4.2 thermostat housing - how can I stop the leaking?

(Austin Gwin) #1

Apologies if this is discussed elsewhere, but I can’t seem to get the search to work correctly.

76 XJ6C with 4.2. Have only had it a short time, but it began leaking after being driven (previous drives = no leaks) and then began leaking any time motor was running. Could not pin down where, exactly, so replaced all hoses. In reattaching the thermostat housing (that complex aluminum crossover piece), it seemed impossible to keep the gasket, thermostat and housing lined up and despite my best efforts, it leaks upon filling (even before motor start up).

Does anyone have experience with solving this? Should I be using RTV on the gasket? If so, which side? Or both? Is it possible the thermostat slips out of place and the housing won’t seal as the lip of the thermostat is caught?

Or if there’s a vintage Jaguar shop in the Temecula, California, area anyone can recommend, as I am moving at the end of the month and the car needs to be running to transport.

Any advice appreciated.

(Paul M. Novak) #2

Austin ,
With a car 40+ years old there could be any number of problems created by age or the actions of prior owners that can have caused the thermostat housing to leak. Can you post pictures of your housing and thermostat. Perhaps they are incorrect parts for your car.

I have owned a few Series III XJ6s with the 4.2L engines and successfully removed and replaced that housing before without having any coolant leaks afterwards. I always use the correct gasket and smear a thin film of thermostat sealant on both sides of the gasket upon reassembly.

I live in the San Diego area and can recommend some shops down here. I don’t know of any shop in the Temecula area to recommend


(Jochen Glöckner) #3


something seems to be terribly wrong with your setup … Some months ago I had the cover off to replace the t-stat and just about everything on my car was very tight and clearly lined up and super easy to fix back with the correct seal. It looks pretty much like this

What does your t-stat housing look like?

Good luck


75 XJ6L 4.2 auto (UK spec)

(Austin Gwin) #4

Thanks for the responses. Understand that the car did not leak at first, but developed this after a few local drives.

All the parts shown on the factory schematic are present and assembled in the order shown. It’s leaking from the joint where you can see the blue RTV (note: won’t use NAPA RTV again. Far too thick to spread properly)

(Paul M. Novak) #5

If I were you I would disassemble it again, clean off both mating surfaces, install a new gasket with a thin smear of thermostat housing sealant and reassemble it again.

I have no idea why it leaked but maybe some work was done in that area before you bought the car and it took a bit of driving before something went wrong. I have had similar “infant mortality” problems with coolant leaks and squealing fan belts that I thought I did right the first time but needed some attention after driving it a few times. Sometimes that attention required retightening bolts or other fasteners.


(Austin Gwin) #6

That;s my plan. Just don’t want to do it all over again and have it leak - again.

I can’t find any torque setting for the three bolts on that housing. it’s aluminum, and I do not want to over torque, but I’ve got to stop this leak.

(Lee140FHC) #7

My guess is the alum housing has already been over torqued and is no longer flat. IF the surface in question can be surface-plate sanded, I would do that…can’t tell you how many housings I’ve seen that were warped by trying to seal a leak with excessive torque.
It could also be the manifold side which is warped, btw. Match 'em up together without gasket and break out your feeler gauges.

(Paul M. Novak) #8

Over torquing the bolts to an aluminum housing could damage it, so your concern is justified.

I just looked in my Series III XJ6 Service Manual and could not find a torque wrench setting for those thermostat housing bolts.

Someone on Jag-Lovers once wrote “snug is tight, tight is broken”.

I find myself repeating that quote when tightening things on our Jaguars to avoid breaking things. It usually works. :wink:

I think the thin smear of thermostat sealant helps prevent leaks and helps with the tightening. I know when it is tight enough when the sealant oozes out in the joint.

But remember “snug is tight, tight is broken”.


(Rob Reilly) #9

What size diameter and thread are the bolts?
My factory Service Manual for '72 XJ12 gives torques for lots of bolt sizes in the front pages, so maybe someone with an XJ6 manual could check there.

RTV is thick because it should be applied as a high bead, circling the bolt holes, not spread around thin, and needs to cure in air for a few minutes before you put the joint together. Then left to cure for 24 hours before you put the coolant in.

(Austin Gwin) #10

I believe they’re 1/2" head. Two short bolts and one very long one, underneath the housing.

