Fuel system help

[idrivea2002golf]

This is a tech question but it's for a GMT 600 so doesn't quite fit the squarebody... but there aren't any good GMC Savana forums. Plus, you guys are a lot nicer than most of the other forums.

So this is a 1996 and it has the L31 5.7 with the multiport injection. The issue I'm having is the fuel pump during start up. From my understanding GM vehicles (i don't know which, but I assume all) have a short fuel pump cycle when the key is turned, to do the initial prime. Once the pressure is there and the engine starts it then hands over the fuel pump operation to the oil pressure switch. All seems easy enough. Where I'm stuck is why I'm not getting that initial prime.

When I turn the key I do not hear the pump prime. So I've checked/replaced (for good measure) the relay and the fuse, no change. The van will start if I hold the key for a bit and allow oil pressure to build. So I know the pump and the oil pressure protection circuit are working. I can kill the fuel pump by disconnecting the oil pressure sender while the engine is running, also good.

What's my next diagnostic? Thanks!

 

[Ricko1966]

This is a tech question but it's for a GMT 600 so doesn't quite fit the squarebody... but there aren't any good GMC Savana forums. Plus, you guys are a lot nicer than most of the other forums.

So this is a 1996 and it has the L31 5.7 with the multiport injection. The issue I'm having is the fuel pump during start up. From my understanding GM vehicles (i don't know which, but I assume all) have a short fuel pump cycle when the key is turned, to do the initial prime. Once the pressure is there and the engine starts it then hands over the fuel pump operation to the oil pressure switch. All seems easy enough. Where I'm stuck is why I'm not getting that initial prime.

When I turn the key I do not hear the pump prime. So I've checked/replaced (for good measure) the relay and the fuse, no change. The van will start if I hold the key for a bit and allow oil pressure to build. So I know the pump and the oil pressure protection circuit are working. I can kill the fuel pump by disconnecting the oil pressure sender while the engine is running, also good.

What's my next diagnostic? Thanks!

Test the fuel pump relay, the truck shouldn't die when you disconnect the op switch. OP switch is a fail safe to get you home if the relay,it's wiring,or signal from the ecm have a problem.

 

[YakkoWarner]

Your understanding of the relationship between the oil pressure switch and relay is very close to right, but there is one key difference. The oil pressure switch isn't a hand-off from the relay, it is a backup for the relay. They relay should always be powering the fuel pump when the engine is running (controlled by the ECU computer). The ECU also is what tells it to switch on for a few seconds when you first turn the key on for priming purposes. The oil pressure switch is there in case the relay circuit fails. From what you describe, that backup system is working exactly as designed. Are you getting any check engine light indications?

Can you hear the relay click when you turn the key on (and then click again when the prime period ends)? If so, the signal is getting to the relay, but the relay is not doing its job of passing power thru to the fuel pump which would indicate the relay is receiving the "turn on" signal, but no power on the input side of the relay.

If you get no clicks from the relay, that would indicate the "turn on" signal is not being received. You could test by momentarily applying 12V to the pin that receives the turn-on signal and listening for a click (disconnect the other wiring first). Check (with key on. engine off) for constant 12V on one of the pins coming to relay. There should be a second pin that gets the momentary +12 when you first turn the key on. If you never get any +12 on the pin that triggers the relay, you need to figure out where that wire goes into the ECU.

 

[CalSgt]

What year is your van?

IIRC my L31 uses a signal from the CPS to the ECM and the ECM triggers the fuel pump with no oil switch. The ECM should have a 3-5 second prime cycle on the fuel pump when it sees the ignition powered up.

My L31 harness also had a test port (single wire) tapped up to the harness near the fuel pump relay. I cant remember for certain, but I think the test port would power the fuel pump when hooked to 12 volts because it would basically bypass the relay when connected like this. It could also be used to run a test light to see when the relay was sending power to the pump.

