Oil pressure issues with relatively new engine

My Car had a professionally rebuilt engine 6,000 miles ago, it has modern Smiths classic electronic dials and senders supplied direct from manufacturers. When driving at 50 mph the oil pressure gauge seems to cycle between 40 and 60 psi.
When I stopped after leaving a fast road at traffic lights the oil pressure fell away completely and the oil warning light came on. I shut down and at the roadside waited a few minutes in shock before trying a re start, pressure was restored immediately ! I drove on home watching the gauge like a hawk. Again near home I stopped at a turn and once more the pressure plummeted and the oil warning light came on. Stoped the motor , opened the engine and checked the oil level. It was 3/4 or more full of clean new oil. A while back it did this and I was advised to replace the oil pressure relief valve which We did with OEM part. Also new oil and Jag filter cartridge , it has a screw on adaptor for an upside down filter.
It has had four oil changes ,Castrol Classic oil ,a d new filters each time as part of running in. I have been underneath and can’t find any leaks or issues. The only clues are there was a small amount of oil on the ground under the car after the last failure , spotted by a follow car, and a brief puff oil oil smoke from under the car, as if oil had fallen on the exhaust. No trace of this can be found on the car. I fear she must go back to the Expert Jag garage when they can take her but I am at a total loss to understand what is going on. I Have confirmed by serial number that we have correct senders for oil warning and pressure gauge, Your wise thoughts on this would as ever be appreciated !


Oil pressure dropping AND light coming on means you have a serious problem.

Oil changes and oil type don’t matter as long as it’s not water and it isn’t.

Relief valve is still a reasonable guess but when mine got stuck there was 20 psi at 1500 instead of 40.

The pressure shouldn’t cycle at 50 mph.

You changed the filter style so the gasket and filter and relief valve have been changed?
Did the problem persist?

Who ever rebuilt the engine should take it in immediately and have a look at the oil pump area… before something drastic happens.

Fluctuating pressure and a drop when stopping are signs of low oil level. I assume the dipstick and tube are correct, 3.4 4.2 dipstick.

Thanks for this, yes oil filter is well fitted an leak free, new pressure relief was professionally done and a new little hose fitted too. As you say the oil pump is a prime suspect , quite how it can cause this fault I can’t imagine. It’s almost as if the oil is draining back to the sump too slowly ? But if that was so fail would be at high not low revs ? Engine is a 4.2. Big thanks for your thoughts.

Dip stick seems correct one by number on it and change of filter made no difference. Sadly this car was rebuilt 25 years ago and stored incomplete, till we bought it and had it completed. I have photos taken in the shop of the rebuild a all bills but builder has now retired !so I am stuffed !

Never say never, but this can’t happen. The oil can flow down from the head via no less than four channels, and the crankcase has no means of keeping oil from flowing down.

Does the tube protrude by ~127 mm?

If this was my problem I would swap in a different, complete oil filter housing just to see, because removing the engine is so much more work. There is another valve in there or indeed the relief valve wasn’t done right.
25 years of storage is never great but if it runs well you‘re lucky!
Maybe a missing baffle plate.
Something like a hole in the supply tube to the pump is what I could think of. Damage, or the o ring where the pump is, and when the oil swaps around the pump sucks air sometimes… try driving with the oil to the full mark and ignore the possibly higher consumption, maybe it helps :slightly_smiling_face:

Thanks for this!
You are correct the oil dip stick tube is 127 and dip stick marked 4.2, so that sound right. I was going to propose to engineers that we drop sump and remove filter housing for inspection and to allow removal of oil pump . Jag specialists should be able to see what is amiss. Car runs sweet and has been well sorted during build up but could be an perished o ring or similar as you say, many thanks will keep topped up fully with oil.
Best Julian

I would round up a mechanical oil guage. Plumb it in and see if the issues remain They may no longer exist.

A few years ago, my car exhibited oil pressure issues . although V8 powered, It uses the Jaguar sensor and guage. I knew that OP was Ok as the lifters did not chatter. I plumbed in an after market sensor and guage electric! No mechanical unit to be found. OP fine…It is in my diagnostic box as a tool

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Many thanks for this, most helpful

Yes, would have been my first suggestion but I figure that if the warning light cones on agreeing with the gauge something else must be rectified because the pressure is truly low.

Removing the sump is not easy, sadly. Engine or suspension must come out. So try what you can to find the problem elsewhere!

You said both events happened at braking, Julian - then oil pressure returned to normal at restart and driving…?

