[v12-engine] Oil Pressure Problem

V12-Engine
1

I’m re-commissioning a V12 Series 1 saloon that has been us
used until very recently but covered very few miles each ye
year for the last few years.

The oil pressure has been measured using the dash gauge and
also a mechanical gauge with the same result.

When the engine is started the oil pressure builds
immediately and is 40+ psi. As the revs are increased all remains
remains normal until at around 3000 revs the pressure falls ba
back to just a few psi. The pressure doesn’t rise as the re
revs fall.

If the engine is stopped for a while and the restarted exactly
exactly the same happens.

Oil temperature has no bearing on the pressure.

The relief valve has been removed and appears to be in good co
condition.

Anyone had a similar experience, or any advice ?–
Rixon Bucknall Special
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2

In reply to a message from mph jag sent Fri 8 Jul 2016:

I’m not that familiar with the oil pressure relief valve of
the V12, however, I am with the one on MGs. The spring can
weaken over time. Try either replacing the spring or what is
done with Mgs is to add a small spacer under the spring.–
Maynard 94 XJS V12 Coupe 91XJS(RIP) 86XJ6 78MGB 67MGB
Palatine, IL, United States
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3

I have not experienced this.

But it sounds like there is a blockage in the sump pickup pipe.

This sounds like the problem people have reported with their fuel pumps when
there is crud in the tank. Starts fine, as the revs rise, it sucks up the
crud which blocks the input filter. It stays there until the engine is shut
down and the crud drops back into the bottom of the tank.

In your case, there are 2 bypass valves in the oil filter block.
The main bypass valve (that you need a big spanner to get off) has a bleed
on the back, back into the sump via a tiny tube. I don’t know what this is
set for. It is reported to be 70psi, but I have measured one closer to
100psi. So there is a question mark over this.

There is also a small flimsy spring loaded flap valve that bleeds oil back
into the pan if the oil cooler is blocked. I think this is described as an
80psi pressure relief valve.

Oil is sucked from the sump, mixed with any return flow from the cooler
(directly at its inlet) and pressurised through the pump.

Output is fed to the filter block where the oil is exposed to both the
filter/galleries and to the bypass valve and relief valve.

Under normal operation, the bypass valve should bypass excess flow (that the
oil galleries don’t use) to the oil cooler, and from there back to the pump
input. If the cooler is blocked, then the relief valve should dump oil back
into the sump.

There is a case where the pump could cavitate, if the sump pickup was
partially blocked. This would put a partial vacuum on the pump inlet leading
to air bubbles forming at the pump output. Air bubbles result in the oil
being unable to apply any meaningful pressure, so I presume the pressure
gauge would drop.

Cheers
Mark____________________________________________________________

When the engine is started the oil pressure builds immediately and is 40+
psi. As the revs are increased all remains remains normal until at around
3000 revs the pressure falls ba back to just a few psi. The pressure doesn’t
rise as the re revs fall.

If the engine is stopped for a while and the restarted exactly exactly the
same happens.

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4

Hi, Its seems I have exactly the same problem you describe with my S1 Xj12, please advise me on wether or not you found and cured your problem… I am waiting in anticipation. Thanks Murray

5

Do you know the accurate mileage of your engine? If the oil pressure doesn’t drop significantly when at NWT then I don’t think it’s a pump wear issue.Which should give you some comfort!
I would get the oil pressure relief valve operation checked out accurately.
If it’s ok, may be a case of sandwich plate off and check oil pipes? A job,that with the problem you have, I’d avoid unless the oil pressure drop off gets really serious!
Ideally I like to see higher oil pressure than the figure you’ve stated, but it’s not unusual for an old V12 to run at that pressure quite happily.

