There is a slow/small leak from the thermostat housing (see photo attached), it trickles out from just under the cap-looking thing and loses about 1/2-gallon over a month of driving. What do I do about it? (besides continuing to add coolant, lol.)

After driving and then shutting off the car you can hear a quiet gurgling from that spot.

 

Very common failure, perhaps 100%. Solution is to replace the plastic housing with an aluminum one which became available a few years ago . . . but not from Jaguar. The aluminum one is significantly less than a replacement Jaguar plastic one and usually includes a thermostat. Check the Sticky above for information on the thermostat housing and the methods of replacement. The rear two bolts are a real chore to get out.

The bright aluminum will look out of place on the otherwise black engine so I painted mine flat black.

 

The aluminum housings are available from several sources, some of which are poor quality. I haven't read of any problems using the Welsh Ent. units

Welsh Enterprises, Inc - Jaguar XK8 / XKR (97-06) Heating/Cooling Radiator, Heater Core & Header Tanks Parts - Aluminum Thermostat Housing - XK8 XJ8

Some of us have removed the old housing by breaking it off in pieces with vise grips to access the rear bolts, or you can use a crow foot wrench. I replaced the rear bolts with socket head screws and used a ball drive to tighten.
If you have a shop replace it, they might want to loosen the intake manifold, not a good idea as it can result in air leaks, unless the seals are replaced.

 

There are some great posts/threads around here telling you how to replace the housing, but if you decide to let someone else do it (like I did) just remember -- they do not have to remove the manifold to get at the rear bolts (that's what my mechanic did) and if they do, then you're in for a lot more money (labor and parts) and you might was well replace the heater hoses and some of the other stuff while they're at it! Good luck.

 

Snap the plastic housing off which will allow easier access to the 2 rear bolts (8mm) and when replacing exchange them for 2 from the actual thermostat housing itself. Same bolts except for their length. The difference in length makes it much easier to put them in place.
Before you snap the old housing off, remove the "cap" and shove an old piece of cloth down the housing tube. This will prevent bits of the old housing from dropping in and getting lodged in the cooling system. Don't forget to remove it before fitting the new housing!!

 

could be as simple as the o-ring under the cap. it comes off with a hex wrench. if not I just replaced my thermostat housing last weekend. it took me most of a day. I got the housing from Welsh and it is a nice part, very well designed. don't make the mistake I made, order the thermostat from them. There are a number of threads describing how to do the job, I used the method of breaking off the old housing and it worked perfectly. Then I replaced the 8mm bolts with hex cap screws just in case I have to take it off again!

 

Thanks all, great info. Going to try Fulton's idea of the o-ring first, if that is not the issue will buy a new unit w/ thermostat from Welsh.

 

Careful about using an Allen wrench to remove the cap. Many a plastic cap has been broken that way. Use a rubber strap wrench aided by the Allen wrench if you must.

 

Welsh Enterprises was about $50. WELL worth the money.

 

Welsh is now installed, all is good - thank you all for your help

FYI, my mechanic figured out a different technique to remove/install it with the manifold in place, he bent a wrench to fit into the space - took 2.5 hours one tiny bolt-rotation at a time, but it worked

 

I attempted to patch the crack by wrapping a piece of rubberized automotive hose tape and wire around that area at the top or the housing. At a later date I was planning on using hot glue and a heat gun for it is PA6 plastic… In the middle of July my A/C went from blowing cold to blowing 99 degree outside air. Idling at a stop light the temp gage climbed to the red zone just as the light turned green and got the water pump to get moving again. In short dont patch just replace and dont let it go. When it bursts coolant all over the serpentine belt it gets slippery and dosent turn the water pump and the A/C, at least thats what I am theorizing because I did not get a low coolant light. I put this project off all spring thinking I could get away with it for a while with the patch and or glue repair. I was lucky I didnt blow the head gasket. I bought a aluminum replacement by Euro Parts and the holes didnt quite line up so I had to file it out. Dont know if it was because I have the AJ26 engine or the Euro housing was made wrong.

 

What fun to see this very old thread resurrected: helped me through replacing my thermostat housing six years ago. Lots of these old subscribers are long gone.

Either you were sent the wrong part, or you forgot to check for the correct engine or you have missed something in the install (incorrect alignment of parts). Welsh fit perfectly on my 2001. Rear bolts were a bitch to tighten . . .

