With the low coolant light shining bright every 100 miles, I am seeking the leak. Pictured is the base of the coolant reservoir showing obvious signs. So I intend to replace all hose clamps with stainless worm type bands. Hoses seem to be in good shape, but should I trash them anyway? Second picture is the coolant reservoir out of the car, showing some coloration along the seam that joins top to bottom. Is this a concern? Do these tanks typically fail after 50k? If I pump 20 psi of air into this tank, is that a valid test for a leaking reservoir? How long should it hold the pressure? Should I try the plastic welding kit that I never used? I hope to gain some insights from anybody who has been there, done that. Thanks so much for your ideas.

Main hose at bottom of coolant reservoir shows obvious signs of leakage.

Tank removed from Jag shows some kind of residue around the seam. Not sure if this is a leak.

 

First thats the expansion tank ...there is also an overflow tank behind the RH plastic wing cover to the rear which must be half full or the system does not work and it looks like you are loosing fluid. You need the wheel off to access it.
Get a new one or pressure test it in a bucket of water and check for bubbles.When you replace it you must fill the expansion tank right to the top and put in the cap. fluid will then go back and forth to the overflow tank as it heats and cools. The caps do go faulty .They have two valves one high pressure to let the fluid out and a low pressure one for the return from the overflow tank.

 

Whether OEM or aftermarket, these substandard plastic reservoirs are well-known for developing leaks at their nozzles as well as at their seams. I installed a new one (from Rockauto) on my wife's 2006 XK8 in September 2017, and I have another new one (also from Rockauto) on my garage shelf awaiting the failure of the current one. They sit there and bake in the engine bay and sooner or later, they will leak....

 

Also agree with Jon and Piston. I know I've fought the same issues several times. A failed cap can easily let enough coolant to blow by in 100 miles to trip the light and never show a leak. Disappointingly the aftermarket replacements are all over the place in quality. The tank itself also looks to be leaking. I would be concerned that adding more heat or chemicals ( by plastic welding) would probably cause a complete failure, probably in a very inconvenient way.

Our engines just don't tolerate overheating. They will warp heads and score cylinders in a heartbeat if run out of coolant. Also be aware that mileage isn't a great indicator of wear on this cooling system. The underhood temps are brutal on the XK (given the way the car is packaged) and some of these pieces maybe almost 20 years old now.

Some things to check besides the Expansion Tank:

• Piston mentioned the overflow tank in the passenger fender well. You can try and see it through the door gap in the body but I've never had luck with this. Otherwise pull the wheel and the fender liner off and you'll see it tucked up against the passenger door hinges. It has to be half full. If its leaking you won't see the coolant as it runs away under the body. The hoses to it also wear as they run through the body.
• The hoses that run under the intake manifold get super hot and can easily leak onto the top of the engine. The coolant pools there and never reaches the ground.
• The auxiliary pump for the heater system is buried under the firewall and on top of the transmission. There are several fiddly connectors down there and they can leak as well.
• The water pump can fail and weep coolant
• I believe that the 04's also have the "octopus" which is what feeds the return and heater core with coolant. It's also buried behind the engine below the throttle body. These will eventually fail and have several places to leak.

Hope some of this helps.

 

30 dollars problem solved

originals fail usually with the float getting stuck aftermarkets are just aftermarket lol

 

Wow, thanks so much for these very helpful replies. I thought the reservoir was NLA but now I see this part is quite common both new old stock and aftermarket. I ordered one today for $30 and will let you know how that works out. While I wait for delivery I may as well inspect the overflow tank and try to inspect as many hoses as I can find. I am a bit saddened to see a URO brand aftermarket reservoir. I avoid them whenever I can as several URO parts have failed on older Mercedes, both mine and many other reports online.
Happy Easter, Passover or Ramadan.

 

With the focus on that tank, I would add the following:

• Folks have reported that the built-in (magnetic?) float is sometimes faulty right out of the box (it stays sunken in). You might want to test it before doing a more "permanent" install.
• The cap is critical to controlling the coolant flow to and from the overflow tank. Look into getting a spare or 2 for good measure.

Best of luck, keep us posted.

 

The tanks are so cheap that I bought one only for the included cap. Sourced from RockAuto.

Two years, 30,000 miles later I’m still using that cap and the spare tank is still in the box.

Z