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Only starts with AFM and Temp Sensor unplugged.

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    Only starts with AFM and Temp Sensor unplugged.

    1989 M20B25 only starts with the AFM and Temp Sensor unplugged, no other combination will work. The temp sensor is brand new, and the afm is known to be working, as I rode in the car I got it from. I am beyond confused, if anyone has any suggestions they would be much appreciated. Sent from my iPhone using Tapatalk

    #2
    Hate to sound like a broken record recently. What happened prior to this problem? Coming out of storage? Car is new to you? Was running fine, then died? All may be hints as to what's going on. Strange that unplugging those would get the car started?????
    2004 525i Manual - 1985 325E Coupe Manual

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      #3
      Originally posted by tomstin View Post
      Hate to sound like a broken record recently. What happened prior to this problem? Coming out of storage? Car is new to you? Was running fine, then died? All may be hints as to what's going on. Strange that unplugging those would get the car started?????
      So I swapped a b25 from an 1989 325i into my 1988 seta. The donor car was in great condition and I took it for a drive. Ran perfectly. I swapped the engine as well as intake manifold, dme, injectors, ICV, afm, etc. basically the only parts still from my original car are fuel pump, wiring harness, and other electrical stuff.

      After the swap it started right up and ran perfectly for about an hour, then just cut out and died. It was really difficult to get it started after this and it died almost immediately when I managed to. I figured since I know the engine and surrounding stuff works, it must be a fuel problem. So I tried to fix it by:

      -New fuel pump
      -Drained the old fuel
      -New fuel filter
      -New main relay
      -New fuel pump relay

      After this it still wouldn’t start, unless the afm and coolant sensor are unplugged. When and only when they’re both unplugged, it idles perfectly, is responsive to throttle/doesn't bog down, and drives until it warms up, then gets funky and dies. I assume because it’s running off the default ice cold temp map with those sensors unplugged. When one of them/neither is disconnected it cranks very strong but never turns over.

      Here’s a list of other recent work in case that helps solve anything:
      -New spark plugs
      -New spark plug wires
      -New cap and rotor
      -New ignition coil
      -New intake boot

      My only guess is either I somehow didn’t tighten the manifold down correctly and there’s a massive vacuum leak (which I’m about to double check) or it’s some weird wiring/electrical thing.
      Last edited by buttery_flakey; 04-02-2021, 06:02 AM.

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        #4
        I think posting on mobile messed up my formatting. Sorry if it’s hard to read, I’ll try to edit it.

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          #5
          You can check biger vacuum leaks with starting fluid, Danger to manifold. Is the AFM door swinging smoothly?

          I was up above it, Now I'm down in it ~ Entropy - A Build thread.

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            #6
            Originally posted by buttery_flakey View Post

            basically the only parts still from my original car are fuel pump, wiring harness, and other electrical stuff.
            I believe the wiring harness is different. Research some of the swap threads.
            2004 525i Manual - 1985 325E Coupe Manual

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              #7
              Originally posted by tomstin View Post
              I believe the wiring harness is different. Research some of the swap threads.
              I’m pretty confident that for the 1988 325, the harness is the same as it’s the Motronic 1.1/1.3. I know that it’s plug and play with the 173 dme from the 325i, as well as the afm and icv. At least this is what I’ve seen in every 1988 Seta “stroker” conversion thread.

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                #8
                Originally posted by moatilliatta View Post
                You can check biger vacuum leaks with starting fluid, Danger to manifold. Is the AFM door swinging smoothly?
                Sprayed some around common problem areas and no change in idle. The afm door is also swinging smoothly. I also went back and checked every bolt on the manifold was torqued to spec, as well as double checking every vacuum line was tight and secure. ICV is buzzing when ignition is on and I checked and cleaned the C191 plug as well as trying a spare. No luck.

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                  #9
                  I would avoid further troubleshooting with the AFM and coolant temp sensor unplugged. You know those parts are good and you are only confusing your DME by leaving them unplugged. I would plug them back in and crank the car for a while to see if you can get it started up. Regardless, you may need to perform some electrical tests since you are confident there are no vac leaks. I would perform the DME electrical troubleshooting tests in the Bentley repair manual to see if you can isolate which signal is not getting there. I would first suspect your TPS is not adjusted correctly and your DME would not ever know its supposed to be at idle. I would also suspect a failing CPS sensor, which is common to fail and cause a no start condition.
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                    #10
                    First check if dme is energizing fuel pump when afm and temp sensor are connected and when disconnected and see DME may have failed. Small Vacuum leaks rarely stop car from starting.

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