Oil pressure issues with relatively new engine

The Royale Motor Co of Preston (1990 to 20000)built about 50 of these including a number of LHD versions for export. Designed by John Barlow who owned the company and still builds stylish one off Jag specials and Bentley specials.Intended as a homage to 1930’s cars but with classic Jag underpinnings, they were popular with the Wedding trade who would send an old XJ6 to Royale who cut it up to recover the running gear, engine etc.totally refurbished it and used their own ladder chassis and a heavy well made GRP re-enforced body. So most were built in the factory. A few brave souls took one as a kit , but it is a fiendishly complex car to build. This one was built by a motor engineer and after reaching rolling chassis and fitting the body he put it in dry secure storage for 25 years. Sold to a north east construction company , I bought it unfinished on the premise that they put it on the road. We added the Lucas P100 headlights ,spotlight and period horns , commissioned the Winged Fury bonnet ornament ( same family as Spirit of Ecstasy used by Rolls) and had a new walnut dash and Smiths Classic instruments fitted instead of Jag/BMC plastic instruments.changed the fan for electric thermostatic one to keep it at 90 deg.
We added a third set of rear lights to make fog and reversing lights integral to design and not clip ons.
Only 6 k on the clock, new manual choke SU carbs, also fitted so starts on the button, power steering servo brakes and the Jag BW3 speed auto box.
A dream to drive . It’s a head turner for poor mans money I suppose.
I think of it as a hand built car , not a special , sadly the words kit car have taken to mean a cheap replica or dodgy thin plastic bodied super car look alike.
We don’t show it it is just a summer shopping and drive out car for the family.
Still we love it! They seldom appear on the market but 20 k should be about right for an ex wedding car dependant on mileage and condition.


The pressure relief valve’s function is to open a bypass, Julian - diverting oil back to the crankcase.

Crudely, pump pressure is applied to a piston, moving against a calibrated spring. If the pressure is too high, the piston moves to open a port to the sump - returning part of the oil to the sump. The action does not block oil flow or reduce pressure applied to the engine - it only reduces the amount of oil flowing through the engine. The oil pressure is measured after the valve and after the filter, as is the oil warning lamp…

It’s sort of unlikely that a failed relief valve can produce the symptoms described - which may more normally appear at very low oil levels; seldom reported as ‘we’ are careful about oil levels. If the pump intermittently sucks air, pressure should come back within seconds once the pump feed is restored - and brief loss of oil pressure is unlikely to cause any damage - but it is of some concern as long as the cause is unexplained…

xj6 85 Sov Europe(UK/NZ)

If it sticks open, it most certainly can. The question is: Can it stick open?

To stick open and divert all oil back to the sump, Kirbert; high pressure must first open the bypass, then stick - and intermittently in this case…

A broken/missing spring will prematurely open the valve, lowering oil pressure, but intermittent ‘0’ - no…?

xj6 85 Sov Europe (UK/NZ)

It can definitely stick.

Say it opens at 45 psi by design, but it sticks (bore not smooth). The piston is not moving up and down smoothly and can’t limit the oil pressure to 45; instead it needs a high pressure difference to get moving.

Oil pressure will rise to say 60, then the valve will eventually open,

and oil pressure will drop below 40 psi until the spring can finally overcome the stickyness and push the piston back.

If it can’t overcome it sometimes, it will not close. Unless the oil pressure falls away completely, engine off, and it then snaps back.

I never heard of this but it seems to be possible and not such a bad explanation. Usually a piece of dirt gets in and the valve will stick open until it is dismantled and cleaned. Nice car. What is the engine number? (On the bellhousing, exhaust side)

Thanks guys , food for thought indeed.
A strip down and thorough clean of all components in the filter ,relief valve. And attendant hoses looks like first stop.
Engine number was requested, this is 7L5684-8 reconditioned about 1995 and dry stored not run till 2017 approx.


The engine number is for a replacement engine fitted when car was rebuilt as a Royale.

That’s a very very early SI engine, I have 7L6926-8, 1969 build (should be cast in 68) and one of the first long stud engines. That means change your coolant every two years. Also, 8:1 compression so any fuel will work just fine as it is.
A SI or SIII filter may not fit hence the aftermarket unit?

Before you attack the oil filter try adding a quart or even a bit more and I wish you very good luck.

Thanks David, will do that with oil and have a test run, the xj6 was a early 70’s Daimler limo version which had an engine change in service, the second motor was rebuilt when it was repurposed but rejected for some reason by the owner and another clearly older engine which was then fully reconditioned was used, I have photos of old Daimler car , and subframes , engine rebuild in a pro shop and much more in the history file.

Was it a Daimler DS420 or a Daimler trim XJ6? Doesn’t matter much, just out of curiosity. Probably XJ6? And if you want to know when the block was cast it’s on the rear of the side of the block just where the head gasket is. All the way up. Forgot which side, I think intake side. They were weathered for a few months and then machined and put in service.
If the quart doesn’t help I think and hope our sticky valve theory is sound. If not you will need to get at the sump I‘m afraid.

