Oil Pressure mystery

[Frees]

This is my first post on a forum so any advice would be very welcome

(When I talk about oil pressure readings these are the readings from a ‘wet’ mechanical oil pressure gauge fitted to the engine via the oil pressure warning light and/or the oil pressure sensor connections and NOT the electrical Smith gauge fitted to the car.)

i recently purchased a Series 3 V12 during the test drive the a good 15 – 20 mins and the car showed decent oil pressure between 40 lbs on the cars oil gauge. Once we got the car home we noticed that over the course of about 10 miles the pressure was dropping off to the point where the oil pressure warning light would illuminate. As soon as this happened the car was in the garage to determine the problem, as the car had been standing in the previous owners garage and not driven for 5 years we decided to give it a service to start the ball rolling in fixing the problem.

After new oil and filter etc the car would start cold with 20lbs oil pressure (wet mechanical gauge) and on getting up to temperature would drop to zero. After speaking to a number of people and plenty of research online we felt the oil pump was to blame and decided to fit a new pump.

After the removal of the old pump we could see that it was indeed worn out and well beyond tolerance we fitted the new pump, new gaskets and O rings throughout the sump, oil cooler and critically the timing cover oil seal etc and for peace of mind checked 3 main bearings and 3 big end bearings all of which looked in almost new condition we were actually surprised how new the bearings and crank looked. We also looked at the Oil pressure relief valve which appears to be working perfectly fine. Rebuilt the engine and started her up, on start up we had 40lbs of oil pressure with the new oil pump however, when revving the engine the oil pressure did not increase it does not go past 40lbs. When the engine is hot and at tick over it shows 20lbs oil pressure but then will not increase when the engines revs are increased?

So today I removed the entire bottom end Sump, Filter housing, and removed all seven main bearings and checked the tolerance all seven main bearings and big end bearings are showing a 2thou clearance again replaced all gaskets and O rings and rebuilt the engine but sadly when the engine is hot we are only getting a maximum of 20Lbs pressure and no increase when the engines revs are increased?

So in short after fitting a new oil pump and checking the bottom end of the engine we are getting very low oil pressure with no increase in pressure with increased revs.

Any advice or help on this would so really appreciated

[MarekH]

Oil pressure is capped by the large spring thing next to the oil cooler/heat exchanger. For a cold start, it ought to show ~70lbs and stay there for a rebuilt engine at any rpm above idle. Idle when warm will drop to maybe 50lbs. Aside from that spring there is another "overflow" spring inside the oil filter housing.

If what you have fitted is within spec, then one of these two springs is being held open and less oil is circulating than you think it is.

The oil pressure light is independent check and is "real". A simple garden insecticide sprayer pumping oil into the gallery generates enough pressure to extinguish the oil pressure light, so you either have left a big hole somewhere or you are not pushing much oil into the oil gallery.

kind regards
Marek

[Frees]

Thanks for the reply however, it doesn't really answer the question. i know what the oil pressure should be and the oil pressure valve has been tested along with everything else. i know i am losing pressure somewhere the question is does anyone know where else i could be losing pressure?

[christopher storey]

I have to say that 2 thou bearing clearances sounds rather a lot to me. Although I am not familiar with the V12 and have no reference material on it, a general rule of thumb would be that the bearing clearance should not exceed 2 thou and that a more typical clearance would be 0.5 to 1 thou. Certainly at 2 thou , and having in mind that there are 19 bearings on this engine for bleed to occur, I think that this well could explain your low oil pressure . Did you measure ovality and/or taper on any of the big end journals ? If these are present it will worsen the effect. However, given that it would require a crank regrind to cure it, there is a definite case for checking the other items suggested and then living with it for a bit , perhaps changing to say a straight 40 or a 20/60 multigrade oil

[chris420sa]

Marek - thanks for your response. Just wanted to ask you if you have fitted a mechanical oil pressure gauge? If so, could you please advise from whom you purchased the gauge, its range and its fittings.
Many thanks
Best regards
Chris

[MarekH]

Dear Chris,
My car came already fitted with a mechanical gauge.

The range doesn't need to extend past 80-psi as the big spring will open and cap the pressure at that level. The fittings fot it to mate it to the "T" at the back of the "V" can be purchased as spares from someone like Pirtek in the UK or any hydraulic shop.

My car has a very thin copper capillary gauge going through the bulkhead to the dash. It has lots of loops where it needs to bend so there is no danger of it work hardening. Most people will probably have a plastic tube instead, but this one is just soldered to the relevant fittings instead.

There ought to be plenty of info on the web, perhaps from other UK classic cars also. The US jag-lovers.com website will almost certainly have covered this also.

kind regards
Marek

[mgcjag]

Hi Chris https://www.merlinmotorsport.co.uk/p/sm ... auge-cb-op are a good starter for all the parts you need.....unfortunatly i think the only similar oil guage is this 100psi unit... they also sell pipes and fittings and are very helpefull...Steve

[Series1 Stu]

I wouldn't worry about having 0.002" of clearance in your main bearings. That is bang on specification with tolerance being 1.5 to 3 thousandths of an inch. See the technical data in the workshop manual if you seek confirmation of this. This degree of clearance is required to ensure you get an oil film for the crankshaft to ride on. It's the basic principle of hydrodynamic bearings. So don't worry about that level of clearance.

I do wonder whether you used good and accurate measuring equipment though. A Vernier caliper, digital or otherwise, is just not the right tool for the job. Use good quality inside and outside micrometers at the very least and ensure their calibration is good. You may be surprised how inaccurate some tools can be.

The service manual does not appear to quote an oil pressure for the V12 although the manual for the 3.8 quotes 40psi at 3000rpm hot. I would imagine that a similar figure would be appropriate for the V12.

Good luck and Regards