Sudden drop of oil pressure

E

erubus

Member
Hi guys, when I started the car today, I noticed the oil pressure gauge hadn't moved. No oil pressure light though.

I tapped the gauge and it slowly rose to its usual 28-20psi. After a short drive though, the pressure was noticeably dropping off to the point it was running at an indicated 5 or so psi *- hard to say because the gauge resolution is in 30,psi increments.

As soon as the engine gets to 700ish rpm the oil pressure light is coming on.

There were no issues last night, there's plenty oil, about halfway between high and low on the dipstick and the lifters aren't rattling except for the one dodgy one on start-up. It takes 30 seconds to a minute to stop ticking.

Any ideas what could cause sudden loss of oil pressure like that? I doubt I've spun a bearing because there's no unusual noise from the engine.

It could just be the gauge but the oil pressure light is coming on as well. It always came on at idle when the engine was hot but only after maybe 10 seconds of 550rpm idle, now it's coming on instantly and the engine wasn't even particularly hot. If the oil filter clogged and it went onto the bypass, the oil pressure should still stay up shouldn't it? Just wondering if there's been a bit of sludge or something picked up, although I doubt it

Looks like I'm going to have to look at the spare engine sooner than I thought.
 
As you say the oil warning light is coming on so it isn't just the gauge. If the filter were to be blocked, the pressure to open the bypass valve in the front cover is less that the required pressure to push the oil through the filter, so the oil bypasses the filter continuing to circulate through the engine as unfiltered oil. There is a pressure relief valve in the form of a plunger/piston that is housed within the oil pump front cover. It is not unheard of for this valve to stick open. When this occurs your engine will be running with very little oil pressure. This would explain what your engine is experiencing. The pressure relief valve is accessible by undoing one of the large flat nuts on the oil pump front cover.

Ron.
 
It's worth checking the pressure relief valve. Id kinda discounted if in my head as the pressure has never been more than 30 on the gauge, so I wouldn't have expected it to have moved.

It's weird that the gauge wasn't working to start with. That's the bit that's confusing me, unless it was coincidence that it started working when I was tapping it and the engine was somehow running with no pressure but no oil light either.

Id had the lid of the brake fluid reservoir off so it's not inconceivable that I'd disturbed an iffy wire in the loom, although highly unlikely.
 
I assume any work on the oil pump, including taking the pressure relief apart to check will require the pump to be packed with vaseline and reprimed? I'll have a look at the pump on the spare engine first. If it's in good condition I ll got the bigger gears if it doesn't already have them (SD1 front cover I think) and just swap the bottom halves over. The dizzy drives will lost likely be different.

I'm actually starting to think it might be worth just swapping the whole engine if the bearings are good.
 
erubus said:
I assume any work on the oil pump, including taking the pressure relief apart to check will require the pump to be packed with vaseline and reprimed? I'll have a look at the pump on the spare engine first. If it's in good condition I ll got the bigger gears if it doesn't already have them (SD1 front cover I think) and just swap the bottom halves over. The dizzy drives will lost likely be different.

I'm actually starting to think it might be worth just swapping the whole engine if the bearings are good.
Click to expand...
I recall reading about an alternative to the standard oil pressure relief valve some years ago. This may well be the way you should proceed.

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If the inside of your existing engine is black, it would be a better proposition to replace the engine with the spare that you have, assuming it is not black inside.

Ron
 
In that sense do you mean black? Burnt oily deposits on the internal block surfaces?

The existing engine is certainly tired, I know that much. All I know of the spare one is what the guy I got it from said - that it was a good runner, excellent oil pressure and no lifter tick. It's a P5 engine but the front cover and sump are from an engine that came from "some sort of land rover"

He'd bought a fully rebuilt engine and fitted it to the P5 with the P5 front cover and sump (and annoyingly the 4 barrel carb that was on it).

I haven't opened it up yet so I don't know what like it is inside.
 
Yes, black as in appearing to be covered in shiny black tar. That appearance is primarily attributable to infreqent oil and filter changes.

Your spare engine with its Land Rover timing cover and sump won't fit in the P6 engine bay, but I assume that you were already aware of that. The only real issue with the Land Rover timing covers is the water pump foot print. It is not compatible with the P6B water pump. The P6B water pump is a short nose design whereas the Land Rover water pumps aside from all being different also featured a long nose.

