87 TBI cranking

[usmdude]

Hey, I am having a problem with my 87 R10, 350. When I let it set several hours and go to crank it, I have to wind on the starter for a few moments until the fuel pump kicks on and supplies the TBI with fuel. I have noticed on the initial crank, the pump does not kick on, it waits for oil pressure or when you release the key, it will kick the pump on. This problem has been on going since I bought the truck a couple years ago.

I have;
Checked fuel system grounds,
Changed fuel relay,
Changed both fuel pumps,
Changed both fuel sending units,
Changed fuel selector switches,
Changed fuel selector valve,
Changed oil pressure switch,
Changed oil pressure sensor,
Pounded head on lift post.

I am nearly ready to tear the truck apart to paint. But I need to to find the problem with this. So I am not chasing down this on fresh paint. Anybody had a problem like this with these trucks? I am at a loss.

Jbo

 

[foamypirate]

Did you replace the fuel pump relay with a used or new relay? It really sounds like you have an open on the 12v signal to the relay or a bad replacement relay. The oil pressure sensor is a backup to that in the case that the relay fails. I'd check at the relay for power while someone turns the key to the run position. It should prime the pump for 2 or 20 seconds (depending on whether you have a hot fuel module).

 

[Jims86]

Hey, I am having a problem with my 87 R10, 350. When I let it set several hours and go to crank it, I have to wind on the starter for a few moments until the fuel pump kicks on and supplies the TBI with fuel. I have noticed on the initial crank, the pump does not kick on, it waits for oil pressure or when you release the key, it will kick the pump on. This problem has been on going since I bought the truck a couple years ago.

I have;
Checked fuel system grounds,
Changed fuel relay,
Changed both fuel pumps,
Changed both fuel sending units,
Changed fuel selector switches,
Changed fuel selector valve,
Changed oil pressure switch,
Changed oil pressure sensor,
Pounded head on lift post.

I am nearly ready to tear the truck apart to paint. But I need to to find the problem with this. So I am not chasing down this on fresh paint. Anybody had a problem like this with these trucks? I am at a loss.

Jbo

If you have a 12v test light, or a meter, go out and probe the positive lead to the tan/wht wire at fuel pump relay cavity "E" and have someone turn the key to see if you get the 2 second reading.
If not, probe the dark green and white wire at Cavity "D" to see if you get a reading when the key is turned. If you do, do a continuity test between the black and white wire at cavity "F" and engine ground to make sure the FP relay ground is present.
If no reading from "D", you will have pull the small plug at the ECM, and probe the green and white wire at cavity "A1"(the pin locations are marked at the end where the wires come in) and run a long jumper wire out to cavity"D" on the FP relay plug, and see if there is continuity.
then get back with us and we can knock our heads around some more!

 

[Jims86]

Did you replace the fuel pump relay with a used or new relay? It really sounds like you have an open on the 12v signal to the relay or a bad replacement relay. The oil pressure sensor is a backup to that in the case that the relay fails. I'd check at the relay for power while someone turns the key to the run position. It should prime the pump for 2 or 20 seconds (depending on whether you have a hot fuel module).

2 seconds on 1/2 tons, 15 seconds on some 3/4 and all 1 tons with the hot fuel module.
1/2 tons ain't be havin the module.

 

[usmdude]

New relay.

I will try the test on the wires as described, this weekend. Seems as I remember the relay clicking when the key is turned. But I will verify with a test meter.

Jbo

 

[bucket]

2 seconds on 1/2 tons, 15 seconds on some 3/4 and all 1 tons with the hot fuel module.
1/2 tons ain't be havin the module.

What is the purpose in the module? Where is it normally located?

 

[Jims86]

This is a guess on my part, because I could never find anythin definitive.
I think that naturally, the heavier trucks run hotter, because of the heavier workload than the 1/2 tons. It would make sense that the longer period of prime, cyles the fuel through the lines, just incase the fuel boils while the workers stop a Taco Bell for lunch.

 

[bucket]

Makes sense. Even if that's not the real reason, it's probably a better reason than what GM had.