(Ian) #11

I noticed the surface of mine was not very good , I have a big flat tile about 12x12 , just right for a sheet of wet and dry paper , 100 grit then 80 with loads of oil , soon got a nice flat surface , by running the faces over it in all directions making sure to keep it flat under pressure .
Ps , I use fuel line clips instead of small jubilee clips for a better all round seal ! s-l1600%5B2%5D

(Rob Reilly) #12

Alright, a 1/2" across-the-flats hex head and going into cast aluminum will be a 5/16-18 UNC thread. The XJ12 manual has one listed.
I would torque them 11-13 foot pounds or 15-17 Newton meters.

(Jochen Glöckner) #13

That’s fair Austin,

you know exactly where it leaks and you can see that the way the joint is mated is not correct. The rear mating between the intake manifold and the housing looks like it’s supposed to.

So just pull the three bolts from the front and the t-stat housing, scrape off that blue stuff - is doesn’t belong there and has proven it - and clean and check the surfaces.

I did that job some months ago and - once you can pull the bolts and don’t shear them off - it is not difficult. As the mating suface is quite big and the three bolts are evenly distributed over that ring, exact torque is hardly an issue as long as you do not damage the threads for the two long bolts in the t-stat housing. Just use the correct gasket, some light sealant and tighten down the three bolts evenly and in turns and with consideraton. This is going to be your easiest enemy …

Good luck


75 XJ6L 4.2 auto (UK spec)

(Carl Hutchins, Jr. ) #14

I’m with those that see distorted parts.

I’ve never worked on this example. but, it is fairly generic in design,

Clean, flat and level mating surfaces is the goal. Gasket plus sealer or even sealer alone will work. Caveat, my experience on other critters.

As stated, snug and a bit is all that is needed. “killing” it with big tools is absolutely counter productive. No stress here, Merely the need to keep the parts in place !!!

Factoid: More care needed in torque on alloy parts than on iron parts!!

Side bar: I am enjoying vastly increased vision in my right eye, My better one!! Cataract surgery went super well.
Many Kudos to the VA team at Martinez, Ca
that treated me so superbly.


(Jochen Glöckner) #15

You’re right Carl,

warping may be an issue, but with the shape of the housing it would seem odd to me. The thick blue sealant keeps the metal parts at least one millimeter apart and if the sealant ring isn’t closed a hole may appear big enough to flush your kitchen sink …

Congratulations to your eye surgery . Short sighted from early childhood and age-related near-sighted for the last couple of years I know what you’re talking about! A nice experience as well that sometimes not everything gets worse over time!

Good luck


75 XJ6L 4.2 auto (UK spec)

(Carl Hutchins, Jr. ) #16

Thanks, Jochen. My brother’s comment was similar. they just keep patching us up and we keep going.

Test: place the housing on a known flat surface. Slight wobble, draw file crosswise. More? toss and replace…

Decades ago, My “T” would only run on two and with luck, three. We used it anyway??? New head gasket only a sight help. Then as the head was on sidewalk, I reached for it! WOWEE. Much like a rocking chair!!! Got another from my “bone yard”. Same test . Huge difference. Put it on the T. Four all the time!!! .Never did resolve why it held water??? .


(Austin Gwin) #17

Thanks for the advice. It was ultimately the order of the diagram that was the problem. The thermostat would slip out of place unless the gasket was applied after the thermostat was placed, keeping the housing from a perfect seal.

This reassembly seems to be holding, at least long enough to get the car loaded for transport.

I’m happy it’s no longer leaking. I am unhappy that I never found the original source of the leak. None of the hoses I replaced were obviously split or leaking, though they were old and some bulged. I would have been much more confident in the repair had I found what hose started this whole mess.

P.S. yes, eye surgery has come a long way. I’ve had multiple on my right eye; it’s never pleasant but they have gotten much better since my first vitrectomy.

(Jochen Glöckner) #18


congratulations! As in many mysterious and unsolved cases of XJ diseases it turned out to be a simple cause, easy and inexpensive to rectify, no bad consequences and a lot we’ve all learned from the discussion - so no need to be unhappy! Plus, as you’ve replaced all those old and bulging hoses chances are your cooling system will now outlast the rest of the car.

BTW, what type of t-stat did you put in? The one I had inside originally opened too soon and the operating temps of the engine typically were way below optimal. Even the first one I ordered new was off-spec. Only the second one opened correctly.

Good luck


75 XJ6L 4.2 auto (UK spec)