 

[idrivea2002golf]

Your understanding of the relationship between the oil pressure switch and relay is very close to right, but there is one key difference. The oil pressure switch isn't a hand-off from the relay, it is a backup for the relay. They relay should always be powering the fuel pump when the engine is running (controlled by the ECU computer). The ECU also is what tells it to switch on for a few seconds when you first turn the key on for priming purposes. The oil pressure switch is there in case the relay circuit fails. From what you describe, that backup system is working exactly as designed. Are you getting any check engine light indications?

Can you hear the relay click when you turn the key on (and then click again when the prime period ends)? If so, the signal is getting to the relay, but the relay is not doing its job of passing power thru to the fuel pump which would indicate the relay is receiving the "turn on" signal, but no power on the input side of the relay.

If you get no clicks from the relay, that would indicate the "turn on" signal is not being received. You could test by momentarily applying 12V to the pin that receives the turn-on signal and listening for a click (disconnect the other wiring first). Check (with key on. engine off) for constant 12V on one of the pins coming to relay. There should be a second pin that gets the momentary +12 when you first turn the key on. If you never get any +12 on the pin that triggers the relay, you need to figure out where that wire goes into the ECU.

I do have a check engine light but I've forgotten what it was for.

I'll get the DMM out and try some of these.

What year is your van?

IIRC my L31 uses a signal from the CPS to the ECM and the ECM triggers the fuel pump with no oil switch. The ECM should have a 3-5 second prime cycle on the fuel pump when it sees the ignition powered up.

My L31 harness also had a test port (single wire) tapped up to the harness near the fuel pump relay. I cant remember for certain, but I think the test port would power the fuel pump when hooked to 12 volts because it would basically bypass the relay when connected like this. It could also be used to run a test light to see when the relay was sending power to the pump.

Van is 1996. I'll see if I can find a test port. I did drop the tank thinking the pump was bad. While I was down there I found the harness was damaged but the pump was ok. So I repaired the harness and reinstalled the tank.

 

[Ricko1966]

The simplest thing to test and the most probable cause,is the relay it's self.

 

[idrivea2002golf]

Did some testing and was about to pull my hair out.

I have 12.6v constant on 87
Jump between 87 and 30 will run the pump (confirmed pump noise)
12.6v between 85 and 86 for about 3 seconds with key on (no start)
Tested the relay out of the car, clicks just fine....

It was the freaking pins in the fuse/relay box... They were bent JUST enough that I could probe them but they were not making contact with the relay. Bent them back out a little and bob's my uncle. Now to figure out the long start and the P1351 "unknown diagnostic trouble code" (i'm using a free scanner and ELM BT reader).

 

[idrivea2002golf]

P1351 looking like a faulty ignition control module. I think it may be a left over code as I replaced it but didn't clear codes.

 

[Ricko1966]

Did some testing and was about to pull my hair out.

I have 12.6v constant on 87
Jump between 87 and 30 will run the pump (confirmed pump noise)
12.6v between 85 and 86 for about 3 seconds with key on (no start)
Tested the relay out of the car, clicks just fine....

It was the freaking pins in the fuse/relay box... They were bent JUST enough that I could probe them but they were not making contact with the relay. Bent them back out a little and bob's my uncle. Now to figure out the long start and the P1351 "unknown diagnostic trouble code" (i'm using a free scanner and ELM BT reader).

Now that the relay is working your hard start should be gone. Verify the relay is actually working disconnect the O.P switch,the truck should stay running. Oh and relay clicking does verify relay is good it should be open fronn30 to 87 with no power applied it should be closed when power is applied. I like to test them with a headlight bulb as a test light. Sometimes they will close and show 0 resistance or light a small test light but not be able to provide enough current to run a fuel pump. If they will light a headlight,they will run a fuel pump.

 

[idrivea2002golf]

I did pull the oil pressure switch and it does stay running. Pump does come on with ignition regularly now.

I recall replacing the ignition control module about 18 months ago. I'm going to see if I can get a warranty replacement on it.