Your shade tree repair . . . Geez, you are lucky you didn't destroy the engine. You miss the class in physics on pressure?

Go to Welsh Auto Parts and compare the part you have from Euro and what their kit calls out on their webpage. Might be useful to burn a few bucks and start over.

 

The Aluminum replacement thermostat housings are so good and so cheap, it is mind boggling. I used a dremel to clean up the flash, but all the the holes lined up on my 96 build date '97 XK8.

 

I installed the thermostat housing over the weekend. Yesterday (Monday) when I drove it home from work, and later wrote the above reply, everything was fine. Ran the A/C for 20 miles at 4pm 98 degree heat. Today (Tuesday) got off the freeway and made it to the 18 mile mark driving thru town with the usual half a dozen stop lights. At the home stretch the A/C went back to blowing hot, came to the 2nd to last stop light on my journey and the temp climbed to the red within a minute. Got it pulled over and let cool for 30 minz. Turned the heater on high to dissipate heat and got hot air for the remainder of the drive with no overheat problems. Got it home and turned the A/C blowing intermittent cold at idle, temp at normal. Turned engine off. Started up again with A/C blowing cold for a brief minute or two and then started blowing hot again and almost immediately, just as before, the temp rose to red within a minute. Driver side radiator hose is hot to the touch, passenger side hose is even hotter so thermostat should be ok. I did see the fans run when the A/C is on but they might be turning off, ill need to inspect. Im not showing any evidence of head gasket failure. No water in exhaust pipes, no oil in water, or water in oil, no white smoke, no coolant level low light, no loss of coolant, 2 inches of coolant in the overflow tank in front of the drivers wheel. No gasses in coolant expansion tank, bubbles that is. I am guessing I have intermittent water pump failure, it should be the updated metal fin version when they did the 3rd generation tensioners, but what about the A/C’s behavior? Unless it needs charging and its just a coincidence or maybe I did blow the head gasket. I realize the thread is about the temp housing and not the water pump and A/C but any advise would be appreciated. Thanks.

 

Yeah, I remember getting this job done, the plastic shell and bolt placement wasn't well thought out, lol - I don't have the car anymore but I'm still in contact with its new owner and the replacement metal thermostat housing is still working fine as of our last correspondence.

 

dont do that, replace the whole thing. Yours is old and has that brittle appearance to it.

 

My earlier theory about coolant getting on the serpentine belt causing slippage to the water pump and A/C was probably wrong so ill . Im probably going to have to start a new thread if I cant figure this out.

 

With your symptoms (AC blowing hot at times and engine overheated), I would first check whether the cooling fans go to high speed. The high fan speed is triggered by the high pressure in the AC gas delivery line (this is when this line gets rather hot) or by the engine temp reaching about 97C. If the high speed of the fans is not activated (due to a bad fan motor or bad fan speed controlling triple relay), the AC system will overheat and stop cooling and the engine temp will clime.

First (starting from cold engine), see if both fans are running at low speed soon after you have switched the AC on.

 

@ M. Stojanovic
Quote: First (starting from cold engine), see if both fans are running at low speed soon after you have switched the AC on.

7/29/19 7:00 pm ambient temperature 75 degrees. 1998 Jaguar XJ8 VDP VIN 835577
Cold start. Turned A/C on and both fans start up with a 4 second delay on low speed (slower speed than before when the engine was hot). I unplugged the coolant temperature sensor and the engine idle stays the same and the fans stay on. I turned the A/C off and the fans stayed on.
Shut down engine fans turn off. Turn key on fans turn on. Start engine fans stay on with A/C still turned off. I unplugged the coolant temp sensor while the fans stayed on and the engine idle elevates from 600 RPM to 800 RPM. Temp guage did not rise above the 1/4 way mark.

Thank you,

Jon

P.S. I am leaning toward starting a new thread. My issue has to deal with coolant temp and A/C while this one is for the thermostat housing repair.

 

Yes, better start a new thread. As for the fans operation, the coolant temp sensor has no effect on the fans until the engine reaches 90C so there is no point disconnecting it while testing the fans responses to the AC while the engine is below 90C. The AC switches the fans on via its pressure switch (Low & High speed). With the AC on, the Low speed will soon come on. When you the switch the AC off, the fans will still continue running for few minutes while the refrigerant pressure slowly dissipates down. If you wait long enough after switching the AC off, you will see the fans going off.

You can see more info of the fans operation in the thread "Workshop manual version".