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Car won't rev past 2500rpm

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    #1

    Car won't rev past 2500rpm

    My car is an 87 325is with a whodwho MS unit (still isn't tuned yet but my old ECU took a dump so it's just so it would be at least running) with 19# M30 injectors, and 2.5" straight pipes all the way back. I got a new exhaust trying to chase a bogging that turned out to be clogged injectors and I didn't have new hardware at the time so it has been massively leaking since then.

    The leak caused the car to run rich and backfire a lot but ran mostly pretty well. I finally got new exhaust hardware and the leaks are gone and it was fine for a whole night and now it sounds like it is missing a little while cruising and won't rev past 2500 or it'll start bucking and missing severely.

    My friend going to school as an auto technician says it's the o2 sensor but unplugging it and resetting the ECU didn't help anything. Tomorrow I am going to see if it'll get past 2500 at WOT since that would bypass readings from the o2 sensor, correct? Any other recommendations? Could the plugs be crapped out or anything else?

    Thanks in advance, Peter.
    Tags: None
    #2
    No ms dosnt care about the o2 sensor, it dosnt listen to it, the tuner will adjust based on his readings the MS unit will follow the tune based on map readings not o2 readings. You don't even need an o2 for MS, having one is simply so you can monitor things.

    Could be bad cps sensor, cps sensor gap, or if your running an aftermarket sensor ditch it. The non bmw plastic sensors don't play with Ms, made that mistake once.

    Sent from my SCH-I545 using Tapatalk 2
    -Build http://www.r3vlimited.com/board/showthread.php?t=295277

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      #3
      Even though the MS still has the motronoic board, it doesn't retain the adaptive features and such? I didn't know that...

      A few months ago the CPS got chewed up a little by the water pump pulley and the insulation was worn through making it misfire a ton but I ziptied it out of the way and it been fine since. I originally thought that was the issue because the symptoms were similar but i checked it and it was still well away from the pulley. I'll check it out again though. Any way to test it to see if that is the issue?

      Comment

        #4
        Ms dosnt retain anything from the stock ecu but he case and connector.

        You could check the air gap and check resistance on the sensor. I'm using my phone but a quick search should give you results but think pins 1 and 2 should be about 540 ohms + or - 20 or so, air gap should be 1mm. If there is any issues with sensor wiring I'd replace it regardless. These are finely tuned sensors and can be affected easily by broken insulation or bad ground causing noise.

        Sent from my SCH-I545 using Tapatalk 2
        -Build http://www.r3vlimited.com/board/showthread.php?t=295277

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          #5
          Get into the logs of the MegaSquirt and see what spark, fuel, and airflow are doing.
          The car makes it possible, but the driver makes it happen.
          Jim Levie, Huntsville, AL

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            #6
            When I checked the CPS, the bolt was loose and it was wiggling around so I tightened it and that didn't really help. I checked resistance at the connector and it was within spec.

            I checked the Coolant Temp Sensor and at the time, with the engine not quite cold but in the blue, it read ~700 ohms and is supposed to be ~3500 according to a thread I saw, so I went and bought a new one but when I checked with the engine fully cold, it was about 2500 so I thought it could still be bad but then read in the Bentley that at 68* it should be 2100-2900 ohms so I guess that isn't the issue.

            The CPS did look a little crooked when I tightened so I tried loosening it and moving it around with the engine running to see if I could get it running better and nothing worked.

            I'm still a little lost. I'm going to try new plugs since the car had been running rich for a while and sounds like it isn't firing all cylinders, but then sometimes while driving, it will randomly sound fine for a few seconds then go back to sucking.

            Also, the car revs up to all speeds now, it's just rough and sounds like it's missing.

            Comment

              #7
              A resistance check of the CPS will tell if the sensor is shorted or open, but it won't tell whether the sensor is good. Furthermore the coax wires in the engine that bring the CPS signals to the DME can be bad. Looking at the CPS signal at the DME connector with an oscilloscope is the only way to tell if the timing data is valid.
              The car makes it possible, but the driver makes it happen.
              Jim Levie, Huntsville, AL

              Comment

                #8
                With MS it's common to not be able to rev past 2500 when the VR signal is not hooked up correctly - maybe something was jarred loose. DIYAutotune's guide says to use a 18k resistor inline with this input. If I remember right I had issues with an 18k resistor and ended up using a 10k resistor instead. Also make sure your TPS signal is grounded (with a 1k resistor) if you are using the stock TPS that came with the car. It should work fine if you swapped in a variable resistance TPS .

                Notes: http://www.diyautotune.com/diypnp/ap...m20b25-mt.html

                *Edit I just saw you fixed the rev issue. for the break-up you're experiencing definitely replace the plugs.

                Comment

                  #9
                  Would an oscilloscope be a worthwhile investment to make or would it be easier to just take it to a shop and have that checked? The cheapest I see new is ~$70 but there are some older analog ones for $40-50 on CL.

                  I'll probably just buy a new CPS anyway due to the worn through insulation as soon as my next paycheck comes in or I get the time to go to the bank and put in money.

                  Comment

                    #10
                    I just wish the auto parts stores around here carried Bosch plugs so I don't have to wait for them to ship here every time I need them... I should probably buy a couple set to have spares on hand.

                    Comment

                      #11
                      I'd order a new one. I feel it's prfentitive maintainence really on these cars. It's a wear item.

                      Go with ngk plugs, usually in Stock and a very good plug. Think the 5's are the stock heat range iirc. Bpr5es I'm fairly certain but double check.

                      Sent from my SCH-I545 using Tapatalk 2
                      -Build http://www.r3vlimited.com/board/showthread.php?t=295277

                      Comment

                        #12
                        So the cutting out fixed itself over a couple days but it wasn't running on all 6 cylinders. I had a problem with the injectors getting clogged after running out of gas a couple times and the same thing happened. I replaced the CPS, plugs, fuel filter, and injectors and now it works!

                        I guess the fuel filter was full and was letting too much debris past, clogging the injectors again...

                        Thanks everyone for the help!

                        Comment

                          #13
                          Guess I am a little late to the party 8^(

                          Glad you got it going, the CPS is a common issue with MS and will can cutout like a rev limiter. Most of the time you can loosen the CPS and push it towards the timing wheel and tighten it back up and it will be fine.

                          Feedback on other comments in the thread:

                          MS doesn't need an O2 sensor but can fully utilize it as it has full EGO control. It will adjust while running but will not adapt and save.

                          I use a 10k resistor for the CPS

                          I have a circuit built that will use the stock TPS and give a idle/cruise/WOT output signal, no 1k resistor mod needed

                          Glad you are up and running, let me know if I can help - you could of hit me up for help 8^)
                          My M20 Frankenbuild(s)
                          4 Sale - Fully Built TurnKey Megasquirt Plug and Play EMS

                          Comment

                            #14
                            Originally posted by whodwho View Post
                            Guess I am a little late to the party 8^(

                            I use a 10k resistor for the CPS

                            I have a circuit built that will use the stock TPS and give a idle/cruise/WOT output signal, no 1k resistor mod needed
                            I should have known you had this all figured out. :)

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