As you brake, the oil surges forward and pump may suck air through the pick-up tube - which is rearwards of the engine. With no supply to the pump, the oil pressure will instantly drop. With proper oil level this should not happen - unless there is a contributory factor. Apart from the unlikely ‘wrong’ dipstick; there is a vague chance that the stopper on the dipstick has slipped - giving wrong level readings. One, somewhat dubious, test is to drain off the oil and see if the amound matches the prescribed refill volume…?

It is highly unlikely that the oil pump is the culprit; it sort of either works or does not. And the coincidence between the braking and pressure drop likely means something - though ‘what’ is of course somewhat hazy. And I agree with David; blocked back drain cannot happen - and any pressure drop from this cause would be more likely at speed…

And certainly; pressure drop indicated by both the gauge and the warning lamp requires some action…

xj6 85 Sov Europe (UK/NZ)

Hi Frank,
I think we are all on the same page now!
Have examined the dipstick and the stopper has not moved and seems solid, its marked as a 4.2 L dipstick and the dip stick tube is 127cm as required. Suspicion is gathering around the after market
“upside down” oil filter cartridge assembly. While a new pressure relief valve was fitted there are I believe other valves in this assembly so it will come off for dis-assembly and inspection.It is relatively new too.
Failing this I fear it may be engine out, as is either a missing baffle or leakage in or around the oil pump or its pipework, gaskets and “O” rings…
Having done four oil changes-in 6,000 miles (OK I am paranoid) I am sure the capacity is correct from drain out and refill amounts.As you say breaking seems to trigger this so possible lack of oil at the pump must be the issue. I am negotiating with my Jag Garage experts for it to go in and have a full inspection, pressure test and professional head scratching session…at worst it is engine out
,I am off for a lay down now!Credit card in the Fridge too.
Thanks for your valued input

I don’t think draining the oil is reliable enough but adding one litre could rectify the situation.
For diagnostic purposes at least.

Should be cheap and easy to put another oil filter assembly in. I blew the gasket to the block and was back in business after two hours, and I was in no hurry.

By the way, a few seconds without oil pressure cause no harm at idle. There’s enough in the bearings and they will not suffer immediately.

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Thanks for the reassurance , I will overfill the sump a little and see is a bit of local testing on a long strait road with no other traffic yields and more info.
One of the joys of living in a remote area (Suffolk)is the ability to go for an hours drive any only see two other cars! My garage for the jag is the one that built SS Jag replicas and they know the XK motor inside out!
Thanks a bunch for reassurance!

I have had a somewhat similar issue when approaching a set of traffic lights at the bottom of a fairly steep incline (facing down the hill). As the RPMs dropped the oil light came on for maybe a second or two and then the pressure came up enough to extinguish the light. Of course the transmission was in D so the idle speed drops to 5 or 600 rpms and I think that it was the combination of events. My oil pressure is good at all other times.

I have noticed the same, coming to a stop downhill. I had no proper dipstick and it told me this way when it was low and the oil hot. Took 5-10 seconds to go off.
And the first long distance drive it suddenly came on coasting down the hill and it was way low.

What I never had was the oil pressure going up and down while cruising.

No David, I don’t have that issue either. When at a reasonably steady engine speed the pressure stays pretty much constant (when the oil gets up to temperature),



IIRC, there was a discussion a few years ago about the pressure relief valve and its tendency not to close quickly enough when the engine speed dropped. I don’t recall whether the cause was the valve itself being sticky or the question-mark-shaped vent line to it being obstructed or something.

Thanks to you all, for your input.
The drop in pressure was off a fast level road braking for a junction.
Both the smiths electronic gauge and the separate low pressure light indicated so some how we really lost pressure. Previously a new pressure relief valve from SCS parts was fitted and a new short hose adjacent to filter was fitted as the old one was in poor condition. The idea that there might be a blockage
In the filter housing or related pipes appeals greatly.Also a slow response from the pressure relief valve might possibly cause it ?
I promise to update you all once it has been torn apart…
Again thanks one and all.

Felt I should fess up about the car….
I attach a photo of car, it’s all xj6 under the skin ,engine gearbox front and rear subframes and suspension plus steering(slightly modified)
Engine as standard except for electronic ignition. Was never fully completed as a Royale Drophead coupe and dry stored for 25 years, paint is original , basically a new old car ! But fun to drive I added the appropriate bright work at the front in lights horns etc.


Well, that’s different! Is this a kit of some sort?