2009 Cooper Base hardtop
75,000 miles
Auto trans

I am having a complete nightmare of a time trying to figure out what is going on with this oil pressure light at idle. It will only come on after about 45 minutes of driving, presumably when every things at full operating temp, at a stop sign. the INSTANT I touch throttle it goes away until I come to a complete stop. If I am rolling in neutral with no load it is not present. This problem developed after I had water pump failure. I have checked compression across the board and everything was within about 6% of each other. I have checked the oil and found no alarming amounts of metal present. I recently recently did a timing chain job and there was no plastic guide pieces floating around in the oil. I replaced the oil pressure sensor on the side of the head with an OEM unit. Checked the seals on the oil cap, as well as the filter top o-ring. the spring is still present on my oil filter housing. The most effective thing i have tried is simply switching to a 10w-40 oil weight. and by successful I mean it comes on after 45 minute drive vs a 20 minute drive.I am not sure how you guys are testing oil pressure in these cars, since there is no way to get a socket around any fitting with the gauge hose present. I have to be missing something here. I cant keep dumping money blindly into this car any longer. I have been desperately been trying to sell it, but I wont let it go until I can sort out this issue. I have done so much countless hours of research into this stupid issue that never seems to get a definitive answer. any sort of direction will vastly help me at this point.

 

Can you upload a picture of the light that illuminates when you’re at idle?I can’t even pull up an oil pressure indication with my ScanGauge II or my Foxwell scanner. I’m not sure the N12 engine has an oil pressure transmitter; I think it only has an oil pressure switch.

If I were in your situation, I would remove the oil pressure switch and install a direct reading gauge so you could see what the actual oil pressure.

 

 

I just did some digging around on the internet. Confirmed, item 7 in the picture above is a pressure switch (on or off) not a sending unit (variable pressure transmitter). The switch contacts open at 8.7 (.6 bar) PSI, pressure increasing and close at 7.2 (.5 bar) psi, pressure decreasing. Install a direct reading pressure gauge into the pressure switch boss on the cylinder head and see what your oil pressure is at idle.

Obviously, the oil pressure light won't work when the switch is removed; if your oil pressure is greater than 8.7 PSI at idle, and the light comes on when you reinstall the switch, it's a faulty switch. If your oil pressure is below 7.2 PSI at idle, you have a problem with the oil pump.

 

Try this kit. Notice how the gauge has a bleed valve so you can bleed the air out after you've connected it. That will make sure you get an accurate reading.

 

 

 

This is oil pressure after 1hr of driving at idle, however, the light is NOT coming on at the moment. After I took the picture it leveled off right at about 10psi. Maybe a hair under but it never dipped under the .5 bar hash

 

 

 

Although I'm new to minis I've been working on engines for almost 40 years. 10 psi per thousand rpm is the usual objective. A combination of a slightly worn engine and a slightly worn oil pump could be the cause of your problem. The easy fix is to either add a couple washers to the pressure release spring in the oil pump or just stretch the spring a bit. From looking at some oil pumps for the n14 engine it does look possible to do. If that doesn't fix it or cause no change then you either have a large internal oil leak or your engine is reaching the end of its useful life.
what are your pressures at 2000 3000 and 4000 rpm?

 

Personal experience from a 2004 BMW 325. Exact same symptoms, just not sure if it crosses over to your car.
Replaced the low oil pressure switch. Problem solved.

 

I worked at a factory, we had lots of brand new parts that were bad out of the box.

You could test your switch by using a tee, small pump, your mechanical gauge, & a ohm meter. Oil too . . .

Or an even easier plan is to put in a tee, have both the guage & switch active, so when light comes on, you can read pressure right then.

 

 

sounds like the motor is getting too hot to me.. there should be no way a 10-40 would cause a low pressure scenario.. and if it did.. its likely too hot/thin

*oil pressure at idle is fine
*switching to the 10-40 just allowed you to maintain your oil pressure better due to the heat (very common for track cars to use 10-60 even for this reason + high temps... a hot oil flows easy, so there is less pressure in the system)

Q- whats your thermostat read? +230c?

 

you're doing everything correctly while searching for an answer. If you live in a hot year round climate 20w40 is fine. I wouldn't use it in a cold winter climate. Do some searches on the oil pump spring. This is the easiest way to bump pressure after thicker oil. You should be able to access the pump spring by just removing the oil pan. What is the oil pressure at all rpm's? With the pan removed you will also be able to inspect the oil pickup screen for garbage. You can buy a new spring, stretch the old one or put washers in it to boost oil pressure. If the spring has weakened it allows oil to dump back to the pan bypassing the engine.