Yes it was an XJ6 trim car longer leg room in rear.
Lord knows where he got that early motor but it was totally rebuilt ,will stick to E5 fuel, this E10 stuff will not suit any of our cars!
Fingers crossed for the filter area solution as it’s lump out to access the sump on the Royale…. The joys of motoring!

I have experienced this for years on my 83 XJ6 which at first scared me. the engine would hold good pressure until I came to a complete stop then drop to near zero and stay there until I revved the engine or shut it off and restarted it. In order to try and determine what was happening I got a complete filter mount assy with the pressure regulator from a local Pik&Pull and took it apart.

Low and behold I found that the piston had worn areas on one side at the top and the other side at the bottom, while the bore matched the wear spots on the piston. On closer examination I found I could push on one side of the piston while in the bore and hang the piston open on a worn step in the bore.
ALAS maybe a reason??? To prove my suspicion I took the Jag for a spirited run with bonnet loose, pulled off on an exit had my grandson quickly get out and wack the housing with a long rod and poof the pressure came back instantly which we did three more times with the same results.

Taking the piston assy out found the same wear as the breaker one. I honed the sleeve bore (it is a hardend steel, Nitrided it looks like) with out much effect but polishing the piston ends smooth with a slight radius on the edges eliminated the problem for about a year then is started all over again, Damn!

Been thinking a ball to replace the piston is a possible solution but a spring change may be required plus installing a ball seat, but must wait for a couple of personal things to get done first.
It looks like the piston may be hydraulically side loaded at the top when passing by the discharge port thus causing a rocking motion with two single point loading surfaces on opposite sides and ends??

Wow , sounds so like my issue, but mine also cycles between 40 ish and 60 ish psi when running at a steady 50 mph. As described above , this could also be a result of the Pressure relief valve sticking, clearing and sticking again.
To clarify , I have already replaced the PR valve this year with a new one, but the problem continues, are there any valves in the after market “upside down” cartridge assembly? As it seems my engine is an early Mk 1 , does it require a specific , possibly different cartridge type filter assembly?
I am minded to buy a total new assembly and Jaguar filter . Are all after market filter assemblies upside down filter fitting ?if not perhaps a conventional hanging filter unit might help, any thoughts?
Thanks so much to you all for helping to tease out a solution to this head scratcher!

Quite a few jaguars had a filter that was not at 90° to the block but pointing downwards. Try SII filters, I think these had that style. A cartridge filter could fit though?

The SI engines are the same as all the other XJ that followed, oil wise. Mk X and 420 engines might be the same. The huge canister of the SI XJ is too large, for sure.

I have done some research on after market spin on filter assemblies,
There seems to be a specific series one variant of housing with also fits late series two. Perhavps more to do with access than oil issues but perhaps the way to goThe original car had a new engine during service and when the strip down was done this went off to be reconditioned. This was I suspect a series 2 motor, and the spin on was probably added then. This motor was rejected and another mk1 substituted and ancillaries fitted to it. It could be that the Mk 2 variant housing was fitted to the Mk 1 motor and that this may have a bearing on the issues.
Just a thought.
Again all thanks for your efforts and ideas.

Hi Folks,
I have the Pressure relief valve out and its only 6 months old.
Clean as a whistle and functioning correctly. No dirt , no sticking etc.
But, looking at the diagram of the "upside down cartridge filter assembly there is a
“o” ring round the top of the PR valve, this is present , but the diagram shows another
washer that sits on tip of the unit and is clamped in place when the valve is screwed in to position.
This does not have apart number and what type of washer it is , is not clear. It was not present when I removed the `PR Valve so this may account for some or all of my freaky oil pressure issues.
So my question is, does any one know what type of washer this should be?
I have gasket material type compressible washers, hard red fibre washers and copper rings which might fit, I assume its purpose is to seal the top of the valve into the filter housing, why else would it be there?
One other point , I have put a quart of extra oil in the sump when the dipstick shows almost full and this makes only a slight increase the oil level on the stick, so I guess it floods up from the lower part of the sump to the much bigger upper part. Any way this may resolve the zero oil pressure on stopping after a fast run… Ofcourse I would have test runtime car but of course the fuel pump packed up at the critical moment and I am waiting for a new one to arrive…the joys of Jags eh!

Well probably not if it’s that full (if that means full - I never overfilled very much, don’t drive far if the crank hits the oil!)

We need pictures of the filter block, there’s so many different styles around that it’s hard to say anything without knowing what to talk about.

Will try and get a cut away pic of the exact unit.

Post a pile of pics please!

As part of your investigation take the filter just removed and cut it open. Inside you’ll find the pleated filtering element, something akin to an accordion bellows. Pull it apart looking into the pleats for metal content. If you find metal, what type/color is the metal? Bearings contain three different metals. Just to rule out any bearing problems.

Stay Well and Happy Trails,