Strange oil pressure issue

nmacc

Hi folks, hoping someone can help here, because I'm stumped.

I was asked to look over a 54,000 mile 2003 998cc K12 at a small dealer's for a cousin of mine. Drives well, feels and sounds like a 54,000 mile car. There were a few small issues which the dealer sorted, but my suspicions were raised as, once he had the deposit, he seemed extremely keen to get it off his hands, to the point of delivering it 25 miles to my house.

The next morning an issue became apparent: when started from cold it takes 12 seconds at idle before the oil pressure light goes out. During this time the engine is very quiet with no top end noise. In contrast, when the engine is hot, the light goes out instantly.

Upon collecting the car from me, the cousin dropped it to his local mechanic who offered the same diagnosis I did: a partially blocked or faulty oil pressure switch, but it seems, not so. The mechanic dropped the sump and cleaned the (already clean) strainer, then he flushed out the gallery, changed the oil and filter and finally replaced the oil pressure switch - all to no avail. He put a gauge on to it and said the pressure was fine, but I suspect the engine was probably hot at that stage. I'm a little vague on this as he's on the other side of the country and I'm getting this second-hand.

He also noticed that the sump and timing cover were sealed with gasket goo. I should have spotted this; clearly I'm getting old.

The car seems fine in every other way and is perfect when hot. There is no evidence of wear or noise. This is really bugging me and I'd be grateful for any suggestions.

Thanks.

porsche boy

What you need to do there is put a mechanical oi pressure gauge on the engine and crank it when cold to see what oil pressure us occurring. Have you considered the oil pump might be going?

nmacc

porsche boy wrote: »

What you need to do there is put a mechanical oi pressure gauge on the engine and crank it when cold to see what oil pressure us occurring. Have you considered the oil pump might be going?

Well, there is an issue that the car is in rural Mayo and I'm in Wicklow, which somewhat limits my options there.

The info above is all I have to work with, and I can't come up with a reason why the pressure would be inferior when cold, but I'm open to suggestions. If this is a characteristic of a failing oil pump it could be the answer; can you elaborate?

There is one very old idea that hasn't been suggested and I say this at the risk of shutting down further discussion, but a cheap oil filter without a non-return valve could possibly give this symptom by draining down overnight and taking time to fill.

I went yesterday to a Nissan main dealer, bought a K12 filter and posted it to Mayo. If that solves it, great, but the filter has already been changed -presumably for a generic - without fixing the problem. On the other hand, if there is an NRV in the block which is missing or broken, then the genuine filter won't help.

Waterson

As above, screw an oil pressure gauge into the pressure switch and crank the engine when cold and check the pressures. Oil pressure itself is generated by the clearance betwwen crank big end and main bearings. Oil pump only provides a volume of oil around the engine.

CJhaughey

Waterson wrote: »

As above, screw an oil pressure gauge into the pressure switch and crank the engine when cold and check the pressures. Oil pressure itself is generated by the clearance betwwen crank big end and main bearings. Oil pump only provides a volume of oil around the engine.

I have never heard of an engine having oil pressure generated by the clearance between bearings, the main bearings are all lubricated by the oil pump, this pumps oil into the galleries, big ends are splash lubricated for the most part or perhaps have oil jets spraying on to them to lube them.
The oil pump provides the oil pressure to allow the bearings to run on a film of oil. Any bearing that has enough clearance to allow a pumping action will not last long.

Ded_Zebra

CJhaughey wrote: »

I have never heard of an engine having oil pressure generated by the clearance between bearings, the main bearings are all lubricated by the oil pump, this pumps oil into the galleries, big ends are splash lubricated for the most part or perhaps have oil jets spraying on to them to lube them.
The oil pump provides the oil pressure to allow the bearings to run on a film of oil. Any bearing that has enough clearance to allow a pumping action will not last long.

Pressure is caused by resistance to flow. The pump generates flow and the bearing clearance creates the resistance to the flow. That's why people say the pressure is caused by the bearings.

I had a Nissan Primera for a while that would take about 4-5 seconds for the oil pressure light to go out on. engine would be rattly too during that time. Similarly it only did it on cold start, hot starts were fine. There was 370,000 odd miles on it though.

When you say it takes 12 seconds is it actually 12 seconds when you measure it? I can tell you that the 5 seconds in my car was plenty long when yu were waiting for it to go out.

jca

CJhaughey wrote: »

I have never heard of an engine having oil pressure generated by the clearance between bearings, the main bearings are all lubricated by the oil pump, this pumps oil into the galleries, big ends are splash lubricated for the most part or perhaps have oil jets spraying on to them to lube them.
The oil pump provides the oil pressure to allow the bearings to run on a film of oil. Any bearing that has enough clearance to allow a pumping action will not last long.

Well you're in the wrong field if you've never heard of bearing clearances determining oil pressure, because that's exactly what shell bearings need and do. Big end bearings aren't splash lubricated, well maybe in a Briggs and Stratton side valve engine but in modern engines there are drill ways in the crankshaft that directs oil from a main bearing out to the big end journal. Op you could be lucky with fitting a genuine oil filter maybe it's just oil draining down overnight. As others have said a proper mechanical oil pressure gauge would tell you a lot. If it was a very worn main bearing that was slowing the pressure build up you certainly would have other symptoms when the engine was hot and the oil at its thinnest, flickering oil light while idling etc.