26 Aug 1996, John Spence:
All Jag trained mechanics here in Phoenix recommend cutting
out the central moving portion of the thermostat and blocking the bypass
hose with a wooden plug on XK engines. There is a high temp Jag testing
facility here. (Our summer temps are 110 - 120F, in winter over 90F in
dec. Frost very rare). My car doesn't overheat (i.e. boil), but the gauge
goes so far up into the oil gage after turning off the engine that I worry
(I had to repair it once from this - ether leak). This suggests fitting
an electric fan to reduce heat soak, but this obstructs air. It seems to
me that a fan in front or behind the radiator should beequivalent, but
an air scoop to reduce pressure behind the radiator should help - avoid
deflecting air into the space between radiator and engine, increasing pressure.
With an external temp of 100F, my car (XK140 SE FH) runs at 95 -100 C in
overdrive at 70mph on level ground. Is this reasonable ? Other suggestions
I've had are an aluminium radiator core, or to fit the more efficient water
pump from later cars. I'm interested in your comment about core plugs.
After having my engine removed from the car and rebuilt recently, the first
thing that happened was a burst core plug. I assumed this was because they
had not filled the heater before the block with water, but now that you
mention it I found that they had also fitted the wrong thermostat. They
fitted a simple one which did not close the bypass line -it had no slotted
collar on the side. (I've since replaced it with the correct one). Could
this have caused the failed welch plug ?
13 Aug 1996, from Rob Reilly:
I guess I got lucky with mine, because when I noticed
a core plug in the block starting to weep, I pulled off the intake and
exhaust manifolds and radiator and removed the core plugs and flushed with
the garden hose and the car runs reasonably cool now. Sounds like you'll
have to pull off the head. When you take off the cam covers stuff a rag
down around the timing chain to catch any stray bolts or tools. Note there
are 8 core plugs in the block, 3 big ones on each side, 1 at the back and
a little one down by the starter motor. I believe early blocks also had
a little one on the left hand side. Remove the gearbox tunnel to get at
the rear one, and even then it's a pain to reach. Better do the ones in
the intake manifold as well.
Aug 96, from ? Dick White ? - (no, wasn't me)
Hearing your story about your engine becoming a coolant
pump after 30 miles reminded me of my XK150. When I bought it several years
ago, it had not been driven for the preceding 13 years. After I cleaned
the carburetors and repaired the gas tank, I actually was able to get it
running. My problem was that it kept overheating. I finally removed the
head and found a blown head gasket. However, I also had a chance to look
down and around the water jackets. The corrosion was not too bad, but I
found a lot of gunk that must have been the residue of coolant that eventually
evaporated. That was as much of my problem as the blown head gasket. I
used boiling water to flush the water jackets. I did this by simply boiling
water in the tea kettle and then running out to the garage with it and
pouring it in the openings next to the cylinders. I eventually got it cleaned
out. Since then I have removed the engine and rebuilt it. When it was apart,
I had the block boiled just to make sure there was no residue of gunk in
23 Sep 1996, from Tom Veale,
Hi John, In looking over your latest letter, I had a few
thoughts about some of the over-heating problems at which were hinted.
In our race cars we try every possible way to remove heat and most of the
methods wouldn't be acceptable for those who show their car. Here's some
thoughts that might help:
1) On our race cars we make up 1/2" blocks for the
bonnet hinges. This raises the back end of the bonnet enough to let a significant
amount of heat out of the engine compartment. Since it's not a permanent
modification, one could use the car this way and then for "show"
take the blocks out. This is about as effective in most cases as louvering
2) Most new cars have "burp tanks" (a non pressurized
reservoir) installed on their cooling systems. Old Jaguars like them too!
Because the cooling system on the XK engine is large in volume, the tank
capacity needs to be large as well. A half gallon would be minimum with
a gallon being better. The reason burp tanks work so well is twofold: 1)
You don't get air back into the cooling system (air doesn't cool anything)
and 2) the oxygen from the air doesn't make hard water drop out as "boiler
scale". The water jacket of the engine must be free of scale to transfer
heat well. You will need a new radiator cap that allows the drain tube
to draw water back from the burp tank. Then you'll need a canister of adequate
volume with either a long tube that goes to the bottom of the canister
or a bottom take-off on the canister itself. After filling the system with
an appropriate antifreeze/anticorrosion material and water you should only
need to top up the burp tank. When the engine is fairly hot the burp tank
will be nearly full but as the engine cools, it sucks back the fluid keeping
air out of the system. If it empties the reservoir when the engine is cold,
then it's not big enough! On our race cars we've usually mounted the reservoir
off in the left wing. A look under the bonnet shows nothing unusual other
than the changed radiator cap and the longer overflow hose.
3) For cars that are driven and not shown, I'd suggest
adding an engine oil cooler. This takes the heat away in a much more direct
way than usual. Instead of heat having to pass from the oil to the water
to the radiator to the air, it goes straight from the oil to the air. This
takes quite a bit of heat load from the radiator.
4) If you've already dispensed with the old mechanical
fan on your Jag. then there is usually room for a deeper radiator core.
We have had deep cores made for our race cars and then add an electrical
fan in "blow through" mode on the front of the radiator core.
Again this might not be acceptable for show cars, but really makes for
happy old Cats that are driven and not shown.
24 Sep 96, from Tom Veale:
You are exactly right on the function of the burp tank.
