Bad connection or bad fuel pump?

[ftpiercecracker1]

Still fighting with the Comanche. Decide to take the hood off so I could get real-time feedback on my fuel pressure gauge while driving around.

 

Haven't started it in probably a month. When I tried to start it today the fuel pressure gauge pegged at 40psi but when the engine actually started the pressure dropped off precipitously all the way to 0psi and the engine died accordingly.

 

After a few cycles of this the fuel pressure finally "caught" and stabilized, holding steady at 30ish psi - 28psi when on the gas.

 

I was able to drive it around ok, but I'm having issues with sluggishness and notable missing and backfiring at higher RPMs and speed. Mind you the fuel pressure did not change significantly during this.

 

Here's where things start to get murky. I came to a stop to see if removing the vacuum line would improve anything, but when I did the fuel pressure again faltered and rapidly dropped zero and the engine died.

 

After a couple of rounds of trying to start the engine without success I noticed a pattern that whenever the starter was engaged the fuel pressure always pegged to 40 PSI. 

 

So I tried starting it again, but when the engine fired up I continued to turn the key for brief moments as if I was still trying to start the engine. This kept the fuel pressure up and kept the engine going. After just a few cycles of this the fuel pressure "caught" again and maintained pressure without further input from me, allowing for me to drive it back home.

 

But, when I got home it did the exact same thing, fuel pressure faltering, dropping to zero, and the engine dying.

 

 

Some things to note:

 

This maybe the original fuel pump to the truck, I do not know.

 

I still have the porcelain thingy, what keeps the fuel pump quiet, on the driver side fender well.

 

 

Thanks

FPC

[Ωhm]

Take the two wires off at the porcelain thingy (fuel pump ballast resistor) and connect them together. Try restarting the vehicle.

[ftpiercecracker1]

Sometimes it's the simple things. Bypassed the ballast and fuel pressure is much better. Holding steady at 31 to 32 psi and jumping up several PSI instantly with the slightest bit of throttle. Rock solid 40psi when WOT. 

 

Although I am still having an issue. I do not think it is related to the fuel pump seeing how that seems to be resolved.

 

Truck is still sputtering whenever I attempt to cruise or hold a steady speed, at any speed. Also doesnt want to maintain higher speeds. Example: I can accelerate to 75mph or so, but when I try to maintain that speed the truck starts to sputter/cough and slow down to 55mph or so.

[omega_rugal]

try a different pump

[cruiser54]

Have you ever done the fuel pump ground upgrade?

 

The 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.

 

 

[ftpiercecracker1]

23 hours ago, omega_rugal said:

try a different pump

 

14 hours ago, cruiser54 said:

Have you ever done the fuel pump ground upgrade?

 

The 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.

 

 

 

 

Thanks for the input fellas, but I'm pretty confident the fuel pump issue has been resolved.

 

Cruiser:

 

I think I have done this, can't remember for sure, was a long time ago. I seem to remember joining the fuel pump and the tail light ground together. I will check next time I'm out there.