GEM code's cause hard start?

Discussion in '2005+ Mustang GT 4.6L Tech' started by JordanBerk, Mar 22, 2023.

  1. JordanBerk

    JordanBerk Junior Member

    Hey all! I had just posted an update to my first thread saying that things were running pretty well since finishing my engine swap and well I guess that was bad luck because now we're back to not running again! :Big Laugh:
    So let me catch you up on what has been going on, for the first ~120 miles the car ran fine with an occasional "lean O2" check engine light, at 120 miles it didn't start at Walmart, had AAA tow it home and when I pulled the fuel pump and hanger assembly out of the gas tank (this had been replaced during the engine swap) I saw the plastic hose coming from the pump, to the output on the top of the tank had burst open and failed!! Rock Auto warrantied the part and all was (almost) well with NO check engine lights for the next 100 miles. I say almost because my fuel gauge was only reading 50% when I knew there was ~15 gallons of fuel in the car.
    I had checked the passenger side Level Indicator when the pump was out and it gave me what looked like a good range of resistance when I played with it but stupid me didn't check the new replacement one with the pump!

    Sorry for being so long winded but we're finally caught up to what my problem is currently. So I put ~100 miles on the new pump until the first Check engine light came on, this time it was only "Fuel Level Indicator open circuit" so I wasn't worried about it HOWEVER I went to drive the car after clearing it and the car was hard to start (Took maybe 4 seconds of turning over) but did start and idle, but died as soon as I touch the throttle. I'll include the forscan log but the only codes It's throwing are GEM B1365-20 "Ignition Start circuit battery short" and GEM B2035-20 "LF Side Repeater Lamp Output Circuit Failure".

    I'm having a hard time wrapping my head around how I could have a short between the ignition circuit and the battery that still allowed the car the start and idle, OR how I could be having fuel pump problems again with no lean codes and no Fuel rail pressure codes?? Unfortunately I didn't have any time to do more diagnostic tonight before I went to work but any ideas or help would be so greatly appreciated! My only ideas of where else to start looking is grounds, and pulling my fuel pump out again, am I on the right track?? Thanks!!

    Attached Files:

  2. 07 Boss

    07 Boss Senior Member

    Some of your symptoms like the wrong fuel level reading and starting but not actually running may be related to the fuel pump crossover tube. I know when I knocked this tube off I couldn't figure out why I was running out of gas when I knew there was fuel in it. Probably a reach but just sharing my experience.
  3. JordanBerk

    JordanBerk Junior Member

    This definitely could’ve happened while I was fighting to get that stupid ring locked back in.. And if so, then filling up the driver-side hump with more gas should make it run again temporarily, probably worth trying at least before I order more parts! Thanks Boss.

    I managed to get some more information from Forscan tonight and I’m only getting ~10 psi to the rails so I’m pretty sure the problems inside of the fuel tank and that the GEM codes are unrelated. Forscan is also showing “error” for fuel system Bank2 status, I assume this is because no fuel pressure but I’m only assuming!!

    Right now my guess is that the rock auto pump gave out and I should’ve went with motor craft. I don’t know how to test the pump itself other than maybe a flow test BUT there’s only so much to go wrong between the pump and the rail and I’ve verified the lines both ways are clear each time I’ve had it apart, and it has a brand new filter as well. I’ll pull the assembly out this weekend and let you guys know what I find!

  4. Pentalab

    Pentalab forum member

    With key on, engine off, fuel rail pressure should be 50 psi. With eng running on idle, fuel rail pressure should be 40 psi...and stay at 40 psi, regardless of rpm.

    Years ago, my fuel rail pressure gagged, but only when I tried to get into boost...and would drop down to just 5 psi. Turned out the issue was the clogged...'sock filter' that goes on the bottom of the oem fuel pump. Once cleaned, fuel rail pressure back to 40 psi, runs like a top.
    JordanBerk likes this.
  5. Laga

    Laga Senior Member

    Next time you have to reinstall the fuel pump ring. Use a pair of pliers to slightly bend upwards, the front edge of the tabs on the body of the car that the ring goes under. This makes installation much easier and still seals completely.
    Also, only use a fuel pump assembly that has the fuel level gauge with the gray plastic body that houses the electronics. They make a white one, and it simply doesn’t work correctly.
    JordanBerk likes this.
  6. JordanBerk

    JordanBerk Junior Member

    Confirmed the GEM codes had nothing to do with my not-running problem! I didn't want to believe I got 2 bad parts in a row but what do you know, I should've went with motorcraft from the beginning!! Just like Laga said the crap I pulled out had the unworking white level gauge, the new one has the gray plastic and worked right away!
    I'm still having the bank2 (Driver side?) fuel system error so all is not fixed yet, but the car's idling really well and running with no missfires or anything so I'm still gonna drive it to work in the meantime until I can get under it and do some more trouble shooting, I'll probly start by swapping O2 sensors to the other side and go from there. As always, thanks for the help! :beer:
  7. john knox

    john knox Junior Member

    get oem fuel pump module your fuel gage will never report the right fuel, scan your ecu about 5 time my car need a new ecu, but reported it as fuel pump code
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