It's a "draw back" reservoir. As the liquid expands with heating
it must go somewhere or hydraulic action will break something or pop freeze
plugs. When the radiator cap bleeds off this expanded liquid in older cars,
it just gets wasted. With a burp tank it gets re-used over and over. My
co-racer David Hinton at TwinCam Sportscars says that the XK150 may have
an unusually deep filler neck and fitting a new style cap may be difficult.
With a normal cap, the pressure relief valve is that part at the lower
end of the cap. It has a spring that will lift when the internal radiator
pressure reaches the rated amount. This then lets the liquid or steam out.
There's also a vacuum break valve there as well. It opened to keep the
radiator from going to negative pressure on cooldown and opens to draw
back air. At the top of the radiator cap is a flange seal as well. On the
older style caps this is just there to keep water or steam from coming
out, but won't seal tight enough to get water to draw back thru the relief
tube. The new style caps have a rubber gasket at the upper flange as well.
You may be able to create a "new cap" out of your old one by
taking some inner tube matl. and cutting a gasket to fit inside the cap.
I've done this successfully on a A-H Sprite. It would be better to get
a new cap with the proper depth seal for the neck of the 150, however.
I'll ask around to see if there are any available. The pressure rating
can be the same as stock. Terry's Jaguar Parts has an oil collar adaptor
kit for XK 150 cars. It uses some parts modified from the XJ6 series cars
I think. It mounts to where the normal oil filter goes. As a bonus, you
use Fram PH8A spin on oil filters in place of the old cartridge types.
That big Fram is available almost anywhere. Mounting the cooler would be
dictated by your own ideas.
I'd suggest mounting it behind the right hand cowl vent
on the front of the car. You may require a bit of shrouding to contain
the air flow thru the cooler, but not much more than that. This is how
the cooler is mounted on our XK 140 rally car. David Hinton mounted his
this way on his XK 120 racer and he's reporting water temp of 170F to 180F
down from 190F to 200F for the same ambient temperatures. He's not using
an oil temperature gauge. On my 120 roadster, I'm running similar water
temps and the oil temp (on the return from the coolmy 120 roadster, I'm
running similar water temps and the oil temp (on the return from the cooler)
is at 210F to 220F under race conditions. This is great. If you can take
heat out of the oil, you will maintain better oil pressure with regular
petroleum based oils. They tend to thin with heat and so oil pressure drops.
Modern plate-type heat exchangers (oil coolers) have very little pressure
drop across the cores. So whatever you gain from cooling the oil stays
as a gain to the system. On my Fiat 2000 Spyder I actually have a thermostat
to keep the oil temp at or above about 160F before it starts to flow to
the cooler. This can be added to any street car. On that car at highway
speeds my return oil temp is 80F above air temp. in warm weather (90F outside
+ 80F differential = 170F return temp, which is very nice for the bearings).
One more idea that I use on my race car is to make sure
that the shrouding around the radiator is as complete as possible. I have
even used hi-temp silicone to glue a piece of hot water pipe insulation
(like you would use in your basement around the pipes) the width of my
radiator top to the inside of the bonnet. When I close the bonnet this
compresses between the bonnet and the radiator top tank and stops air from
going around the radiator instead of thru it. On my 120 there's a space
in front of the bottom of the radiator that has the drain valve attached.
In that space between the frame and the radiator I've cut another piece
of that same foam insulation and fitted it in there. Again it forces t
same foam insulation and fitted it in there. Again it forces the air to
go where it will do the most good. The shrouding and use of the foam rubber
was good for between 5F and 10F lower temps at race pace.
Hope this helps.
29 Oct 1996, from Bill Schorse, Ontario, Canada:
I replaced my original rad core with a modern type. The
chap who did the job for me used the type of core that hot rodders use
on high performance engines. Another problem is the availability of the
original type of thermostat. It has an external ring that shuts off the
bypass hose when it opens. If you use a standard design thermostat some
of your coolant will not go through the rad but return back through the
bypass hose to the block. I have blocked my bypass hose to stop this as
I cannot find the proper thermostat. It runs about 10 degrees cooler than
before. The Mark9 sounds interesting but I live in Northern Ontario about
a half a world away. Too bad it isn't closer. As I want to get a closed
car to take the grandchildren out in. Got to get them appreciate the finer
things in life when they are young.
30 Oct 1996, from Cleo Bay, Texas, XK120, XK140:
I got an original type of thermostat from XKs Unlimited
not so long ago. (6 months?) They do work. Hopefully you haven't totally
blocked off the bypass. You run the risk of having hot spots in your motor
before the thermostat opens. Those hot spots could cause accelerated engine
wear or head warpage (worst case). Until you can get the correct thermostat,
you could use an insert in the bypass that has a hole in it to allow some
16 Nov 1996, from Jim Canedy:
Adding an expansion tank to a 120 does not require any
changes to the radiator. Modify your existing cap to seal completely above
the overflow tube and attach the tube to the overflow tank. I used a series
II E type on my 120 racer and it works well.
Mike Plechaty, 17 Nov 1996:
Before strapping on external electric fans and whatnot,
be sure the cooling system is clean (consider having system professionally
power flushed). Be sure thermostat is a *stock* Jaguar unit, not a generic
aftermarket unit from the local parts store, and it is working properly.
Be sure the radiator cap is new. I would also recommend a stainless steel
flex fan in place of the stock fan; it made an amazing difference on my
120, as it pulls much more air thru the radiator at low speeds. Pricey,
but worth it (XKs Unlimited).