With contributions from
Wim Groenendijk
(wgroenen@odyssey.apana.org.au),
Randy K. Wilson
(randy@taylor.infi.net),
Lawrence Buja
(southern@ncar.ucar.edu),
Gunnar Forsgren
(Gunnar.Forsgren@eua.ericsson.se)
and Kirby Palm P.E.
(palmk@freenet.tlh.fl.us).
Problem
Wim Groenendijk once asked...
When driving long distances at reasonable cruising speed (ca.
100-110 km/h) the engine temperature goes up to the red area on the
temperature gauge. When I drove the car around Perth it always stayed
on the cool side, staying at the end of the white band of the
temperature gauge, not even getting up to "normal." Some time ago the
cooling water header, water pump, and fan unit were replaced, as
there were leaks. The engine itself runs beautifully, hardly uses any
oil, exhaust color is a nice grey.
Solution
There are three areas of attack before things start getting
serious.
Radiator Air Flow
A lack of air flow through the radiator is usually caused by
debris (like bugs) blockng the radiator or a/c condenser maxtrix. At
the speeds being traveled, the fan should be out of the
picture.
Restricted Water Flow
Water flow restriions can be caused by a clogged radiator, a bad
water pump or ....
Thermostat Failure
A thermostat that has failed, and is stuck partially open would
account for the abnormal around town temperature as well.
Bad Radiator Cap
A hard or expired rubber seal on the radiator caps or an
under-rated (3 lb or 8 lb) pressure cap on the overflow
tank. For a Series III XJ6, replace the header tank cap with a flat
non-venting cap and the overflow tank cap with a 15 lb pressure
cap (The header tank is located on the front of the coolent rail on
the top, intake side of the engine, the overflow tank is on the
fender wall).
Coolant Hose Leaks
Leaks, even pinhole leaks, in any of the coolent/heater hoses (not
only the main big ones but also in any of the little ones like those
running to the throttle housing in a Series III XJ6).
Lawrence Buja adds:
A bad radiator cap or leaking hoses allow your system to boil
over much easier by lowering your cooling system pressure to zero
psi. As I'm sure you know, the boiling point of water is directly
related to pressure (at sea level, water boils at a much higher
temperature than it does here at 5500 feet above sea level where
the air pressure is much lower). The same goes for your cooling
system.
When your system operates at it's rated 15 psi pressure
(XJ6 Series III) the boiling point of the coolant circulating
inside of your head and block is much higher than if your system is
operating at ambient pressure. If you've got a bad hose or radiator
cap, the internal pressure of your cooling system falls from it's
rated 15 psi pressure down to zero psi.
Once this happens, your coolent can start boiling in the block
and the head on a large scale and bad things start to happen as the
expansion of the water in the block into steam forces coolent out the
overflow tank and then the new steam now in the block is very
inefficient at dissipating the combustion heat, so you really start
to overheat. I'm sure that somewhere along the way you start to get
reduced coolent circulation due to cavitation in the water pump
too.
I've heard that the real killer is that your temperature gauge
sensor may not give you a clue about what's going on because it's now
trying to read the temperature of the steam (which it can't do)
rather than the coolent temperature.
Gunnar Forsgren seconds this:
Pity I did not have these distinctions when I had exactly those
problems with my cars a few years ago. At the time I did not
understand the importance of a correctly pressurized system.
I have had these problems on my XJ6 Series I, a leaking
radiator header cap caused pressure reduction and increased
sensitivity to overheating. The problem with the kind of overheating
you describe (the long wait for boiling to quiet down and coolant
loss, etc) is that there is a risk for cylinder head warping (or
simply a gasket failure) or (as I assume) contribution to
between-bore cracks in the block. After such damage occurs you cannot
be sure if the problems are simply due to a low pressure, other
problems may have arised as a result. Another possible reason could
eventually be a bad fan coupling.
Bad Fan Coupling
A ceased fan viscous coupling, e.g that the fan is not running at
appropriate speed, but this is the least probable suggestion of the
ones suggested so far.
Ignition Timing
It has been Kirby Palm's impression that faulty ignition timing is
a major contributor to Jaguar's reputation for overheating:
If the timing of the V-12 is retarded it will overheat BIG TIME. This is supposedly true of most cars but I
have never seen it happen the way it happens on the H.E. V-12 5.3
litre engine.
Unfortunately, many problems result in retarded timing:
1) If the centrifugal advance seizes it will usually seize in
the retarded position.
2) If the vacuum advance is NOT
disconnected when setting the timing it will be retarded -- ALWAYS disconnect when timing!
3) The timing procedure for the V-12 (3000 rpm) is
excellent but unusual, many don't seem to understand it.
4) The official repair manual lists both timing specifications
and distributor advance charts. Some folks may misinterpret these and
attempt to substitute an incorrect value for idle timing when they
don't want to use the 3000 rpm method.
5) In addition, several of the distributor advance tables in
the Jaguar repair manual are just plain wrong
which can also lead those trying to derive idle timing conditions to
arrive at the wrong numbers.
6) If any of the myriad vacuum lines fails and vacuum is lost
to the distributor, guess what? Retarded again.
Folks, if you get a tank of lame gasoline in your H.E. it will
knock gently at almost any speed or throttle condition above idle.
This is a sign of an EXCELLENT arrangement of
advance curves, since the objective is to keep the timing just OUT of the range of knocking.
The engine is designed for 89 octane (usually referred to as
Plus here in Florida, midrange between Regular and Super), and
apparently will knock on 88-1/2 octane. Now, some of you use Super
anyway, but if you use Plus from different stations and have NEVER heard your engine knock, you might want to
look into your ignition timing -- especially if it runs hot.
Conclusion
Wim Groenendijk replies:
Thanks to all who responded to my message. The suggestions were
very valuable. The mechanic in Exmouth (at a gas station) has done
the following:
1) Remove the thermostat and put a port plug in; put a bypass
in containing a 3/4" hole in the flow path as apparently there should
be a 'slight' obstruction in the flow (advice from Jaguar dealer in
Perth: can anyone tell me why? Is it because the velocity of the
fluid would become too high?).
2) Renew cap on radiator header.
3) Flush the radiator.
4) Add an additive to the cooling water that should cure any
pin holes (I think its called "Irontight" or something?)
Given the symptoms I think my money would be on the
thermostat.
A test drive over 26 km @ 120 km/h was showing the temperature
to stabilise around the 'r' of 'normal' on the temperature gauge. So
hopefully the above "shooting from the hip" has actually hit the
target!
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