Lost oil pressure on freeway

[idaho944]

Engine started running a little rough, oil pressure gauge was on zero so I turned the car off and coasted to the side of the road. There’s still oil in the engine, might be low but it was full this morning and there’s no sign of a leak. Tried to restart and it started briefly, oil pressure made it to the top of the gauge then dropped back down to zero and the car shut off. First time the 944 has put me on the side of the road

Any guesses?


Thanks!

[idaho944]

Left out a critical bit that there’s charging light on the dash started flashing right before we shut down. I’m suspecting an electrical issue or maybe serpentine belt tension too loose after driving for hours

[kdjones2000]

Sounds electrical to me...

[jmitro]

running rough and no oil pressure makes me think it's a rod bearing

[Jfrahm]

The belt drive system could be affected by a loose crank bolt, which would also kill oil pressure.

[9FF]

Its usual for the charging light to flicker when the engine revs drop and it’s about to stall. If your oil level is now low and it was full when you started off this morning like you said, then you are losing oil from somewhere so check around for leaks. If you don’t see any external leaks then there may be oil in your coolant or in your combustion chamber. Check for milkshake in your reservoir and oil on your plugs.

[walfreyydo]

Originally Posted by jmitro

running rough and no oil pressure makes me think it's a rod bearing

+1, although that is worse case scenario

Do you hear a rapid, pronounced tap that increases or decreases with engine rpm? (spun bearing)

Other than that, it could also indicate an issue with the oil pickup tube, or faulty oil pump/oil pump drive gear (rare on these cars).

Best case scenario its just a bad oil pressure sending unit. I would test the existing one, or replace with known working unit and see what happens.

[9FF]

Originally Posted by walfreyydo

+1, although that is worse case scenario

Do you hear a rapid, pronounced tap that increases or decreases with engine rpm? (spun bearing)

Other than that, it could also indicate an issue with the oil pickup tube, or faulty oil pump/oil pump drive gear (rare on these cars).

Best case scenario its just a bad oil pressure sending unit. I would test the existing one, or replace with known working unit and see what happens.

Confused at some of these answers. Op said he lost oil and it’s now low:

“There’s still oil in the engine, might be low but it was full this morning”

Please explain how would rod bearings, oil pickup tube, oil pump or oil pressure sender be the cause?

...Maybe I’m missing something, or we need to know how low it is compared to it being full earlier that day, idk.

[walfreyydo]

He actually never confirmed if it was low, (said it might be) but that would be good to know as a first step before moving further.

Anytime you have low oil pressure scenario its either a false reading from the sender, low oil, or some internal mechanical issue- such as an issue with the oil pump itself, oil pickup, etc.

1) Bad oil sending unit would (obviously) not be transmitting the correct pressure to the gauge in the dash, leading one to believe a false reading of zero pressure on dash.

2)The rod bearings themselves act as a restriction which (intentionally) keeps oil pressure high, due to the small hole which the oil flows from the crank journal onto the bearing surface. When that restriction is removed, such as when you spin a rod bearing, or bearing clearances are too high (for many reasons), oil pressure drops because there is no longer that bearing as a restriction. This restriction is necessary to keep the oil pressurized as the oil pump needs something to pressurize against.

3) Oil pickup tube crack should be self evident as to why you would lose oil pressure (try to drink something with a hole in your straw)

4) Oil pump or oil pump drive gear stripped = not pumping oil, nuff said. Crank pulley could be loose, but unlikely.

"There’s still oil in the engine, might be low but it was full this morning and there’s no sign of a leak."
Burning that much oil in 1 day would be very evident. Oil mixing with coolant although bad, wouldnt show a drop in oil pressure (although it could cause damage to bearings).

The very first thing the OP should do is to make sure you have adequate oil by checking the dipstick (not just a little bit low, but so low its not even registering on dipstick) and then second, eliminate false readings from a potentially bad sender by hooking up an oil pressure gauge (or known good sender) to confirm whether or not its the sending unit. This is probably the most likely scenario assuming oil levels are sufficient.

However... if oil pressure is confirmed to still be low with secondary sensor, then next steps are to check the other possible causes of low oil pressure which I have already mentioned. Losing oil pressure can be catastrophic even for short periods due to bearings now coming in contact with the crank journal and not having that thin layer of oil to float on. Hopefully that is not the case here.

Keep in mind that when these cars are cold, oil pressure should be high (~4-5bar) and as the oil warms up, pressure drops (~2.5ish) due to the lower viscosity (see point 1).

[azbanks]

Originally Posted by Jfrahm

The belt drive system could be affected by a loose crank bolt, which would also kill oil pressure.

I agree.

With a loose crank bolt, you will lose oil pressure and the drive pulley for the alternator won't turn with the crankshaft. = no oil pressure and no electricity

Check to see if the oil is actually low.