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    #1

    Fuel pump fuse keeps blowing

    I already have a project thread for my car, but I decided to start a thread dedicated to this one issue in the hopes that it will get more views. My build thread contains the full background on the car, but here is a short synopsis. The car is a 1985 325e, completely stock, manual trans. It did not run when I bought it, the PO told me it "needed a fuel pump" and that he had tried to do some basic diagnostic work on it. I found that the whole in-tank pump and sending unit had been removed, so I replaced it with a brand-new (eBay) unit, but that didn't fix the problem. After I bought and stripped a 1987 325is parts car, I installed the main fuel pump from that car last night, and it did not fix the problem.

    So here's where I stand:
    - The car will run on starting fluid, so it's getting spark
    - Fuse #11 blows as soon as I try to crank the engine
    - The fuse does not blow if I just turn on the ignition, only if I crank
    - I am not hearing any kind of clicking or the pumps priming or anything
    - The PO replaced the fuel pressure regulator. The fuel filters have not been replaced.

    A few pics, in case something jumps out. The new in-tank pump and sending unit:


    The "new" main pump from the parts car, installed:


    The fuel pump wiring and ground point under the rear seat. All appear to be in excellent shape and all connections are tight and corrosion-free:


    I did do a search, and this thread seemed to be the most relevant to my problem, however his problem turned out to be loose connections, which I have ruled out. The Bentley manual's test procedures all revolve around the fuse being intact, and since that's the first thing that goes wrong, I can't really get anywhere with those.

    Any help would be greatly appreciated. Thank you.
    Tom - 85 325e for sale
    Tags: None
    #2
    Disconnect both pumps and check for a short to ground at fuse 11. I suspect that will be what you will find.

    The fuel pump relay is controlled by the DME and only engages when the DME sees timing data from a rotating engine. That is why the fuse blows as soon as you crank the engine.
    The car makes it possible, but the driver makes it happen.
    Jim Levie, Huntsville, AL

    Comment

      #3
      Originally posted by jlevie View Post
      Disconnect both pumps and check for a short to ground at fuse 11. I suspect that will be what you will find.
      That makes sense, but what procedure would you follow? Check for continuity from the hot side of the fuse to the hot side of one of the pump connectors?
      The fuel pump relay is controlled by the DME and only engages when the DME sees timing data from a rotating engine. That is why the fuse blows as soon as you crank the engine.
      OK, let me try and make sure I have the whole system understood. As far as I know, it works like this:
      1. Ignition key is turned, completing starter circuit
      2. Starter engages and starts turning the engine over
      3. The DME (via the crank position sensor) senses that the engine is rotating and sends a signal to the fuel pump relay
      4. The fuel pump relay closes, completing the electric supply circuit to the fuel pumps
      5. Current flows through fuse #11 to the pumps
      6. The pumps turn on and fuel flows

      Is that correct? Just want to make sure I understand it completely and have it right.
      Tom - 85 325e for sale

      Comment

        #4
        The first step is to disconnect both pumps. Then use an ohmmeter to measure resistance from the fuse to chassis ground. One side of the fuse socket goes to the relay and the other side to the pumps. I don't remember which side goes where, but both should be an open circuit. If you get a low resistance there is a short someplace in the wiring.

        Yep that is how the pump circuit works.
        The car makes it possible, but the driver makes it happen.
        Jim Levie, Huntsville, AL

        Comment

          #5
          Remove the electrical tape from the fuel pump power wire to see why it was taped. You might see a short there.

          Comment

            #6
            Originally posted by jlevie View Post
            The first step is to disconnect both pumps. Then use an ohmmeter to measure resistance from the fuse to chassis ground. One side of the fuse socket goes to the relay and the other side to the pumps. I don't remember which side goes where, but both should be an open circuit. If you get a low resistance there is a short someplace in the wiring.
            OK, gotcha. I'll give it a try, thanks.

            dnguyen1963: Good point, I didn't think to look at that.
            Tom - 85 325e for sale

            Comment

              #7
              dnguyen1963 gets the prize! Pulled the tape off and this is what I found:


              The pic is a bit overexposed, but basically, it's two of the ugliest solder jobs you're ever going to see, and there was no effort made to separate them, they were wrapped together nice and tight. Hillbilly engineering at it's finest!

              However, even after fixing that, it still won't start. It no longer blows the fuse when I crank it, but it won't fire, either.

              But anyway, I wanted to completely verify that my fuel system was all good, so I took a couple of pics under the hood. It looks like the PO did a bit of monkeying around with the lines feeding the fuel rail and warmup circuit. Does this look right?

              Coming up from the fuel filter:


              Looking down at the top of the intake, you can see a "T" and what looks like a damper on the line?


              The front of the intake with the pressure regulator:


              And, in a new development, it won't fire on starting fluid anymore, either. So now it seems to be a spark issue?

              The last thing I was going to do was attempt to verify that the pumps were working. Using a test light, I verified that both pumps are getting power when I crank it, so hooray for no more electrical problems! Just to make sure they were actually working, I decided to pull off the line at the "T" in the picture above and see if it would pump fuel into a container. Well, I loosened the hose clamp, gave it a tug, and the "T" broke and sprayed fuel all over the damn place.

              It's a good thing I enjoy working on cars..... :razz:
              Tom - 85 325e for sale

              Comment

                #8
                that whole motor and those lines look a little ghetto rigged
                i had the similar problem with my fuel system wasnt getting power to 11 and then when i tried to crank it my fuse #11 literally melted and blew up
                so after trying to diagnose what was wrong with fuse #11 and why i wasnt getting power when the key was on (when it only gets power when your cranking ) didnt know that till later
                soo to try and fix the problem i swapped ecu's and then i tried to crank it but when i did nothing
                so i pulled the spark plug i had no spark
                i then re pulled the ecu and swapped my original one in and wahhhmmmoo
                spark again
                one thing i that i did that made me feel like a idiot for wasting a week worths of my time trying to get my car fire on its own juice and not ether was
                i had my breather line crossed with my fuel line make sure thats not happening to you
                im not sure why you dont have any spark but best of luck

                Comment

                  #9
                  i put in an aftermarket fuel pump and the fuse kept blowing too...turns out the TRE pumps run at higher current, so I bumped the fuse to 10A...no problems since. IIRC the wire is 2.5mm which has an amp threshold of 14 I believe, so you're still safe with the 10.
                  1991 318is --- currently not road worthy
                  1991 318i ---- 308K - retired

                  Originally posted by RickSloan
                  so if you didnt get it like that did you glue fuzzy oil to the entire thing?

                  Comment

                    #10
                    I actually started another thread where I'm trying to diagnose my no-spark condition. Consensus there seems to be that I need to replace both crank position sensors. I've been working on the interior, so I haven't done that yet, plus it ain't cheap.

                    Here's the thread: http://www.r3vlimited.com/board/showthread.php?t=262333
                    Tom - 85 325e for sale

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