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88 325I - installed 89 325is engine - rpm and tach issues

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    88 325I - installed 89 325is engine - rpm and tach issues

    Hello everyone,
    I have been having a very rough time trying to get my daughters first car running. We purchased a nice 88 325i from the neighborhood, it had been sitting awhile but seemed ok otherwise. I knew we would have to change some things to get it running. in the end, after getting it running and trying to get it smogged and registered the shop said the engine had very low compression. had it checked out and the rings and head gasket were bad. I found a rebuilt engine from a 1989 si locally and purchased it. I swapped this in and have been having a tough time getting the car to run correct and the check engine lights to clear.

    issues: When the car fires up and i rev it, once it hits 2000 rpm it drops. even if i am holding the throttle open. it drops to about 1000 and then start reving up again. always exactly at 2000 rpm. I have changed the AFM, CPS, throttle body, TPS, ECU, changed the coil, checked the relays, fuel pressure. it has a new fuel pump and pressure reg. new plugs. the IACV is new and the rubber boot is new as well.

    Also, when i plug my OBD with adapter to the big circle plug i get no reading at all. it says to turn the key on (even though the key is on). and now the tachometer does no move at all anymore.

    Do you think this could be because I used my old 88 325i wire harness on the newer 89 325 si engine? The 89 did come with the manual getrag trans, but i kept my automatic from the 88 engine.
    Any help or advice would be cherished at this point. Any shops local to Orange County Ca that could help me would also be welcomed info. The rebuild 89 engine did come with a wire harness but i used the one in the car since it was already in place. TY in advance. Mike

    #2
    There is a round wiring connector, under the throttle body that is know to corrode. I would start there.
    2004 525i Manual - 1985 325E Coupe Manual

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      #3
      yeah that one was bad. greenish blue build up. i disconnected that and tried to use a small nail file to clear it. got all the green/blue stuff off it but no change in behavior.

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        #4
        I would verify the TPS is indeed adjusted correctly. Also be sure your AFM is not another used 30 year old unit as that would be like using a 30 year old spark plug in your motor, never trust it. Test your AFM with only an oscilloscope, not a meter. See my AFM thread for info about how the signal should look and what a bad signal looks like. Be 100% there are no vacuum leaks. Lastly, your tach issue may be related to a failing SI board which will not cause the engine issues. Tach failure is typical on non refurbished clusters.
        Owner - Bavarian Restoration
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          #5
          TPS was doing the same to me when cold only it drove me crazy to figured it out although it was adjusted and clicks and idle switch and wot pass continuity test but part throttle test was giving me resistance when the tps is cold only.
          Gave it a deep cleaning the idle switch (white switch inside tps) socked in alcohol then blow it with air.

          Tps rarely needs replacing since it is only a switch and oil gets in due to worn lower butterfly shaft seal.

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