Morning Team,
While deciding to aquire my SIII XJ6, I came upon this article entitled "Fighting the flaw in the 4.2 engine block" Article below.

Fighting the Flaw in the 4.2 Engine Block | Jaguar Car Club of Tasmania

The author recommend modifying the thermostat to reduce what he describes as the intense "thermal shock" when the thermostat opens each time the engine is asked to warm up.

It seems to make some sense provided the bleed holes are not too large.

Anyone have any comments on this mod. I'm planning to replace my thermostat and was wondering if I should do this mod, before I have the dreaded Head Gasket Failure issue.
Ian

 

Thermostats dont snap open but I have always drilled four or five 9/32's holes in thermostats of all my vehicles. These few holes do tend to increase warmup time a bit and allow the thermostat to be even more gradual. I put this one in the category of 'it couldn't hurt' so I always do it.

 

As your car is 1985, it will have the "slotted block", which removed the design defect in the previous block that caused the cracking. So you don't have to worry about cracking, but even so, head gasket failure can still occur, generally after about 85k miles.

 

I've seen this other places and agree it shows a fundamental misunderstanding of how thermostats work. They very gradually open and are fully open at their advertised temperature. So if you suddenly got on the highway with a slightly warmed engine and greatly increased cooling, the stat would close up. It is actually a very brilliant thing in how well it regulates temperature, there is no thermal shock.

 

Hi,
I agree and if true every engine with a thermostat would be similarly affected. I'll still drill an extra hole though.

 

Hi all !
Zombie thread back from the dead alert !
I was about to make a new post asking almost the same question after reading the article linked above. I'm going to service the cooling system on my "new" 1985 XJ6 Sovereign with 37,000 that was parked for around 15years - so DIY flush system in car, new hoses, belts, plastic fan ( has age cracks ) and new thermo. I bought the new thermostat from SNG which has the back flow tag. Should I remove that tag and drill another hole as recommended to help reduce the chances for block problems ? I've not really driven the car much since I just finished cleaning out the entire fuel system and replacing all the 1984 hoses ( have insurance but no tag yet too ) but I don't think it has any over heating issues ( fingers crossed ).
Opinions welcome !

Cheers,
Brian

 

As I said before, your engine has the 'slotted' block which doesn't crack between the bores, so thermostat is not an issue. Just bear in mind that the 4.2 was really a step too far with the XK block dimensions; the bores are just too damned big for the block !. With the slotted block (a spatchcock late in the engines production life), there is very little metal for the headgasket to sit on around the bore, so headgasket life is not all that long. You'd be looking at HG failure around 80-90k miles. Of course

 

Yeah, every time I see a picture of an XK 4.2 block I see what Fraser sees. Not a new issue. Exists elsewhere. Bored engines to get more CI's.

The stove top process for t'stats tells a lot as to hw they work. gradual, not slam/bang.

I'd be telling tales as to reveailng the source. Severe engine over heat. fortunately, it is fine,. Super caution. flushed new belts and hoses. New coolant, of course. and a new t'stat.

SOG. Too cold, engine will not reach operating temperature , Triggers side issues. Why? bum new stat?

Naaah, solved. In back wards. Why ??? Reversed. all is good...

Carl

 

first time I hear of this issue.

Both my 4.2 engines are 1983 and 1984.
are these years affected?

 

I think both your engines will have the slotted block, but you'll only know if you lift the head as it is unknown what engine number starts the slotted block mod. Jaguar should have really replaced the 4.2 XK engine in the 70s, but went off and did the V12 instead, A fat lot of good it did them, as it used far too much fuel and sales were never that high, so the XK had to carry on. The aluminium engines didn't come along until the mid 80s. They were pretty good and got better, not worse, as time went on.

 

that is relaxing news Fraser!

though I've never had overheating or head gasket failure in my 1984. I have replaced the thermostat ONCE since 1989 ! Knock on wood ! The good news? I did not break the studs when I replaced the thermostat years ago.

 

Jose, what is the trick to not breaking these studs. Why do they break?

 

Hi Iramphal,
when I was going to replace the thermostat, I asked J.D. of J.D. British Cars in Nashville. He sold me the correct thermostat. He owns a 1985 XJ-6.

he warned me to first soak the nuts for a few days with PB Blaster. (smells horrible).
Then, on D-Day, to slightly tap the nuts with the wrench or ratchet with socket in the opposite direction, i.e., as if to tighten the nuts. Very gently tap them tighter. Just a little tapping, not literally tightening them.

Then to gently tap them in reverse to loosen the nuts. then to repeat the tapping by gently tightening the nuts again, and again in reverse until the nut is freed.

the problem is the steel nut and stud reacting with the aluminum thermostat housing, there is a name for it but I can't remember, electro-something.

J.D. is an excellent mechanic and instructor.
He is or was president of the Music City (Nashville Tennessee) Jaguar Club.

 

Hi Jose, Wow, your up early! Thanks for the explanation. Will follow your directions. I'm replacing my 85's original radiator with an alum. one as well as all hoses so I thought I'd change the Thermostat.

 

Iramphal,
yeahh I'm up at 4:30 am every day. Nasty habit I know!

why spend on the aluminum radiator? the factory spec radiator is good enough as it is.

 

cuz they are really cheap!!!

Carl

 

I've never understood why they used studs at the thermostat housing.

Why not use three bolts?

is there a reason for using studs instead of bolts like in other cars?

 

Jose, I think the word you were looking for is electrolysis. But in actuality, it’s called galvanic corrosion that’s the two dissimilar metals joining to form corrosion.

 

British practice was to always use studs where a part would be subject to removal. Removal means taking off steel nuts on steel studs, so wear was not a problem. Removing/replacing bolts into aluminium housings means the tapped threads can wear quickly. Studs are also used on the inlet and exhaust manifolds and the cam covers which are also secured into aluminium, (the cylinder head).

 

Addicted2boost and Fraser,

Thank you.