I would say the thermostat should be changed, and see what happens
afterward. On both of my Jaguars, I have had intermittent problems
with this component. One failed closed, resulting in overheating, one
stayed open resulting in low operating temperatures, and poor
heater/defroster operation. If this is the problem, it is the
cheapest and easiest one to fix.
When did you loose coolant? When you opened the cap, or did you loose
it in the course of operation? In some instances, excessive pressure
within the cooling system is and indication of other problems.
I do not think it is the water pump.
Regarding your change back to the pressure switch, why did you do
that? My thoughts are if you are in traffic, why not run the fans all
of the time, rather than playing “catch up ball” with the fans after
the engine gets a bit hot? Also, many XJ40 owners have fans that
freeze up due to lack of use.
Let me know if the T-Stat does the trick,
From: ZORAN1 [mailto:ZORAN1@prodigy.net]
Sent: Wednesday, November 20, 2002 10:03 AM
Subject: Re: [x300] Spontaneous Over-heating
I was thinking it might be the thermostat but I’m not sure the
symptoms are in-sync with that scenario. The coolant in the radiator
definitely gets hot and the pressure is enormous as the first time it
had happened to me I had lost a considerable amount of coolant out the
expansion tank cap.
Also, on the freeway there is no problem, I can turn off the heater
and the temp stays at normal operating temperature.
Like I said, the two times that this has happened to me the
circumstances were the same. Long extensive downtown driving with an
occasional stop here and there. Perhaps it is the thermostat since I
believe it begins to happen after the engine has been stopped for
awhile allowing the stat to close.
Another list member suggested the water pump but isn’t there an
indicator if it’s failing? Like a drip out the front hole?
I’m aware of Jaguar’s so-called “fix” with the jumper cable.
My car already had it however I found that it was unnecassary to have
the fans on all the time regardless of engine status. SO I went ahead
and installed the original design pressure switch which appears to be
working fine. I doubt this is the cause of my problem as the engine
temp switch should be telling the fans to come on. Besides, from my
recollection a failed pressure switch would blow the fan fuses. I’ve
checked the fuses and they are fine.
In reply to a message from ZORAN1 sent Tue 19 Nov 2002:
Your problem could be a questionalble thermostat. If you have not
changed yours, I would do it right away. They have a tendency to
Also, there is an X300 upgrade in that a wire jumper is installed
enables the fans to run on low speed all of the time. A pressure
switch (that controls the function) in one of the A/C cooling lines
removed, and a one wire jumper is installed where the wiring harness
from the pressure switch connected to the main harness… This
take a dealer about 5 minutes to do.
There is some debate about this jumper, but it will solve two major
problems. 1) Eliminate the problem that the fan motors freeze up
lack of use in the colder months and climates due to lack of
use. 2) In warmer climates it is a continual pre-emptive step to
Now, if you are in Finland or some other very cold location, where
your problem is gettimg the engine up to temperature, perhaps this
is not for you.
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–