6

Gday Nigel
My engine is brand new and hasnt done any Km’s…There is talk of blocked pick up pipes around but it was cleaned to hospital standards like everything I touched for the rebuild, the oil pump was well within spec…and all the "o"rings for the plumbing were renewed and checked they were going to do the job properly.
There is also talk I might have a blocked oil cooler but would that give me these symtoms ? That been said it was washed out, should I of gone to the trouble of tank removal or have I infact loosened up the silt / rubbish in it and now its blocked ? and once again is a blocked cooler GOING to give me this symtom…
It fires up and straight to 40lb and by the time the chokes off (5 mins) I have scary low pressure, like 5 to 10lb, the oil light I must say has not yet flickered AT ALL ! and yes I have a wet gauge on it at both the transmitter hole AND straight into the gallery aswell to rule out a cam feed pipe blockage.
If I put the filter housing gasket on wrong even I believe I would still have oil pressure just not filtered oil Yes or No ?
I was susspect on the by-pass and relief valves but i am absolutely SURE they are AOK !
What I can’t find anywhere is an ‘Jaguar V12 oil flow chart’ remembering this is the “by-pass” system and not the later “full flow” system… If anyone could flick me one of them or direct me to one I’ll shout the first and second round.

7

Hi Murray,

My problem turned out to be the pressure relief valve. It’s difficult to
understand why it behaved in such an unusual manner I can only assume it
was sticking at a certain point.

With a new valve fitted the oil pressure is fine.

Regards

Mike

8

My problem turned out to be the pressure relief valve. It’s difficult
to understand why it behaved in such an unusual manner I can only
assume it was sticking at a certain point.

This is the thing with the question-mark-shaped tube connected to it? I’m
guessing sticking open would cause low oil pressure. Sticking open could be
caused by junk stuck in it, but I guess it could also be caused by an
obstruction in that vent line. Either way, glad you got it sorted.

– Kirbert

9

This is what I think the “Bypass Flow” system looks like.
I assume the oil filter has a bypass if blocked (although perhaps this is not the case on early types of filter).

Oil Flow.JPG1591×785 187 KB

Note that the pickup pipe is sized to handle only the flow demanded by the engine (main bearings etc) NOT the pump flow capacity. The reason for this is that excess flow from the pump is bypassed back to the pump inlet via the cooler.

If the pump were to try to get all its flow from the pickup pipe, it is likely that it would cavitate at higher rpm (due to the vacuum formed trying to suck more flow than is possible through the small dia pipe). The effect of such cavitation is theoretically aeration in the pump output which would lead to loss of pressure (air bubbles can not support high pressure).

So , my theory is that a blockage in the cooler could cause cavitation and a sudden loss of pressure at higher rpm. Note that such blockage might be the result of the Pressure relief valve jamming, not necessarily a blockage in the cooler.

Perhaps a hydraulics engineer or fluids engineer could comment?

Cheers
Mark

10

Note that the Pressure Regulator valve is normally fully closed. When closed it will divert all pump output to the engine galleries.

As the pump output exceeds the engine demand, the excess flow should crack open the regulator and this flow then goes through the cooler and back to the pump inlet.

In theory the pump is delivering 2 - 3 times the flow required by the engine. Thus 2/3rds of the pump flow is short circuited back to the pump inlet via the cooler. (And 2/3rds of the oil is being cooled).

However, the sump pick up is only having to handle 1/3rd of the total pump flow capacity. I believe it is sized to optimise the oil velocity based upon this expected demand.

If anything happens to restrict the flow through the bypass circuit, the sump pickup will have to handle increased flow rate. Think - sucking a thickshake through a thin straw. I would guess it has about a 50% safety margin, before the poor old pump will suck so hard that it cavitates.

The funny thing is, that in the full flow cooling system, all the engine demand is cooled, while the regulator bypasses directly to the sump. The sump pickup is larger diameter to cater for the increased flow. BUT, only 1/3rd of the pump flow is now being cooled. I believe this enabled the oil to heat up quicker and run hotter for improved emissions.

The interesting question is “What happens at idle rpm?” And it is here that perhaps the bypass system suffers, with little flow being cooled, while the full flow still maintains at least some flow through the cooler.

Always open to correction or discussion.
Cheers
Mark

11

If an oil pump were to cavitate and thus aerate the oil, where would the air
to do that come from?

