Diagnosing Low Oil Pressure

Let me just list what is going on.
2008 GMC Envoy and I have already changed the sensor - no change. Apparently mine was the last year that did not use the mesh filter.
I have an oil leak that is getting worst and the readings are getting lower. I will be getting the leak fixed with my tax refund. I think it started slowly getting worse after I did an oil change with Rislone … maybe.
Despite the low pressure reading (like 10 psi), I don’t hear noisy lifters et al. I does run a little rough when reading 5 psi.
I hooked the diagnoses tool into it and it read as low voltage from the sensor. The exact error is P051 Engine Oil Pressure Sensor/Switch ‘A’ Range/Performance. Which is either honest low oil pressure or a bad sensor / harness open or shorted / bad connection. Question: this means the gauge is OK and reading the pressure as per the sensor (which may be wrong itself) correctly?
As a general rule, oil pressure rises with engine RPM.
On a cold morning (sub-10 degrees F) the oil pressure reads as 0psi until the engine warms up a little. I use 5W30 synthoil
It has been quite few months. Am I correct in that it is probably not the oil pump? My understanding is that the pump goes from starting to fail to complete failure in a very short time.

I get that you need to diagnose my car to give me the full answer. I guess what I’m looking for here is when I take it in to have the oil leak fixed, what are the chances it is a cheap repair like
The oil leak itself
New sensor is bad
Wiring issue
And not an expensive repair like bad oil pump or a bad gauge.

The sensor is a pain to install so is there any way I can diagnose the wiring myself with my multimeter?

@Gary_T usually has great ideas in threads like this.

It’s not impossible that both your old oil sensor was bad and the new one too but it seems unlikely. You can test the wiring to the sensor, which could also cause the problem, with a digital multimeter. My guess though is that you actually have low oil pressure.

It could be cheap or very expensive. A leaking oil line would explain both the low oil pressure and the oil loss. That’s usually not too costly, particularly if your mechanic can find it quickly.

If could mean worn piston rings or bearings. To fix you would have to remove the engine and rebuild it. Probably not worth it for a 2008 Envoy. It would probably be cheaper to drop in a used engine. Or to find a new truck.

You might want to check whether your truck calls for lower viscosity in very cold weather. That might reduce prolonged oil starvation in very cold temperatures although it might also, ironically, make the low oil pressure problem slightly worse as the truck warms up. Good luck!

My Audi A4 (2002) the giveaway was after an engine flush, new oil, the oil light came on when you lifted your foot off the accelerator to coast. The though was by then it was too late, clogged oil screen or passages from burnt oil kept the back pressure acceptable until it was flushed clean. The wear and tear from poor oil feed had already worn the parts of the engine that should provide the proper back pressure (Esp. crankshaft bearings?)

For the first few years the dealer had not used synthetic oil because the factory specs at the time did not call for it. Solution was either a new engine (cost as much as the car was worth) or a new car.

You’ll want to get a mechanical oil pressure gauge and test it that way, then go from there.

There is a ton of oil on the lower passenger side of the block just below the water pump.

I don’t know what the odds are that it’s one cause rather than another. I do agree with steronz that it makes sense to test the oil pressure so as to know what the actual value is.

As far as testing the wiring and the gauge, it’s done by finding the specs and checking the gauge reading with a known resistance in the circuit. For example it may be that with 200 ohm resistor in place of the sender it should read 15 psi, and with a 1500 ohm resistor should read 25 psi (numbers made up to illustrate concept).

And it is disappointing in multiple ways. I took it to my mechanic that I have trusted for years. Note he has a bunch of new hires. The diagnosis from one of the new guys was two leaks: the timing cover I knew about and the oil pan I didn’t. Yes it needs a new oil pump. Total cost $6000 BUT I could have them pull an engine out of someone’s (who’s?) truck and intalled in mine for $4500. How can I go back to that shop after that quote?

Of course I said F that. I called a local mechanic with a great reputation and the quote for a new oil pump is $2750. I was expecting around $1500 because some online sources talk about jacking up the block like 2 inches and going through the timing cover. This shop will drop the whole engine because (thank you GMC) that’s the only way to get to the pickup. Refurbed engine from this shop starts at $1500 + labor with 3 year warranty.

So I’m having the control arm and bushing replaced (I trust that diagnosis and it is a safety concern). As for the oil, I’ll use a thicker oil like 10W-30 and make sure it stays topped off. If and when the oil pump fails, I assume my engine will be toast and then look at my options at that time. My hopeful time frame is 2.5 years before getting another car so fingers crossed.