Low oil pressure

I have a 1999 Suburban, 135,000 mi. Lately after long running time,‘I travel a lot for work’ the oil pressure drops and the engine rpm with it. I change oil regularly, and added an additive. any ideas as to what’s going on?

Can I assume that you’re posting because you have a “low oil pressure” warning light and it’s been coming on at idle when the engine is hot?

If so, what’s going on is that your engine is wearing out normally. Oil pressure is created by the oil pump forcing oil through the small orafices and spaces between your parts, like your crankshaft wear surfaces and their respective sleeve bearings. This process creates a high pressure fluid barrier that seperates (for example) the crankshaft wear surfaces from the bearings. When the wear becomes too great, the spaces become too large and the pump has difficulty maintaining pressure…the oil simply moves through the spaces too easily.

The best solution is to simply try an oil of a higher base weight.

No warning light!

Do you have a gage?
How low is it dropping to?
What does the engine RPM drop to?

Let me suggest that the oil pressure dropping is because of the RPM dropping rather than the other way around.

Gauge, yes. rpm drops to approx.500

It sounds like mountain is correct. When the RPM drops it is normal for the oil pressure to drop, especially when the engine is worn and is no longer able to hold the pressure at low RPM. There are other possibilities, but I think the bike has nailed it.

500 is too low.

So the real question becomes why is the RPM dropping so low. It’s possible that the oxygen sensor is bad, the fuel pressure is not being maintained at idle (read: weak pump, bad fuel pressure regulator, perhaps plugged fuel filter), the idle air control motor is malfunctioning, or perhaps the EGR valve is sticking partly open, mass airflow sensor is sending a bad signal, or ???

The point is I’d look for fuel metering at idle and the sensors and systems that comtrol it. The weak pump I threw in, but I’d not put that high on the list, as a weak pump would also show up as poor power when driving.

My sense is that an ignition problem would light up the CEL.

Guys. any other thoughts on a low idle when hot?

I’m gonna bump this thread myself because I’m trolling for better ideas than mine on possible causes for low idle on this vehicle. I’m not happy with mine.


You have handled this thread very well…

The idle could be dropping too low due to a vacuum leak or an Idle Air Control valve acting up.

However, you did not state exactly how low the oil pressure drops.

Rev the engine up to about 700 RPM and see what the oil pressure is.
If it shows 10-20 PSI or something like that then it could be:
Faulty oil pressure sending unit. (fairly common with age and mileage)
Worn engine bearings. (Oil pressure can be verified with the use of an external gauge, not the factory one.)