Mike Eck
New Jersey, USA
www.jaguarclock.com
'51 XK120 OTS, '62 3.8 MK2 MOD, '72 SIII E-Type 2+2

If the pump were to try to get all its flow from the pickup pipe, it is
likely that it would cavitate at higher rpm (due to the vacuum formed trying
to suck more flow than is possible through the small dia pipe). The effect
of such cavitation is theoretically aeration in the pump output which would
lead to loss of pressure (air bubbles can not support high pressure).

12

If an oil pump were to cavitate and thus aerate the oil, where would
the air to do that come from?

Cavitation is not air. Cavitation is drawing so much vacuum that you
vaporize the fluid, in this case the oil. It can be quite harmful to hardware, as
the vapor bubbles sometimes collapse quite suddenly to liquid once again
and can beat on pump impellers like a hammer.

– Kirbert

13

Hi Mike,
I’m no expert, but apparently the same place the air comes from in a soda bottle. It’s already there. The oil in the sump contains fuel, water and air. If pressure drops below the vapour pressure of the contiminant, then vapour (bubbles) will form.

This link is quite good. I’ve quoted from it below …

"> Cavitation is the formation and collapse of air cavities in liquid. When hydraulic fluid is pumped from a reservoir, pressure drops in the suction line to the pump. Despite what many people believe, the fluid is not sucked into the pump. Rather, it is pushed into the pump by atmospheric pressure.

The movement of the rotating elements causes a drop in pressure at the suction line. The resulting pressure difference between the reservoir and the pump inlet causes the fluid to move from the higher pressure in the reservoir to the lower in the pump’s suction line. As long as the pressure difference is sufficient and the flow path is clear, the operation goes smoothly, but anything that reduces the inlet flow can cause problems. Whenever the pump cannot get enough fluid to fill its inlet chamber, cavitation occurs.

Hydraulic oil contains approximately 9% of dissolved air. When the oil cannot flow into the pump fast enough to match pump’s intake volume, air is pulled out of the oil. The air bubbles travel into the pump, and when they reach an area of relatively high pressure, they collapse (implode). The resulting shock waves cause a steady, high-pitched whining sound and damage to the insides of the pump.

Fluid velocity is inversely proportional to the size of the hydraulic line. Most pumps have a suction line that is larger than the pressure line. This is done to keep inlet velocity low, making it very easy for oil to enter the pump. Any blockage, such as a plugged suction strainer or filter, can cause the pump to cavitate.

In the case of the bypass flow oil pump arrangement, the inlet “pipe” consists of the sump pick up AND the oil cooler return line. Both are combined at the inlet of the pump. It is presumably quite efficient, because any excess pump flow is returned (at pressure) back to the pump inlet. As I understand it, the pump was sized to deliver about 3 times the normal flow requirements. This allowed bearing wear to double engine demand and still leave a 1/3rd safety factor.

But if something were to block off the cooler, excess flow would short circuit to the sump through the relief valve, but the suction pipe would have to handle 3 times the normal flow.

Why not use a bigger suction pipe? I think hydraulic engineers use design targets for minimum velocity, as well as maximum velocity. I had in my mind between 3ft/s and 8 ft/s, but I’m not sure where those numbers came from!!

ME

14

JEEZ, lot of table conjecture here, seems nobody knows EXACTLY what the REAL problem was, and fixed.

years back when teaching Vocational school Engines, daytona FL.

i had students(18/40 yrs old), some times when they brought in there vehicles for a problem, thet had no idea what was wrong!

so they would do things under the hood, most of the time they cured the problem!

i came along with a saying, “GOOD THING YOUR HANDS ARE SMARTER THAN YOUR HEAD”

there head didnt fix it ,there hands did!! yeah i know everyone is gonna rationalize it there way.

15

Hi Murray
From what you’ve said I think it can only be either the oil pump efficiency due to internal wear or sloppy tolerances.Or the wrong size bearings fitted to big ends and or mains?
An oil pressure drop off to the degree you now describe, is not acceptable and needs resolving ASAP . I wouldn’t run it again until I’d found the cause! As I see it cavitation would only occur if created by another fault i.e. Incorrect assy or maybe excessive tolerance.oil pressure loss is always the result of a fairly basic fault!.