Ron
 
Try replacing the oil filter as first move. A dodgy non-return valve will let the oil drain back out. Then check your pressure relief valve isn't stuck open...
 
mikecoombs said:
Try replacing the oil filter as first move. A dodgy non-return valve will let the oil drain back out. Then check your pressure relief valve isn't stuck open...
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Hi Mike,

Sorry, but I do not agree. The non-return valve in an oil filter is the rubber section beneath the ring of small holes (the oil inlet). Even if the rubber didn't spring back after the engine was switched off, for oil to exit from those holes it needs to flow uphill against gravity. Any issue with the oil filter will not result in next to no oil pressure in the engine.

Ron
 
My reasoning is the oil can still syphon through as the oil level in the sump is generally lower than that in the filter and pump. Air only has to get to pump level before a worn pump will lose suction. The valve isn't on all filters, only certain ones where drainback is an issue.
 
SydneyRoverP6B said:
Yes, black as in appearing to be covered in shiny black tar. That appearance is primarily attributable to infreqent oil and filter changes.

Your spare engine with its Land Rover timing cover and sump won't fit in the P6 engine bay, but I assume that you were already aware of that. The only real issue with the Land Rover timing covers is the water pump foot print. It is not compatible with the P6B water pump. The P6B water pump is a short nose design whereas the Land Rover water pumps aside from all being different also featured a long nose.

Ron
Click to expand...
Yes, it's the long nose water pump timing cover on the spare engine. The engine itself is from a P5B and a 10.5:1 engine. The engine it was swapped with in the P5 was from "some sort of land rover" and I suspect, probably an SD1 originally. It was certainly late enough not to have provision for a mechanical fuel pump.

The front cover and sump were swapped over to allow that engine to fit the P5. The later cover is only loosely bolted to the engine and if be swapping the front covers before fitting the engine.
mikecoombs said:
Try replacing the oil filter as first move. A dodgy non-return valve will let the oil drain back out. Then check your pressure relief valve isn't stuck open...
Click to expand...

I don't think the (K&N) filter that's fitted has a non-return valve. I've yet to find a filter with the correct thread, which is why I'm thinking to swap the filter section off the spare engine and fit the bigger pump gears while I'm at it. I can then fit one of the ¾" filters I've got.

I'm still dubious as to how the pressure relief could lift enough to stick on an engine that struggled to get close to 30psi cold but it's also the only reason I can think of for it to be fine when parked at night and not the next day.

Air somehow getting into the pump does fit the circumstances, assuming the gauge starting to work when I tapped it was just coincidence. I would have thought the oil pressure light would have come on though if it was sucking air.

I'm not sure how much time I'll have today to look at it but hopefully I can get the bits stripped off the spare engine ready to fit on Monday as I've got other stuff to do tomorrow.
 
erubus said:
I don't think the (K&N) filter that's fitted has a non-return valve.
Click to expand...
It does have the non-return valve. The filter manufacturers make it sound fancy but it is nothing more than a rubber flap. When you look at the face of the filter you will see a ring of small holes and in the centre the large threaded hole where it attaches to the spigot. The inlet to the filter is through the ring of small holes and directly beneath them is a rubber flap. The pressure exerted by the oil pushes the flap aside, passing though the filter media and then returning via the central chamber. When not running the flap springs back to cover the small holes thereby helping to prevent oil from leaking back out from the direction in which it came.

Ron.
 
The filter fitted to mine doesn't have the one way valve. You could see right through the holes on the top. The Mahle and Mann ones both have the NRV just the wrong size thread.

I just started the engine. Pressure gauge stayed at the bottom, started moving after tapping it. Got to 25ish and did a couple of weeks jumps as if it was skipping teeth (I know there aren't any gears or teeth to skip) and stayed at that level.

As soon as I let the choke in a little and the RPMs slowed down to about 1200, the needle dropped like a stone and didn't come back up with a few revs. Oil pressure light didn't come on.

Ill fit the gauge out the parts car and see if there's any difference but the needles have been painted with a god awful fluorescent orange paint so they won't be staying!
 
So you're not using a HP 2003? What type of K & N filter are you using?

