5.1 - Cooling Info for the MK1/MK2/S-type ( Paul Saltwick,  )

Do not expect the temperature gauge to sit still in the middle
like a modern car, it never will. The correct thermostat is 74C,
it is is normal for the running temperature to vary between 74C
and 95C. The engine is not overheating until the coolant boils.
The 3.8S suffers from a marginal cooling system, an undersized
radiator, a packed engine compartment and a small grill. Jaguar
spent a lot of time improving the cooling system in the saloons
because of overheating complaints, but the effective mods didn't
really arrive until the 420. The water pump impellor was redesigned
several times, the intake pipe enlarged, a cross flow radiator fitted
and the speed of the pump at idle increased several times.

It is not "overheating" until the coolant boils. If your system holds
pressure and you use a reasonable coolant mix, it should not boil over until at
least 110C. If your engine is in good condition, 90C or even 100C for a short
time will do no harm. Modern engines of the same construction operate at 120 C
with no engine damage. If you take a look in the archives you will find many
saloon owners whose internal thermostat turns red after 70C because of the
location of the gauge needle. This is a problem with the owner, not the car.

If you think you are having a problem and find yourself staring at the gauge,
first check the accuracy of the gauge, an IR thermometer is one way. The
comments about the thermostat are correct, it doesn't even start opening until
74C and isn't fully open until 85-90C. On a cool day you should be able to see
the thermostat open on the gauge as the temperature momentarily falls back, if
everything is working properly. To properly diagnose the cooling system you
need to first check that the system holds pressure. Then drive the car at a
reasonable speed for at least 10 miles on a hot day. The temperature should
stabilize somewhere between 74C and 90C. It may exceed 90C while sitting still
but it should drop down once you get air flowing through the radiator. If you
find that the temperature slowly climbs above 90C and keeps climbing and
never stabilizes, there is a problem.

The first thing I would do is to check the accuracy of the coolant
and the oil pressure gauges. They are both notoriously inaccurate.
I would remove the oil filter head and clean out the pressure
relief valve, check the seat and spring and fill up with 20/50W
oil. Then I would pressure test the cooling system. The 3.8S was
fitted with both a 4 psi and a 7 psi cap during production, the
limit is the water pump seal, not the radiator. Your radiator could
probably take more, but I wouldn't go above 7 psi. If the system
holds pressure, I would use a 7 psi cap and fit a 1 quart overflow
tank, it gives you a little extra coolant capacity and lets you
identify any internal leaks. A 70/30 water coolant mix with a
bottle of Water Wetter is a better idea than 50/50. The 3.8S uses
a by pass thermostat designed to allow coolant to circulate during
warm up while the thermostat is closed. It is designed to close
the by pass when the thermostat opens, but the manifold is often
corroded to the point where too much coolant by passes the radiator
when hot. If you don't have the correct themostat, try one, but
the solution is to fit a plug with a small hole in the by pass hose
to limit the flow. The factory recommended a 3/16'' hole, I use a
1/4'' brass NPT fitting.
There is no magic bullet for the 3.8 cooling system. The usual fix
for rising temperature at idle in hot weather is to fit an electric
pusher fan in front of the radiator. There are many options, I use
a 14'' Permacool that comes on at 95C and off at 90C. It only comes
on after prolonged idle in summer, but it stabilizes the
temperature. You can also replace the heavy stamped metal fan with
a modern nylon fan that moves more air at idle. Another good idea
is to add some ventilation to the engine compartment, the most
effective short of hood louvers is to vent the triangles in the
wheel wells in the rear lower engine compartment. The ideal set up
is to follow the factory's lead and fit a 4.2 pump with the larger
intake pipe and change the pulleys to increase the pump speed at
idle, but you also need to change the timing cover.