What could cause a voltage drop to the fuel pump? The fuse is good, 12V there, but when checked directly at the fuel pump the voltage is only 6V. The car cranks fine but the pump won't pump on 6V. BTW, this is an aftermarket GM pump. When jumped directly from the battery, the pump runs and the car will start. The connection is good and through the floor pan under the backseat looks good. Where does the line follow from there to check for problems?

loose or broken connections would be where I would start. next I would visually inspect the fuel pump wires from the pump all the way to the fuse.

voltage drop generally occurs when a run of wire is too small for its length. As am example I've installed really long runs of wire for street lights and to prevent voltage drop we start with a wire bigger than your thumb at the begging of the run and end up with a wire that is no bigger than a pencil led.

 My point is that if anywhere along your fuel pump wire is partly cut or has damaged connectors that would reduce the amount of surface that current can be carried on it will result in voltage drop

MY 87 ea82 spfi fuel pump relay(Std Mtr Prod # RY51) is located above ECM alongside ignition relay(Std Mtr Prod # RY56). Both mounted to same bracket held with 1 screw after moving ECM out of way.

Relay contacts can & do fry lowering voltage thru contacts.

Where is the fuel pump relay? I've looked online and it seems to be behind the instrument panel? If it is, how easy is it to get to? 

33 minutes ago, soobie87 said:

Where is the fuel pump relay? I've looked online and it seems to be behind the instrument panel? If it is, how easy is it to get to? 

IF yours is spfi it will be mounted on a bracket held with one screw to a dash support behind the instrument cluster. BUT you can get at it by removing the plastic panels on between steering wheel & foot pedals, then lowering your ECM(3 12 mm nuts hold this in place) to get at the screw. Fuel pump relay is the round one - Intermotor(SMP) part # RY51.

Don't confuse that one with 4 relays on back of dash above fuse panel. They are interchangeable though.

If yours is carbureted someone else will have to chime in.

 

Yeah, we need to know if it's SPFI, SFI or carby.

It is carb

2 hours ago, soobie87 said:

It is carb

Well that would've been nice to know..........there's a rev sensor in the circuit for fuel pump & I don't know where that is.

Search function does work tho....

I'm out.

Edited September 3, 20187 yr by czny

On 9/3/2018 at 8:59 AM, czny said:

IF yours is spfi it will be mounted on a bracket held with one screw to a dash support behind the instrument cluster. BUT you can get at it by removing the plastic panels on between steering wheel & foot pedals, then lowering your ECM(3 12 mm nuts hold this in place) to get at the screw. Fuel pump relay is the round one - Intermotor(SMP) part # RY51.

Don't confuse that one with 4 relays on back of dash above fuse panel. They are interchangeable though.

If yours is carbureted someone else will have to chime in.

 

Just wanted to comment and say I'm currently diagnosing a FP relay issue on my 87 SPFI ea82 GL and this was extremely helpful, thank you czny.

@soobie87 - did you get the issue sorted?

My thoughts would be the fuel cut relay/engine revolution sensor box. It’s a black box somewhere above the driver’s feet. 8 pin plug from memory. 

You should be able to throw a jump wire in to fire up the pump whenever the ignition is ON. Only recommended to do for diagnostic work. 

If the box is dead get another known good one

Cheers

Bennie