OPS,. What the heck does this indicate?

[waynep]

Hello All: Well my 3/4 ton 1997, sub is stumbleing and stalling after it seems to warm up. I am monitoring the fuel pressure while driving and it seems to increase while coasting and decrease while driving it hard, and jump eratically. What is weird is that I totally disconnected the oil pressure sensor switch and nothing changes.IE the fuel pressure acts exactly like when it is connected. Does this mean that the fuel pump is bad? I thought that the OPS ran the fuel pump when warmed up?
Thank you for your suggestions.

[john8662]

The Oil Pressure Switch operates the lift pump and the oil gauge inside. If you're seeing no change in fuel pressure that could indicate that your pump is either bad or not receiving power.

Re-connect your oil pressure switch on the back of the engine, then start the engine to verify oil pressure on the gauge inside. Then with the engine still running, crawl underneath the burb and grab the fuel pump located on the drivers side frame rail, you should feel it pulsate if it's working. If not, start looking elsewhere, like check for voltage at the connector going to the fuel pump, just disconnect and use an voltage/ohm meter to verify you're getting voltage to the fuel pump. If nothing or very low then you can point towards replacing the OPS.

Where are you tapping off for fuel pressure readings?

[waynep]

Hey John: Thanks for the quick reply. I am connecting a guage to the fuel drain T at the front of the engine. I have read from other posts that the pressure guage is in parallel with a relay on 97 and up, and used only as a backup to the relay, which sounds kinda odd that GM would spend the money on this design.The fuel pressure measurement takes some "hits" when it starts stunbling and stalling, but does not go to zero. Same measurements with switch in or out of circuit.

[john8662]

Yes, the 96+ oil pressure switch system is different, but ultimatly, the OPS still drives the relay which still activates the lift pump. So when you unplug the OPS, the power to the lift pump should be termintated, because the relay isn't getting power to engage. Thats odd that you get pressure from that location with no lift pump operation, I would think you would get no pressure.

Still check to see if the lift pump is operating in the above conditions.

[rjwest]

Oil Pressure sw is not needed in a OBD2 vehicle....

PCM senes RPM and turns on lift pump....through a relay. Oil pressure sw is in parrelel to the relay. When you shut down, PCm opens relay but lift pump will run untill oil pressure drops.

[waynep]

Further diagnosis: Well it seems it is not the lift pump. Started truck and totally disconnected the lift pump power connector. Truck ran just as good, or bad, as it did before. Measured a vacuume in the fuel line but again, truck ran OK.

Looks like a bad injector pump or FSD, and the dealer said that they "absolutely will not warrantee it" if it id just the FSD. Is the FSD covered or just the pump??

[autocrosser]

If it's the Injector pump it's under warranty PMD included. They should actually change both under the warranty. I have had two complete ones changed out under the warranty. They need to read the documentation that describes the warranty. It's real clear that it's covered. The PMD is part of the pump assy. Have them call the GM folks if they don't believe you.

[waynep]

Hello All:
I had the same dealer repalce the pump and driver at around 80,000 miles and it was a battle convincing them not to charge me the $100 deductable. Now it has 112,000 miles. This is not good.
Another problem is that it is very intermittent, no codes and not totally dead.
Any ideas on what else it could be, sure acts like it did last time it died.
Any good ideas on how to kill one besides just driving the darn thing... LOL

[patrick m.]

there is a method to get the needed codes to set before going to the dealership. (the codes must be there for pump condemnation).

Eventhough we all know the pump can be bad and not set the codes.

someone else will have to explain the procedure.