Dropping oil pressure, a riddle for you...

[3xpendable]

My brother has finished building a wet lined 998cc engine for his Imp. It runs fine and he's done about 100 miles so far in it but is very concerned with the oil pressure.

Before fitting this engine he retrofitted a few gauges including an oil pressure one. The old engine ran about 20psi (indicated) when warm and at idle. The new engine is fine whens tarted up, but when it gets warm its only showing about 20psi when driving and one time he was idling in traffic the gauge just sudenly dropped to zero! (but the warning light didnt come on).

As I've said, the engine runs fine with no signs or sounds that you'd associate with a low oil pressure. He's changed the relief valve, and the gauge/sender unit are the ones he was using on the car before and the oil pump is new. Any ideas folks?

[TerryG]

Has he / you put a proper gauge on it to see what pressure it is actually generating or are you relying on the dash one?
I bought a really cheapo gauge on ebay a while ago and it has been used a dozen times since on various cars.

[Luxobarge]

As above. Also, what oil specification is he using?

[JPB]

Is the relief valve the one with the seat directly set into the aluminium and held there by Araldite or is it the cassette type? If the former, dig it out, throw it in the furnace and fit a cassette one, job done.

[3xpendable]

Well, for once he realised his little brother knows what he is talking about and decided to listen to me when I said what you guys have, which was the have another gauge/sender or a capilary unit, here is his response:

Thanks for that. I fitted a mechanical oil pressure gauge yesterday and only got to the bottom of the village before it blew the pipe off and left an oil slick 50 yards down the road and redecorated the engine bay! I did manage to get some static readings off it which indicated the pressure to be nearly 50psi at 2500rpm when hot which is about right so I think the sender may be dicky.

Thanks all.

[JPB]

Yes, it must be kaput, it should require a darned sight more than 50psi to blow a capillary off, especially as the area is only around 1/300 of a square inch. Unless some one attempted to make their own replacement with rubber pipe or fitted an end without the correct dies. But the actual pressure is still unknown and the reason it's so high can only be the anti draindown valve sticking (unlikely if he's followed best practice for tuned Imps and fitted the spin on Volvo filter, slightly more likely if it's an unbranded one) or the relief valve sticking. Low pressure shows that the seat has become loose, high and the ball is sticking, both symptoms demonstrate that the valve is the old separates type. The cassettes are like hen's teeth though, so prepare to make a temporary fix to the four piece by finding a ball the right size and/or using some high temperature epoxy in place of the factory Araldite around the seat.