Sparatic idle after fuel pump replacement

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92rcsb

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Ok, so I recently had a crank, no start issue with my 92 rcsb. Sprayed some starting fluid and it fired right up, checked fuel pressure to find it was only getting about 8-9 psi. Changed out the fuel pump and it ran great for the day, next morning dead again. After some troubleshooting found out the pump relay was shot so changed that out and it fired right up, went for a test drive and all was good ran better than it ever has. Parked it, then went to run some errands this afternoon and now its idling sparaticly, and at close to 1500rpm when it decides to steady out occasionally, as well as surging when I'm coming to a stop. So now I'm at a loss, it almost seems like its overfueling itself. I know the ecm is pretty slow in these old tbi trucks but I figured after a few drives it would sort itself out and get used to actually getting the proper amount of fuel but no luck so far. Has anyone else had a similar problem or have any idea what I might be looking at here? Any thoughts would be appreciated, thanks in advance.
 

Schurkey

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1. 9 psi is at the bottom end of what GM calls acceptable. The truck should run just fine on 9 psi, provided you still have 9 psi at maximum load.

2. Did you change the fuel filter? The filter sock in the tank? Did you clean out any rust or debris that was in the tank? Were the in-tank wires to the fuel pump corroded? (Common problem!)

3. What is your fuel pressure NOW?

4. How old are the cap, rotor, plug wires, plugs, O2 sensor, etc.

5. I would be very likely to connect a scan tool and look at the data stream.
 

92rcsb

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1. 9 psi is at the bottom end of what GM calls acceptable. The truck should run just fine on 9 psi, provided you still have 9 psi at maximum load.

2. Did you change the fuel filter? The filter sock in the tank? Did you clean out any rust or debris that was in the tank? Were the in-tank wires to the fuel pump corroded? (Common problem!)

3. What is your fuel pressure NOW?

4. How old are the cap, rotor, plug wires, plugs, O2 sensor, etc.

5. I would be very likely to connect a scan tool and look at the data stream.

I'm at right around 11 psi now, fuel filter is only about 2 weeks old, new sock on the pump, wiring and grounds are good, tank is clean, plugs and coil are about a month old. I have been considering the cap and rotor but it was all close to new when I purchased the truck as well as the distributor although I havnt ruled it out yet. 02 sensors are definitely not new and very well could be the factory units though
 

Urambo Tauro

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How healthy is the battery? Will it pass a load test? Does the voltage drop severely while cranking the engine?
 

DerekTheGreat

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I had my fuel pump replaced about 2 months ago. Drove fine when I picked it up, then wouldn't start after I got where I needed to be. Turns out the connector inside the tank was bad, all melted and such. Truck has been great after that.

Erratic idle might not be related to the fuel pump. Like Schurkey said, get a scan tool (I love my MT2500) and see what the sensors are feeding to the ECM. They might be slow, but they're good enough for what it's for. I had an erratic idle, didn't think to connect my scan tool to the truck until I was ready to pull my hair out, long story short, the TPS was bad and was reporting voltages of 0.59-0.80 when at idle, with no driver input.. So when that happened the IAC counts would increase. As Eric O. would say, "There's your problem lady!" Went to junkyard, found another one and bam, idle has been good ever since.
 

92rcsb

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I thought it might be a problem with the connection in the tank as well, so I did pull it out again but everything looked good in there. As for the idle, I have kind of expected the iac but a scan tool wouldn't be a bad thing to get my hands on. Could there possibly be something in the ignition like the crank position sensor that might not be telling the fuel pump that the engine is cranking?
 

Schurkey

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The ECM turns on the fuel pump relay for a two-second prime when the key is first turned on. After that, the pump runs when the pickup coil in the distributor sends an "engine is turning" signal to the ignition module, which passes that signal to the ECM for timing adjustment. The ECM turns on the pump relay when it gets that "engine is turning" signal from the ignition module.

The oil pressure switch bypasses all of this, and powers the pump separately when it "sees" enough oil pressure. Some of the wire harness is shared, but either the relay or the pressure switch can power the fuel pump.




Do the injectors have power? They should have power any time the ignition is "on" or "crank". The ECM controls the ground side.

Will a noid light flash when cranking, on both injector harnesses?
 

DerekTheGreat

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...I have kind of expected the iac but a scan tool wouldn't be a bad thing to get my hands on...

In my case the IAC was NOT bad, although since it is responsible for idle speed adjustment (in a perfect world) many start by firing the parts cannon there by replacing it. Scan tool allowed me to see that it wasn't actually the bad part, that the TPS was, which by reporting values over what the ECM considered "idle" caused the ECM to react by opening the IAC a bit..
 
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