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M20B27 Fuel System Problem Determination

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    M20B27 Fuel System Problem Determination

    I have a problem of long cranking after the car sits overnight. I have a fuel pressure gauge coming, but first the back story.

    I recently replaced the fuel filter as a normal maintenance item along with some of the hose from the steel line to the engine. Started the car up, took it on a brief test drive, and all looked well. On the way to work the following day, started smelling gas. Found a hose clamp a bit loose, tightened it, all good. Filled the tank up, smell gas again. Put the car up on jack stands, can’t find a leak, strong smell of gas. Strong enough that I put a fan in the garage door to air out the garage. The fan happened to be by the right rear bumper. Continue to smell gas, now stronger. Finally traced the smell back to cracked hoses in the fuel tank vent system by the filler neck (my post about cutting those original equipment stainless steel crimp clamps). Replaced all the rubber hoses from the tank to the overflow can and to the filler neck.

    So, the smell of gas is gone. BUT, now I get extended cranking after the car sits for several hours. Seems to start right up after sitting about an hour or so. Runs fine after starting. And after work last night, with the car sitting for about 90 minutes, I decided to check, in the off chance, some debris got into the cold start injector. Plenty of pressure in the hose going to the cold start injector. No obvious debris.

    The in tank pump (lift pump?) seem to be working. I can feel it vibrating and hear it buzzing with the back seat out and access cover removed. All the hoses look good.

    I would rather not simply throw parts at this problem. I’m looking for problem determination steps where this thread can be updated for the next guy. I’ve searched and found plenty of replace this or that without any objective problem determination process.

    My plan now is to test the fuel pressure at the cold start valve (easiest to get to). If that is good, see if the pressure bleeds down over night. Cold start valve could be bad? Fuel pressure valve bleeding down or leaking? Drain back from the lift pump? There seems to be a pretty long list. I would like to pin it down!

    Thanks for any suggestions. Or directions to a good problem determination process that I may have missed in my search.
    2004 525i Manual - 1985 325E Coupe Manual

    #2
    Update: I installed the fuel pressure gauge inline prior to the fuel rail. Pressure is fine but lost 5 pounds in an hour of sitting. Re-pressurized the system and clamped the hose after the fuel pressure regulator. Lost a fraction of a pound over an hour. I think that rules out leaking injectors and points to the fuel pressure regulator. I have one on the way with some fuel hose. Fingers crossed!
    2004 525i Manual - 1985 325E Coupe Manual

    Comment


      #3
      What I think I’ve learned. Maybe this will help the next guy.

      After replacing the fuel filter as regular maintenance along with the hose from the filter to the fuel rail I started having a problem of long cranking before starting. And it seemed to get worst by the day.

      I installed a pressure gauge before the fuel rail. Pressure was good but bled down about 5 PSI an hour after shut down. After shut down, I clamped the hose just past the fuel regulator and the system held pressure. I leapt to the conclusion that this was caused by the pressure regulator bleeding down. Installing a new pressure regulator did not solve the problem.

      After spending some quality time with Mr. Bentley, I discovered the fuel pressure regulator AND the main fuel pump are responsible for holding pressure in the system after shut down.

      I ordered a new fuel pump, 8x13 mm hose and hose clamps. When the pump arrived I loosened the “pressure” side hose and there was no pressure in the line. After about 4 days of sitting, the pressure had bled off to next to nothing.

      The in-tank lift pump should pump 4.3 PSI through the 12 mm hose to the main fuel pump. The in-tank pump as no ability to hold pressure in the system. Shut off the pump and the pressure drops quickly. You can check for proper operation of this pump at the top of the pump or at the main fuel pump. Since I was replacing the main pump, testing at that point was the obvious choice. I installed the fuel pressure gauge and the in-tank pump seemed to be working as intended.

      After installing the new pump I started replacing the 8x13mm hoses from the pump to the fuel rail and back. New clamps and hoses all around. The car now starts like it should!

      Opinion on:
      At the beginning of this journey I used 5/16 hose as that is all the local retailers stocked. Do yourself a favor, order the proper 8x13mm hose! The 5/16 WILL work, but it is more difficult to clamp securely. Also, after chasing a few leaking joints with worm gear clamps, I prefer the crimp style clamps.

      The crimp style clamps are a bit more difficult to find and you need to get the right size for the hose. They are single use, seal very consistently and look very tidy in the engine bay.

      The interlocking clamps are almost as good as the crimp style for consistent clamping pressure. You also need to get the right size but are a bit more forgiving size wise and are reusable.

      Finally, the worm gear clamps are very functional, reusable, available and more versatile when it comes to size. But since we’re dealing with fuel here, proper sealing is critical. Every leak I had was with these clamps. They are now my last resort.

      Bottom line one more sub-system on this car that has been overhauled! After all, it is 30+ years old and probably overdue!
      2004 525i Manual - 1985 325E Coupe Manual

      Comment


        #4
        I used 5/16" in my car and it clamped fine. It's slightly smaller in inside diameter than 8mm, so it should seal itself better with the same clamping force. Could there be another reason why you had an issue with it? It's only smaller by a few thounsandths of an inch

        Sent from my XT1053 using Tapatalk
        Budget E30 Parts - Used and Reconditioned parts for your BMW

        Comment


          #5
          I've never had a problem with the BMW gear clamps, only with the American style ones where the rack is made of holes in the clamp. Those kind tear up the hose and it's just a bad situation, but the BMW ones are solid, and the rack teeth are impressioned in, they work a lot better.

          I've used the reusable nylon clamps without issues, too (this style https://www.amazon.com/Heavy-Double-...dp/B00938915K/).

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