Low oil pressure at Idle

[Cakey]

My oil pressure light lights up at idle & the car starts to sound like death.

Just replaced the head after overheating & cracking it. Been having troubles replacing it since i dont know wtf im doing but i enjoy "fixing" it. Most days :)

Changed the head gasket and milk shake still poured out.
Took it in and they quoted me 3k to change my head.
Bought a head from CL but it had 33mm intake valves instead of 30mm.
Think its from a M44, Could this be the cause of my low oil pressure?

Tried to take off my lower oil pan but failed after 20minutes and bolted everything back up. Starting to think i might of fucked up the gasket and losing pressure there.

Might of blown a gasket somewhere else when it overheated but where?

Also read you need to prime the oil pump but i'm pretty sure im getting pressure, not that ive tested it with a gauge but the light is off on cold starts & Sounds pretty good then starts to lose pressure & sound shitty after it heats up. Idk what that could mean.

Thinking about changing the lower oil pan gasket, the oil filter housing gasket, thicker oil and oil pressure switch. Anything else that i could try?

[ttrousdell]

Chances are if your have run it with the oil light on and it is making awful sounds you have already done major damage to your bearings and engine.
It makes sense that the light would not be on at first start. The oil is cold and the pump has to push hard to get that viscous oil through the engine. You can expect startup pressures of around 65 at idle. When the oil is hot that pressure will be around 25psi at idle. I'd say throw a pressure gauge on and if the pressure starts to drop below 20 at idle then it's confirmed that you have a low oil pressure condition.
I don't know if mating a m44 head to an m42 block will cause a oil passage to not line up or something to cause lower pressure. Take the oil cap off when it's running. Oil should be splashing everywhere. Is a leak causing the loss of pressure? Idk are you loosing oil out of the sump? Is the coolant a milkshake? If the oil level is slowly going down maybe. If the oil level hasn't changed then a failing pump or clogged pickup would be my guess.

[varg]

Most common causes of lost oil pressure in M42s:
Failed oil pressure relief valve
Failed oil pan gasket allowing the pump to suck air

M42 intake valves are 33mm.

[Cakey]

Oil splashes out when its idling.

Sounds like shit for 3 seconds and then idles fine after a cold start. I let it sit and idle for 25 minutes and nothing but the 2 times ive been out driving with it the light would come on and start to idle like shit at stops.

Going to start with the lower pan gasket , oil filter gasket, oil pressure switch and 20w50 oil
Running 5w30 right now.

Looks like m42 are 33mm, the head i cracked had 30mm. :)

[e30davie]

what oil does bmw recomend for that engine?

My initial guesstimate is that 20w50 might be a bit thick. m42 is a pretty trick little engine compared to say the m20. I would guess maybe 10w40 for the m42?

[roguetoaster]

OP 20w-50 is fine, as is anything within the spec sheet listed in your owner's manual. It probably won't be enough to fix your issue, but it's worth a shot.

I'd also take off the valve cover and have a look at the cams before you spend money on the engine. If they're scored badly the bottom end would probably look the same.