I have a 1991 Convertible Classic that I've probably got $25K invested in. Great car- my fifth Jaguar over the last 40 years-drives like a dream, but still have one problem that's making me crazy.
When the ambient temperature is greater than about 80 degrees F the oil pressure drops noticeably after a 15-20 minute warmup. I have installed a mechanical oil pressure gauge with the sample port directly into the oil galley. Took one of the plugs (1/2" X 13) and tapped it for a copper oil line to the mechanical gauge which mounts neatly in the little blank area just forward of the ignition switch.
Start the vehicle (NAPA 10/50 oil) and oil pressure is in the 70's as expected. It stays there regardless of the RPM, typically less than 3200) until the oil gets really hot at around 20 minutes, Here's the part that has me scratching my head: climbing a hill (and there's a nice 1500" one with all the winding, twisting switchbacks one could possibly hope for just south of Prescott) the oil pressure drops to 35-40 psig , and when at the top, goes to about 15 psig at idle.
Turn the car around and the 50 psig is restored halfway back down the mountain. When I got home after another fifteen minutes on surface streets the oil cooler was at approximately ambient temperature, making me believe that there wasn't any oil going through it.
I have searched off and on for several months for an explanation of how the oil system works, and have had zero luck.
I've got the big green official shop manual and still can't figure it out.
Questions:
1. Does anyone have any idea where I can get a schematic of the oil system, and
2. any ideas about how to force the oil to travel through the cooler?
Thanks in advance
-Bill

 

The oil pressure chnages are NOTHING to worry about, all quite normal.

You car has the full flow oil system. In principal, all the oil gets cooled, except that there is a bypass valve within the oil cooler head, that actually lets oil short circuit the cooler in the event of a pressure buuld up in the oil circuit.
There is a huge amount of oil in the V12 system; and a not very hot oil cooler is quite usual unless you have been seriously loading the engine at very high revs on a very hot day.
If you take an infra red thermometer and point it at the cam covers, engine running, as soon as you stop the car, this will tell you the oil temp at the hottest part of the oil circuit. It takes a long hard drive to get the oil really hot.
On my car, which has the bypass system which may or may not be slightly less efficient, the oil temp at its hottest has never been over 88°C. Meanwhile, once warm the pressure on the gauge drops to about 20psi at anything under 2000 rpm. 100% ok. On later cars, Jaguar replaced the guage sender with, in effect, an on/off sender; just to stop owners fretting!

 

Thanks very much for the kind words, but I am still really interested in seeing a diagram or schematic of the oil system.
I had an 86 Hardtop XJS that did not exhibit this problem.
Really I would like to understand the oil system, and although I used to think I knew how to use search engines, this search has convinced me that I do not.
If anyone has any information about this, I would be grateful to be pointed in the right direction.
Bill

 

Your 1986 had the bypass system. The full flow works like this:

1. oil pickup in sump to pump
2. pump to filter and through the filter (but see below in red)
3. filter to cooler
4. cooler to filter head casting and NOT through the filter but straight to main oil gallery in block
5. oil gallery to crank and external take off to cam feed at rear of heads via distribution casting at rear of V that also carries the oil guage take off.
6. oil drops by gravity down to sump.

Within the oil filter head is a sprung valve that is continuously opening and shutting to allow excess oil pressure in the filter casting to bleed off. This valve spring is not large and gets tired and starts to open more easily than it did. This may well be the cause of the pressure oddities.

 

Merci beaucoup!
In your opinion is this spring performance heat-related? I ask because the oil pressure is 60+ until suddenly it is not.
If this were your engine would you replace the spring?
BTW this is the first time anyone in my experience has described the full-flow oil system.

Bill

 

Bill,

I have never had the full flow experience, we got the by-pass system. Maybe full flow in later years, not sure.

That same valve is located on top of the upper sump panel (AKA Sandwich Plate). The Full Flow is the same valve, but, IN the filter head.

Some parts book schematics below.


Full Flow Parts Schematics.
These Parts Drawings show a "spring" as #10, not sure of that, see below Part Numbers.

#11 is the item in discussion = C37888.
#10 is named a "Retaining Ring", EBC3343 = NLA.





V12 By Pass Valve C37888.

This my "spare" new valve, in my shed, from when I had waaaay too many V12's.

 

Attached a photo of the inside of the FF filter head, showing the valve. If the oil cooler circuit gets blocked, this valve can open and thus allow oil to short circuit the cooler system and go directly to the oil galleries.
This sprung valve can weaken. Also, I should mention, the overall pressure relief valve, 3 in Grant's diagram above, can also weaken and be affected by heat, which would also reduce pressure.
The important point is that your engine is not at any risk with the pressures you are recording. The spacer with the large holes is there to ensure the valve cannot fly out of its seat, if the keeper ring holding it there fails.

 

Again thanks.
According to the SNG Barratt parts list the pressure relief valve is P/N EAC 7149 and is basically unobtanium (Very rare element generally associated with older British vehicles. I first encountered this substance when working on my long-departed 1967 BSA Lightning.)
However, I would very much like to replace this valve, and wonder if anyone has any suggestions as to where to find one. Secondarily, are they re-buildable?

Best,

Bill