I’m having a seriously weird issue all of a sudden. My fuel pump is randomly getting only 7v out of the 12v it should get.

I’ll start from the beginning here. I got my engine back into place, got everything bolted up, fluids filled up, grounds tightened, and went to go start it. All it would do is crank. I confirmed I had good spark, re-timed my distributor again for good measures, verified I have a cam and crank signal via an oscilloscope, and I had good compression because I just rebuilt the damn engine.

After all of those checks, I checked fuel pressure, and I had none. 0psi. I could hear the pump prime, but even taking the line off of the fuel rail, no fuel would come out. Thinking I had a bad pump, I did what everyone should do, and check for power. And I had 7 volts. I manually applied power to the pump, and fuel sprayed out of the line. I then plugged the connector back in, hooked the fuel line back up, primed the pump, and I had fuel pressure. I went back to the connector again, and I had 12 volts. I never touched anything, and all of a sudden it was working just fine. So, I cranked it up. It started perfectly, and then after about 1 second, it hiccuped, stumbled, and died. I then went back to the fuel pump, and it now had 7 volts!

I then started looking at the wiring diagrams, and it seems the pumps ground is somewhere in the fender, and it’s power is supplied by the fuel pump relay, which is energized by the ASD relay. I swapped the ASD relay with the starter relay, and the fuel pump relay with the compressor relay, and it didn’t change anything. I checked the voltage at the relays, and there were 12 volts there at both of the relays.

So, why? I’m inclined to think that it has to be something I’ve done recently, since I never had any fuel related problems before putting my engine back in. But, the grounds in the engine bay don’t ground the fuel pump. The only thing that would have some sort of effect would be the noise shield wire coming off of the front of the engine bay, grounded to a head stud to ground the instrumentation.

I’m really baffled, and so is everyone in my shop. I did run out of time to check resistances and do more testing, but I have a feeling that’s not going to get me anywhere.

Also, to clarify once again, I’m randomly getting 7 volts at the pump, and then getting 12 volts, with no rhyme or reason. So, if anyone can help me out here, I would be ecstatic

 

ASD relay or a damaged wire or connector would be my guess.

 

CRUISER'S MOSTLY RENIX TIPS

IMPROVING THE FUEL PUMP GROUND

JANUARY 3, 2016 CRUISER54 57 COMMENTSThe fuel pump and fuel tank sending unit ground at a sheet metal screw up behind the spare tire on an XJ, and behind the driver’s taillamp on an MJ.

Not only is a sheet metal screw a lousy way to ground things, this ground path is long and travels through some connectors that are prone to corrosion and moisture.

Locate the black wire on the HARNESS side of the fuel pump/sender 3 wire connector. Remove a 6″ length of the split loom covering. Strip back about 1/2″ of insulation from the BLACK wire.

Take your new ground wire, preferably at least 14 gauge and 12 to 18 inches long as needed, strip it about 3/4″, and wrap it around the exposed part of the harness plug wire.Solder the connection. Tape it up and reinstall the split loom covering.

At the other end of your new ground wire, add a crimp on eyelet. Attach the eyelet under a bolt that goes directly into the chassis. Be sure to clean the attaching point til shiny and apply OxGard to the contact surfaces.

 

Thank you Cruiser. That’s what the issue was.

The ground was corroded because I sprung a tiny leak from the rear right glass, so sitting out in the elements didn’t help in the least bit.

I also took the liberty of improving all of my engine bay grounds, which would be the dipstick ground, PCM ground, chassis ground, starter ground (not too sure if it’s for the starter, it’s an 8 gauge solid wire that grounds to the ignition coil), and the noise shield wire. Each had new 12 and 8 gauge wire run, except for the dipstick ground. That’s just 3 18 gauge wires that I put into an eyelit. After all of that, perfect fuel pressure!

 

Good job. Also on the ground refreshing.
And thank you for letting us know C's tip fixed this.

 

I’ll be completely honest here, I, and most people, frequently forget that having ground is just as important as having power, and most of the time, a power issue is a ground issue.

Seriously though, thank you again Cruiser.

 

Yep the man has saved a few souls and sanities here over the years..