Low Oil pressure - just confirming my worst fears !

[George's Keeper]

Hi and happy New Year to all for next week.

I'm just posting in the hope someone may have some better ideas than me before I take my engine to bits..

Car is a 1098 with mildly modified engine built by Geoff at Autosprint in Birmingham a few years ago but used only very moderately since.
Flowed head, 266 cam, lightened and balanced etc etc. always run up to temp carefully and regular oil changes with 20 / 50 oil.

Low oil pressure light has started to show regularly and bottom end sounds rough. Car has not been run on the road with this issue as I'm worried about the main bearings so have tried......

Fitting oil pressure guage - Reading bears out the low pressure shown by the light
Compression test about equal across all 4 cylinders ( about 80 - 90 PSI )
no obvious leaks or puddles under car
When I prime the oil pump by removing copper banjo to the spin on oil filter, filling the block gallery with oil and hand turning engine over ( backwards), then quickly refitting pipework - the oil light goes out and pressure gauge shows about 50 PSI oil pressure.
When I come back a couple of days later the oil light is back on again and I have to go through the whole process again!

I'm concerned that a bearing has gone but am not sure why priming the oil pump would temporarily solve the problem....or could it be the oil pump itself is at fault although it was a new replacement pump when the engine was rebuilt by a reputable chap about 1500 miles ago?
Unfortunately this engine rebuild was about 7 or 8 years ago and the car has been off the road for 5 of those years while I restored the shell.. so its well and truly out of any warranty period.

My next move is to strip the engine unless anyone out there has any bright ideas! I learnt about the oil pump priming trick from the MMOC forums and was delighted when the light extinguished first time round...but obvs this was fooling me into false hope ! (
Cheers all
Dave G - Staffs

[oliver90owner]

Does it knock or rattle at start up, after standing for a day?

What is the oil pressure?

Does the pressure drop as the engine warms?

Is the low pressure problem solved by increased revs?

A leak-down test would better determine if the problem is bearings or oil pump related (may be both by now, of course).

It may also be the PRV leaking or simply stuck open.

[George's Keeper]

Hi Oliver,
Thanks for the reply..

When I've done the oil pump prime rigmarole it sounds OK and engine seems to run A1. I've though of doing the prime process, getting the engine up to temp and then running at high revs to listen for bearing knock but to be hones I'm not sure at what point the sound goes from the modified engine / fat exhaust / induction noise to bearing knock - will it sound very obvious if big ends are gone or can it be more subtle but still cause oil pressure issue ?

When left for a day or two it sounds rough on startup and oil pressure doesn't register on Oil Gauge - Obviously oil light is on !
I don't really want to go all out on revs but, I've tried giving it a few seconds to settle after start up and then increasing revs to about 2000 and this has no effect on oil pressure or noise as long as oil pressure is up and running first. I'm obviously wary of doing this for too long although I suspect that if the oil isn't circulating for some reason, then I'm probably looking at a toasted engine anyway..

I've also had the PRV and spring out and all seemed to be moving as normal although this would tie in with the fact the problem first appeared after the car had been standing unused and not started for about 6 weeks - I though the spiring may have got gummed up but it looked OK.

hope this helps...?

[philthehill]

If you have no oil pressure showing on the gauge YOU SHOULD NOT RUN the engine - you will only make things worse.
You need to grasp the nettle and check the bearings. The big ends can be checked without taking the engine out and they should be checked first, To check the front and rear mains you need to take out and dismantle the engine. The centre main can be checked at the same time as the big ends.
There may be a problem with the oil pump which means taking out the engine.
The gear of the oil pump can come loose on drive shaft if not keyed. This is a well known problem.
As regards priming the pump to get oil pressure you are bypassing the suction function of the pump. It could be that the oil pump is not drawing up oil from the sump possibly because of a loose gear.
What ever happens you do need to get that engine out and stripped and examined and the sooner the better by the sounds of it.
As an aside those compression figures are terrible and should be in the region of 150(plus) lb/ins sq
Phil

[George's Keeper]

Thanks Phil,

Everything you say makes absolute sense and I agree - TBH I had assumed the engine needs to come out for inspection and refurb already - I was just hoping against hope that there may be another explanation as I know how fiddly getting the sump off can be on axle stands ! I'll probably do the engine out job rather than try going in from underneath as I don't really trust the oil pump / drive any more and would prefer to see the whole picture..

I've not run the engine any more than was necessary just to confirm the oil gauge is working - the pressures were checked some weeks ago when I initially had the first light come on and I hadn't put two and two together ! Obvs the car is off the road at the moment and not in use. ...!

Thanks anyway - At least I'll feel it's justifiable setting to with the spanners now as there doesn't seem to be any other obvious answer !!

[les]

Unless it’s something silly, it’s not what you’d expect from a rebuilt engine that’s done 1500 miles. Play safe and do as Phil suggests.

[oliver90owner]

He’s got 50 psi oil pressure if the pump is primed and the engine started. Certainly if the engine has been run with no lube, the bearings may require changing. But for diagnosis, 50 psi, if the engine has not been run with no pressure (it only needs about 5 psi to lube plain bearings) would seem reasonable. I suspect there may be a leak on the inlet side to the pump thus losing prime. But better to be very conservative, if needing to post a thread on the forum, I suppose.

I once drove my 1650cc Ford Escort 70 miles on under-sized OD main bearing shells by not stressing the engine and not letting the rpm drop too low. At the time, I had no idea of the problem but needed to get home (after building the engine in a hurry and fitting it in a cowshed on a Sunday afternoon). No damage done.

[George's Keeper]

Thanks all, Looks like I'm getting the spanners out again !

At the moment It seems to me like the oil pump is the culprit due to the change in symptoms when the car is left for a day or two. As Les said though I was just hoping someone may come up with something daft that I'd not thought of.

I'll strip it down and repost with an update when I get to the bottom of it.
Best wishes
Dave G

[philthehill]

I would also suggest that the oil is draining back down from the oil pump into the sump and the pump is unable to create enough suck to pull the oil back up from the sump. That is why you are having to prime the oil pump each time.
When you have the oil pump out I would measure the clearances of the rotor, end float & body clearance.
Also check the paper gasket between oil pump and block. It must be in good condition and the right way round. You could be sucking in air from the crankcase if the gasket is not in good condition.

These are the dimensions you should be looking for when checking the oil pump condition:-
Outer ring end float................................................... 0.005".
Inner rotor end float...................................................0.005".
Outer ring to pump body diametrical clearance.............0.010".
Rotor lobe clearance...................................................0.006"

Any scoring of the parts will not help either.
It should be noted that there are good oil pumps and there are mediocre oil pumps. Fit the best you can afford as it is the beating heart of the engine.