Oil light on.

[Mtrike]

Was out today and the oil light suddenly came on but only if the revs drop below 1900. There was no warning of this no flickering light etc just it came on at the first junction when it went to idle. Oil level was ok and now topped up with no change and it's the same now it's cooled down. There is no knocking or signs of oil starvation but I'm not pushing my luck in testing that! My first thoughts in order of likelihood are
1. The sender switch has failed - but why does the light come on at higher revs?
2. The pressure release valve is sticking open slightly
3. The pump is failing but again why suddenly with no warning but working at higher revs?
4. A shell has failed but why isn't it knocking?

Any other ideas?

I should add that because of the hot cam idle speed is 1500.

[Hans Efde]

I have had a serious leakage problem (hose to the oil cooler came loose) and I completely hadn't noticed it. My friend Ed saw a white cloud of smoke coming from my car and stopped and so prevented my engine seizing. There was only 0.5 liter of oil left, but I still had oil pressure and no oil warning light. Half a year later this engine was stripped down and I found the oil pump to be in bad shape (lots of scratches) and 1 bearing shell in pieces. Oil pressure was low at idling and maybe 25 bars at motorway speed. But never had an oil warning light.
So judging on this experience 3 and 4 are unlikely. I would guess a sticking pressure release valve (better ones are available from Minispares). Check it for dirt and damage. Also (but not probable) is an earth problem of the pressure switch.

[Mtrike]

Yes Hans I'm favouring the pressure release I'll take it out next week and flush/clean. Earth on the pressure switch is ok already checked. I'm sure I would have some more symptoms if there is any thing more serious. There is the possibility of a wiring short but unlikely as it would to be engine speed sensitive however the loom is shared with the rev counter so it might be possible. Mind you I'd rather do an engine overhaul than try to work out the wiring on a plastic kit car.

[ACourtney]

My money would be on the pressure relief valve too. From what you say the warning light goes out when you rev the engine, but it is insensitive to oil temperature. When the pump starts to wear you notice the pressure dropping off as the temperature rises, or at least you do if you have a pressure gauge. If the relief valve is slightly open then you will get a drop in pressure, which will be overcome by the pump as the revs build. This is usually due to dirt between the valve and its seat, but the aluminium ones can become worn or pitted. Some engine builders prefer the old ball bearing type of valve as they are harder and therefore less prone to damage. The other side of the coin is that they have a smaller contact area and therefore more sensitive to dirt.
I have also known the spring to go soft, but that showed a pressure drop at all revs.
I would be inclined to buy a replacement valve and spring as there are only three or four pounds. You're supposed to bed the valve in on its seat with some fine grinding paste, which is fine if you are rebuilding the engine, but not such a great idea if you are doing it in-situ. I would be inclined to have an engine cleaning brush to hand to try and some carb cleaner spray to clean the seat area with, just in case there is dirt or a build up of deposits.

[Mtrike]

It was the relief valve stuck open. It pushed back and is operating ok now. I couldn't get it out so will fashion an extractor that I found on the mini forum and then put the new one in. Oil and filter change as well. Unfortunately I still won't be at Donington because I'm taking part in a 125 mile cycle ride in the Cotswolds. Hopefully I will be at the Stanford Mini day though.