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Changed fuel pump now no start? M20B25 Turbo

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    #1

    Changed fuel pump now no start? M20B25 Turbo

    Hi all,

    Just finished turbo-ing my M20B25 which (admittedly) wasn't running amazingly as it's still on the stock ECU and was having significant spark blowout(?) past 3k rpm / heavy load. (Single coil OEM setup)

    Anyhow - was still running albeit slightly rough at times whilst i wait for my MS2 PnP standalone to arrive and me to set everything up correctly. Completed the swap a week ago and cold start had began to get a bit worse (taking 10-15 secs to crank and sometimes having to kill power and start again)

    Yesterday I decided to replace the fuel pump for a high pressure Walbro 255 in-take. This is where things went wrong - I accidentally swapped the terminals to the pump and fed a ground to live and vice-versa. After realising my mistake and trying to crank a few times with no pressure to the rail, I swapped back in the old pump and hanger which then didn't help the car to start and gave me a persistent no start issue.

    Pulled the plugs which were now black and changed for fresh set - after this checked and getting decent spark to each plug. Checked that still getting fuel pressure to the rail which I now am. Voltage at the pump is 12V which drops to ~11.5 when cranking. Fuel pump relay works correctly and even bridged pins to prime fuel pump without cranking.


    Does anyone know what I could've done wrong or why I'm getting no start? I'm planning to put in Megasquirt in the next week or so to be able to measure conditions and see if leaning out / overfueling or other problems.

    update: Car now starts but doesn't idle and I have to regulate the throttle in order for engine not to fall flat and die. Will try post video when I can.
    Last edited by alpinaseeone; 11-20-2023, 09:08 PM.
    Tags: None
    #2
    check fuse #11 it needs to be atleast 10a in order to not pop with that pump

    88' Seta 2.7i Zinno

    https://www.r3vlimited.com/board/for...430-my-88-seta

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      #3
      Initially had a 30amp fuse from PO, replaced this with a 7.5 when I was checking everything and had reinstalled the OEM pump

      Comment

        #4
        Update - Just tried another ICV that I had lying around to see if this was cause of the bad idle to no avail. I should mention motor is freshly resealed (less than 1000kms) so don't believe there would be any significant vacuum leaks. Might still be worth a smoke test!

        The thing I find weird is that the car ran fine before I attempted to change the pump over and now refuses to idle after.... been scouring the forums for the past two days - possible things to look into at the moment are bad grounds and potential voltage drop at fuel pump off throttle? Going to try splice in a multimeter to confirm that voltage stays consistent at the pump when starting the car and if it changes off / on throttle.

        Comment

          #5
          Need pics of the pump, year, model. Assuming you have an early 325i, have you checked that the small reducer hose hasn't split? Honestly, it will if you haven't shortened it. I also had the clamps loosen which has the same result. You stated you had fuel pressure, how much?
          "I'd probably take the E30 M3 in this case just because I love that little car, and how tanky that inline 6 is." - thecj

          85 323i M TECH 1 S52 - ALPINEWEISS/SCHWARZE
          88 M3 - LACHSSILBER/SCHWARZE
          89 M3 - ALPINEWEISS II/M TECH CLOTH-ALCANTARA
          91 M TECHNIC CABRIO TURBO - MACAOBLAU/M TECH CLOTH-LEATHER

          Comment

            #6
            It's an 89 (Facelift) coupe, originally M20B20 but the new motor is a M20B25 (also facelift but unsure of year). I've only got the in-tank pump (no external high pressure pump)

            I've just bought a fuel pressure gauge which I'm going to hook up when I get a spare hour or two! Plans to test pressure at the pump (apparently ~45psi?), pressure post fuel filter (for any significant drop), pressure at the rail (any other drops? according to forums should be somewhere ~43psi) and lastly pressure of the return line (~15psi). I pulled these values from an earlier R3V search, hopefully they're consistent but will update when I have news!

            Comment

              #7
              Theres a whole lot of swapping of parts going on here with not alot of thought.

              what injectors have you put in? if you are running significantly bigger ones on stock ecu I doubt it will run good or even start. You'll just be flooding it.



              Comment

                #8
                I agree, been a bit of a rushed build as I daily the car and wanted to get it running asap but alas maybe I should've waited and planned further.

                I kept the stock Bosch 280150715 Injectors 14.5lbs. The car was running for the past week or so which makes me think that I'm missing something with the fuel pump replacement I messed up or it's just a huge coincidence and it chose that exact moment to not run properly after the swap

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