 'Stant' brand with the correct 15lb rating. I pressure tested the system and discovered a very slight leak at a hose but that was easily cured by simply tightening a clamp. My system will now hold 15 pounds of pressure for over an hour! With that, confidence was extremely high! However, I am STILL loosing coolant someplace and damnit, my low coolant light will come on after 30-35miles at temp at speed yet I can see NO coolant leak anywhere on the engine and have NOTHING dripping once home in the garage.

Is the issue with the fender well mounted atmospheric recovery bottle or the firewall mounted expansion tank? Where and/or why is my expansion tank blowing out to my recovery bottle? Pressure test is of the closed system without the cap fitted. Assume both the old and new cap are holding 15lbs. However, pressure testing does not test the interaction I guess! Is it someplace between or in the function of the expansion tank/pressure cap/recovery bottle? How did your repair/mod to your recovery tank discharge tube port fix your slow loss? As much as I appreciate a workaround repair, would simply replacing the expansion tank fix it as well.

 

Any pics? I'm fighting the same slow coolant loss issue and just replaced the cap with a Stant brand. I think I understand your 'fix', but need to be sure before attempting something similar.

 

I replaced the slightly-leaking factory coolant expansion reservoir in my wife's 2006 XK8 in September 2017 with a new $57 one from rockauto. I threw the factory reservoir away but kept its cap because it was a relatively new official Jaguar one (September 2016). I sealed that cap in a plastic bag and stashed it on my garage shelf thinking that at some point I would probably need it again. I check the fluids and tire pressures in all of our vehicles every weekend and a couple of months ago I noticed that the XK8 reservoir level had dropped about a quarter-inch below where I typically keep it. I decided not to top it up and wait until the next weekend's check before taking any action. The next weekend the reservoir level had dropped yet another quarter-inch or so. There were never any visible coolant leaks in the engine bay or underneath the vehicle, and there was absolutely no smell of coolant anywhere around the vehicle. So I assumed that the rockauto reservoir cap was in the process of failing and was no longer allowing coolant to be returned from the recovery tank in the fender well back into the reservoir tank at the top of the engine bay. As a result, I threw away the cap that came with the rockauto reservoir and replaced it with the official Jaguar cap I had purchased in September 2016 and saved on my garage shelf. I then topped up the reservoir to my typical fill level (just a fraction below the bottom of the reservoir fill hole). Presto - no more coolant draw-down. The reservoir coolant level has remained exactly where I typically keep it ever since I threw away the rockauto cap and replaced it with the September 2016 official Jaguar cap....

These caps are critical to the proper functioning of our entire cooling systems. Probably a wise idea to always keep a new or almost-new spare cap on hand....

 

I believe the expansion tank cap is the key to a functioning recovery tank. It looks like my coolant loss is from HOT coolant flowing FROM the expansion tank TO the recovery tank as designed but NOT being drawn back into the expansion tank when cooling. In short, I'm discharging coolant to the recovery tank little by little with each heat cycle where after a few heat cycles, I get the stupid 'Low Coolant' light again and I need to top off the expansion tank again. In fact, when inspecting my inner fender mounted recovery tank, it was overflowing. Ha! So THAT's where all my coolant was going!

I've got a breach someplace in the return COOLING cycle. Fill is easy being discharged under hot, expansion pressure but the coolant return is drawn from the recovery tank by a low vacuum created during the slow cooling/contraction process. I must have an air leak in the line that is bleeding off enough vacuum to where it will then not draw the heavier coolant up and out of the recovery tank. I see it as a trying to gently suck on a drinking straw with a small air hole in it... or hitting the pipe with your finger off the hole.

The 15lb Stant (#10263) is an OEM quality cap that has two O-rings. The smaller, lower O-ring seals the engine side of the expansion tank. I checked my engine's cooling system's integrity by pressure testing and it can hold exactly 15lbs for over one hour! The larger, top O-ring defines the blow-off/recovery section. A pressure tester will only test the cap's release pressure here at it's rated 15lbs. Accordingly, simple static pressure testing alone will NOT prove the integrity of the dynamic vacuum recovery side of the system.

Evidently the now 20yo original plastic expansion tanks are prone to cracking and failure and now simply a routine replacement item. As the original poster discribed, his had a leak at this critical plastic barbed port at the cap that he was able to repair by inserting a piece of copper tubing at no cost. The alternative is a $45-$55 parts house replacement that typically comes with a new cap and at least a one year warranty. (As a bonus, they also do come with a new $23 'low coolant' sensor too) I have one on my eBay watch list now but today I plan to inspect every inch of line, connector hose and every damn clamp before hitting 'Buy it Now'.

 

I have a similar issue with coolant disappearing, and no apparent leak or smell. I have replaced the coolant tank - the one on top of the engine, and the cap, and I suspect coolant is not being returned from the recovery tank. Mr Bentwrench or Burl, do you have a picture of the fix, or a link to the parts replacement ? I hate to sound lazy, but I'm having a little trouble with the specifics of the possible fix. TIA.

 

The recovery tank port is located on the very top of the expansion tank and faces rear towards the firewall. There is a small hole on the inside rear of the neck of the expansion tank that exits straight through a barbed plastic 3/8OD port. I read Burl's original post that when he covered the inside hole with his finger and blew into the expansion tank the from the port side with a hose, he had an air/pressure leak. Yeah, an internal or external crack would kill the needed vacuum. Furthermore, I read his repair as simply reaming out the port and sleeving it with a short section of copper tube and JB Weld. However, my similar test of the port integrity proved it air tight and sound. Crap! I was honestly hoping for a failure and was anxious to just buy a new tank for the fix.

My test of the line between the expansion tank and recovery tank was two steps. First, I blew into the line from the expansion tank end and blew bubbles in the recovery tank. Check. Second, I sucked on the line and drew a taste of coolant! Yuck... but check. Now, I'm reexamining my cap.

I replaced my original cap as a matter of course with my recent metal thermostat housing upgrade. Although I quite specifically specified a 'STANT #10263' from the parts house, I now notice that I received the cross-referenced MotoRad brand. Although probably perfectly functional, I'm quickly running out of options and they will have a Stant replacement in on Wednesday. Jeez! After this, I got nothing!

Just for some Saturday morning trivia, Stant has been around since like 1893 and are still based just outside of Indianapolis in Connersville, IN. They also now offer an advanced design 'Superstat' thermostat that is REALLY facinating!

 

I have also been swapping out all my old 'worm screw' drive hose claps for the fuel injection 'band' clamps on ALL my cars. They don't cut into the rubber hose and seem to clamp more securely. I guess so if used for high pressure fuel injection! You can get an assortment of 30 in different sizes for $8.88 on eBay.