Now we are getting somewhere. No oil pressure but oil light not on indicates the pressure transmitter is the problem.

Ron.
 
It is an HP 2003 but it doesn't have an NRV, unless it's broken up and gone into the filter cannister.

The oil pressure light was coming on yesterday, I just wasn't giving it a chance to today and switched the engine off when the gauge plummeted.

I'll have to try it with the gauge out the parts car and see if there's any difference.

I presume if I take the pressure relief valve out (which looks incredibly awkward die to the cross member) I'll have to pack the oil pump with vaseline and reprime it?
 
erubus said:
It is an HP 2003 but it doesn't have an NRV, unless it's broken up and gone into the filter cannister.

The oil pressure light was coming on yesterday, I just wasn't giving it a chance to today and switched the engine off when the gauge plummeted.

I'll have to try it with the gauge out the parts car and see if there's any difference.

I presume if I take the pressure relief valve out (which looks incredibly awkward die to the cross member) I'll have to pack the oil pump with vaseline and reprime it?
Click to expand...
If you remove the front valance you can then remove the radiator support crossmember, two bolts each side, this will give better access to the pump.
 
erubus said:
It is an HP 2003 but it doesn't have an NRV, unless it's broken up and gone into the filter cannister.

The oil pressure light was coming on yesterday, I just wasn't giving it a chance to today and switched the engine off when the gauge plummeted.

I'll have to try it with the gauge out the parts car and see if there's any difference.

I presume if I take the pressure relief valve out (which looks incredibly awkward die to the cross member) I'll have to pack the oil pump with vaseline and reprime it?
Click to expand...
Genuine K & N HP 2003 filters all have the anti drain back valve. I had not heard of this until now but it looks very like you have bought a fake (or rather been sold a fake). The image is for the HP 2003.
KNHP2003_3_L_800x.jpg

Yes, pack it with vaseline.

Ron
 
SydneyRoverP6B said:
Genuine K & N HP 2003 filters all have the anti drain back valve. I had not heard of this until now but it looks very like you have bought a fake (or rather been sold a fake). The image is for the HP 2003.
View attachment 28673

Yes, pack it with vaseline.

Ron
Click to expand...
It looks genuine enough. I have no idea how old it is though. The car was sitting in a hedge for over a decade so it's at least that old.

I bought a Mann filter today at a motor factors which does have the correct thread but the filter itself looks touch and go if it will fit lengthways.

If I need to pack the pump with vaseline anyway, I might as well take it apart completely and potentially swap it for the other one if it's in better condition.

I'll try the spare gauge and see if I get a more favourable reading on the off chance I've just got a gauge that's gone bad. I'll not get to any of that until Monday though probably.
 
There's so much counterfeit stuff about that you need to get items from reputable dealers that will stand behind their wares. If they can counterfeit the items, whatever it may be, then they can do the same to printing and packaging. I know it's wrong and a shame, but we need to protect ourselves and wallets.
 
TL/DR: It was a dodgy gauge.

I spent the day helping do a few jobs on my friend's P6, 2000 and P4, 90. I also stripped absolutely everything that could possibly be useful off the parts car before it's scrapped next week.

I wanted the rear calipers but everything was seized up. I cut the boot floor out and still couldn't get them, so I took the whole diff out. Then I thought the de Dion tube looked like brand new ends and the training arms were heavily surface corroded but good and solid with a judicious twatting with a 3/4lb ball pein, so off all that came too.

Plus I took the rear wings to make repair panels and the whole front corner either side to weld up into repair panels for mine.

I found the proper air box and got a pair of brand new old stock rear wings. Few other odds and ends.
20260628_170117.jpg20260628_172353.jpg

Anyway....

Got all that loaded into the shed and decided to try the oil gauge from the parts car. It didn't work.

I looked closely at the mechanism on the original one and the needle wasn't quite sitting right. Don't know how that could have given it the symptoms it had, but I carefully rebalanced it on its tiny pivot and all appears to be well again.

I also adjusted the idle a little to keep the idle above the point where the oil pressure light comes on in traffic. It's a tad fast at 750 indicated but better than the oil pressure dying.

Panic over and I don't have to rush an engine rebuild on the spare engine.
 
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