Hard start when cold. No start once warm.

[Georgeb]

That would come directly from the distributor.

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The Reference High signal #430 correct?

Shouldn't there be a trouble code? Perhaps the OP should jump A-B on the OBD port and check for one.

 

[yevgenievich]

It should trigger a check engine light and throw code 42 or 41. But if it runs with fuel pump wire jumped it would not be an issue as ECM will not fire injectors if it sees no rpm signal.

Main thing that would make it clear is if the fuel pump runs when jumped in no start condition and will the vehicle run in that configuration after the fuel pump is running

 

[Georgeb]

It should trigger a check engine light and throw code 42 or 41. But if it runs with fuel pump wire jumped it would not be an issue as ECM will not fire injectors if it sees no rpm signal.

Main thing that would make it clear is if the fuel pump runs when jumped in no start condition and will the vehicle run in that configuration after the fuel pump is running

According to the OP yes it will run. He has indicated that if he applies power to the test lead on the plug for the relay the truck starts and runs. It will also start and run with the oil pressure switch jumped.

 

[Mbyoung424]

As of my last testing the green/white wire goes from 12v with key on to 0v with key in start position. Just doesn't make sense when looking at the wiring diagrams. Shouldn't it be 0v with key on and 12v with key in start to close the relay and allow the orange wire to provide power to the pump? This makes me think ECM issue, but I had not error codes when last tested.

I don't believe there is a heat soak issue with the ignition components as it'll run all day once started, but the fuel pump won't run once it's turned off and still warm unless you run power to the test lead or jump the oil pressure switch connector.

I was beginning to think there was an oil pressure issue as when you crank when cold the oil pressure gauge will start to increase then, presumably, the oil pressure switch kicks on power to the pump and it starts. Once warmed up the oil pressure gauge doesn't register anything and no power ever makes it to the fuel pump. Granted the oil pressure sensor feeds the oil pressure gauge with a signal and doesn't directly affect the oil pressure switch, but it's something I noticed.

 

[yevgenievich]

Check the green/white wire for 12V while running, while hot, while in the jumper bypass mode.
If it is not hot, then check it at the ecm. If dead there, then check all the power and ground terminals at the ecm with key in the run position. If that is all good, then with no other issues being present, ecm is likely at fault.

 

[Mbyoung424]

I'll started checking all that tomorrow. Thanks for the direction.

 

[Mbyoung424]

With oil psi switch jumped there is no difference for the grn/wht wire at relay connector or ECM. 12v with ign on, 0v when ign at start or while running.

With oil psi switch connected and not jumped the grn/wht shows 12v at relay connector or ECM when on and 0v with ign at start or while running.

With oil psi switch connected and relay installed you no longer have 12v at ECM with ign on.

The orange wire always has power at both ECM and relay terminal. The Brown/white wire always has power at both ECM and Relay when the oil psi switch connector is jumped.

With the oil psi switch connected, the brown/white wire reads 0v when you turn ign to start, climbs to 10v prior to the start as the oil psi gauge starts registering and stays at 12v once running. After the no start issue it stays at 0v and the oil psi gauge doesn't register.

 

[Georgeb]

We may need some help here. Hopefully @hatzie or @chengny can chime in and help out too.

 

[cl2sparky]

Had this trouble once with a 91 Blazer. Finally isolated the trouble to a faulty ECM.

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[yevgenievich]

It is leading towards being a fault in the ecm. One last check woukd be to probe the wire directly at the ecm to rule out bad wire

 

[Georgeb]

I have gathered that the issue is not related to heat outside of the fact that there isn't sufficient oil pressure to complete the circuit through the iol pressure switch when the engine is warm. Where I get lost on this is my understanding of the sequence of operation. Is the ecm supposed to turn the fuel pump on when cranking? If so what tells it to? Obviously there is a signal from the distributer because there are injector pulses. I say this because with the oil pressure switch jumped the truck starts right up. I have reached the limit of my understanding on this one.

 

[yevgenievich]

Ecm turns the fuel pump on when ignition is first turned to on position for few seconds and then when in run position. If ecm sees engine turning, it will pulse the injectors. It is worth checking power and ground wires going to the ecm as well.

 

[Mbyoung424]

I only checked A1, B1 and B2 at the ECM yesterday. Along with verifying the grounds on the A/B side of the ECM yesterday. When I get a chance I'm going to check the grounds on the C/D side of the ECM as well as the other terminals with the IGN on and while running.

What I think is odd is that the green/white wire has 12vs with ign on at the ECM as long as the fuel relay us disconnected, but once the fuel relay is connected there is no longer power at the ECM. I'm am leaning to the ECM at this point.

I need to read up on engine upgrades as this 5L TBI is under powered and I want to make sure I'm not wasting money on a 5L TBI ECM and then needing to get a different ECM once I get around to completing an engine swap. bypassing the oil psi sensor and using the fuel tank selector switch as a kill switch by disconnecting one fuel pump wire seems to work for the time being. I just hate knowing there is a problem and simply ignoring it.

 

[Mbyoung424]

GeorgeB,

As I understand it, the oil psi switch and fuel relay are in a parallel circuit with either one being able to power the fuel pumps.

There is a 12v orange wire One the ECM B fuse with power all the time. Power is transferred to the tan/white wire at the fuel relay one of two ways. The first is when the green/white wire is energized by the ECM once the IGN is on. This closes the circuit on the relay allowing the orange wire to provide power to the brown/white wire that goes to the fuel tank selector switch inside the cab which then gives power to the left or right fuel pump. The other way is by manually putting power to the red test lead which is connected to the brown/white wire inside the relay when the switch is open.

Once oil psi is registered by the oil pressure switch, that closes a circuit sending power that connects the brown/white wire with the orange wire. This again sends power to the fuel tank selector switch inside the cab which then sends a signal to the fuel pump.

 

[Georgeb]

GeorgeB,

As I understand it, the oil psi switch and fuel relay are in a parallel circuit with either one being able to power the fuel pumps.

There is a 12v orange wire One the ECM B fuse with power all the time. Power is transferred to the tan/white wire at the fuel relay one of two ways. The first is when the green/white wire is energized by the ECM once the IGN is on. This closes the circuit on the relay allowing the orange wire to provide power to the brown/white wire that goes to the fuel tank selector switch inside the cab which then gives power to the left or right fuel pump. The other way is by manually putting power to the red test lead which is connected to the brown/white wire inside the relay when the switch is open.

Once oil psi is registered by the oil pressure switch, that closes a circuit sending power that connects the brown/white wire with the orange wire. This again sends power to the fuel tank selector switch inside the cab which then sends a signal to the fuel pump.

Got that thanks. But since the oil pressure doesn't rise enough when hot to complete the circuit with the oil pressure switch during the no start condition what is the fix? My question is "Is the ECM supposed to turn the fuel pump on during cranking or not? If we are only relying on the oil pressure switch then is there a different switch to use that makes contact at a lower pressure?" Or is the engine cranking too slow to create proper pressure?