55
to prevent wear, but gets hot enough to melt the soft bearing material because the same oil is staying in there and
getting hotter and hotter, rather than cool oil flowing through.
All engines tend to display a drop in oil pressure at idle; the pump moves less oil when the engine is turning slower, but
the openings through which the oil flows are the same size no matter what the engine speed. The Jaguar V12 is no
exception, and sometimes shows lower oil pressure when hot and idling, especially when it has a few miles on it. The
V12, when running on all cylinders, can idle very slowly, causing even lower pressure. This causes many XJ-S owners
to panic, and some to take poorly conceived countermeasures.
There is no magic value for oil pressure. The only real need for any pressure at all is to get oil to all points in the
engine, and this would only require a couple psi. Oil pressure is monitored simply to insure there is oil flow, which is
essential.
If your car has always had low pressure at idle when hot, and it gradually over the years gets a little lower, don’t worry
about it. If it suddenly has much lower pressure than it used to, you may have a damaged bearing that is allowing oil to
flow through too fast; the cause should be investigated. If your car suddenly has no oil pressure at all, stop
immediately, do not drive it one more minute until the problem is located and corrected. Many motorists fail to
understand the importance of this, so I will emphasize: If the red light on the dash comes on indicating that you have
no oil pressure, it is not good enough to “take the next exit.” You should pull over immediately, possibly even shutting
the engine off while still moving. A towing fee, and even an illegal parking ticket, is a minor expense compared to an
engine replacement, which is the inevitable result of driving with no oil pressure.
Do not add oil thickeners to your oil. While these may increase the indicated oil pressure at idle when hot, they do no
real good and can do considerable harm. In particular, when cold the oil may be so thick that very little flows and most
of the output of the oil pump is wastegated through the pressure relief valve. While there is good pressure, there is little
flow to the bearings, and they may fail due to lack of cooling flow before the engine and the oil warm up. This is also a
good reason not to run the engine too hard until it is fully warmed up.
OIL PRESSURE SENDER: The oil pressure sender on the pre-’92 XJ-S is notorious for leaking; see page 46. It’s also
fairly well known for electrical maladies. Tom Mackie says, “My oil pressure sender is getting flaky. I connected an
ohmmeter and found that rather than nice varying resistance readings, it sometimes goes open. Which makes me
wonder how accurate this particular unit has been all along.”
If the connector on top is loose, Mike Morrin provides some insight: “The connector moving around is due to some
little plastic pegs broken inside, and will cause the sender to read low. You can uncrimp the top and get inside to glue it
down.” Since uncrimping is difficult, others suggest cutting right through the circumference of the can to get the top
off, fixing the innards, and putting the top back on with aluminum tape or JB Weld or some other mechanical means.
Ed Sowell provides electrical data to aid diagnosics: “Here are the measurements from my sender and gauge, for what
it's worth:
Oil pressure sender (markings: PTR 1001/10/EC 700kn/m
2
, 29-7 Part #46272):
Engine off: 350 ohms
Cold engine idle: 80 ohms ( I believe the oil pressure is about 60-70 psig under these conditions)
Oil pressure gauge:
290+ ohms = 0 psig
55 ohms = 50 psig
10 ohms = 100 psig
short to ground = off top of scale.”
I’d like to be able to tell you what commonly available sending unit can be used for a substitute, but I can’t; nobody’s
worked one out yet. The pre-’92 Jaguar V12 uses a 0-100 psi oil pressure gauge while Jaguar 6-cylinder cars use a 0-
80 psi gauge, so senders from other Jags won’t work. Before trying a sender from any other car, make sure it has the 0-
100 psi range. Electrical oil pressure gauges with 0-100 psi ranges are very common as aftermarket items, though, so
perhaps the sender from one of those will work. Summit (page 717) and Jeg’s (page 714) offer many such gauges, or