Hard start when cold. No start once warm.

[Georgeb]

If this were mine I would have already run a fused lead from the starter to the fuel pump prime to power the pump while cranking. I wish we knew the answer as to if the pump should run during cranking regardless of the oil pressure switch.

 

[Mbyoung424]

You got me. No schematic shows any other power source. Once primed by the fuel relay it looks like the pressure from the oil pump created by the cranking of the motor must be enough to close the circuit in the oil psi switch before the fuel pump gets any more power.

After checking the terminals at the ECM my next step was going to look at how the oil pump works to see if that's a possible issue.

 

[yevgenievich]

pump should run during cranking.

 

[Georgeb]

pump should run during cranking. ECM does not see a difference from crank vs run position. it gets the power source that is 12v in run/crank key position.

Ok so lets see if I follow. You turn the switch to run and the pump primes for two seconds and stops. Then when you turn to start the ecm should tell the pump to run continuously, not for just two seconds but for as long as you crank correct? Then once the engine starts the oil pressure takes over and maintains pump power until the ngine is shut off. If it's supposed to do this it obviously is not powering the pump while cranking. But why?

 

[yevgenievich]

It should prime for few seconds, then keep pump on once cranking/runing. Oil pressure switch is more of a backup in case relay fails. It might be worthwhile to connect a computer to it and see what ecm is doing. If ecm comanding injectors to fire, it should be also doing the same for the pump.

 

[chengny]

I didn't see any mention of a code being thrown - have you scanned the ECM for intermittent codes? An intermittent code will reset itself (and extinguish the CHECK ENGINE light) after 10 seconds. I think you are looking for a 54 code - Fuel Pump Circuit Low Voltage.

This input is kind of different from other inputs - it kind of generates itself. The way the ECM checks for voltage in CKT 120 (i.e. the tan/white lead) - which supplies the fuel pump - is by detecting power at terminal B2 of the ECM. So in effect, it is just sending an output (the green/white lead) to the FPR on pin A1. It then waits for a response (that verifies the relay has pulled in and is supplying the pump motor on pin B2). IOW, it basically just checks relay operation.

Start by ascertaining that the CEL is functional and then scan for codes.

Here's a link to an easy procedure on how to read the old GM 2 digit codes - without any special scanner. Just jump the appropriate terminals on the ALDL connector under the dash and count the flashes:

https://www.obd-codes.com/faq/read-gm-2-digit-obd-codes-free.php



If the CEL is working, but not illuminated, that could indicate a self clearing intermittent type of code. Remember it will go out after 10 seconds - without help from you. It might clear so fast that you don't even notice it.

Attached are several pages from the 1987 GM Light Truck Fuel, Driveablity and Emissions manual.

CEL operation and diagnostics:

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Fuel pump circuit wiring diagrams and diagnostic charts:

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Code 54 (Low Fuel Pump Circuit) explanation and diagnostic:

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Crank Signal explanation and chart:

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One thought: have you considered that the starter motor/solenoid may have a ground fault. One that pulls the system voltage down - only while cranking the engine.

It's a longshot, but you might want to check voltage on CKT 440 at the FPR during dynamic conditions - you know, while the starter is actually cranking - not just when the key is in START.

I only read the thread quickly but, if you can consistently reduce cranking times when power is supplied through the CKT 490 test connection (and have changed out the relay)... I don't see many other possible causes other than:

1. Low voltage in the orange 440 circuit (check the fusible link in that leg) or

2. The ECM is not operating the relay as designed

3. A mechanical wiring problem in the FPR control circuit.

 

[yevgenievich]

Ft232 based usb board goes for about $5 on ebay. It and tunerpro rt is all it takes to connect to ecm and see what it does. Tx and rx tie together and connect to M terminal on obd connector and gnd connects to A.
How to set up soft:
http://www.gearhead-efi.com/Fuel-In...ter-Tutorial!-New-users-this-is-a-must-read!-)

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Ignore the missing key. Top right read out on display is fuel pump volts

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[Georgeb]

More on that here too. http://www.gmsquarebody.com/forum/showthread.php?t=13194

 

[yevgenievich]

More on that here too. http://www.gmsquarebody.com/forum/showthread.php?t=13194

nice write up there

 

[Mbyoung424]

I have family in town so won't have a chance to do any testing until Wed.

I had previously checked the ECM and there were no codes. The code for no faults kept repeating itself.

 

[Mbyoung424]

Bringing this post back to life to explain all the issues that I figured out.

The ECM was bad. Turning the key on wasn't sending power to the fuel relay for the initial start prior to the oil pressure switch assuming the responsibility of powering the fuel system. This is why the previous shop bypassed the oil pressure switch with a jumper cable utilizing the power from ECM B fuse, but that was killing the battery.

The engine had a blown head gasket and due to the other issues I found from the previous mechanics who installed the engine I pulled it in favor of a crate 383.

I also went with a FiTech system so I was able to pull the ECM all together. The FiTech needed a 58 psi pump and instead of replacing two in tank pumps I added one inline pump. I had left the fuel selector switch unplugged from my previous fix since the left tank didn't work anyways. Today I rewired the fuel tank selector switch to recieve power from a fuse that is hot with ignition on. You can hear both in tank pumps run, but nothing is pumped out of the left. In line pump will still suck gas from the left tank and run the truck st idle. Never took it for a test drive.

Even though it worked I wanted to solve the issue so I dropped the tank and the internal rubber hose between the pump and sending unit was bad. The pump was also going out.

Now that the tank is out Im going to get it cleaned up then add a longer hose with a strainer instead of getting a new pump since I have the in line one anyways. I'll cut the power to the right pump, but keep power to the sending unit for the fuel gauge.

All in all there were a whole slew of issues, but everything has been identified at this point and the truck runs like a champ...minus the transmission.

Thanks for all the help for those that were following this last year.