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1989 325i - Split Second Loss Of Power When Driving

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    1989 325i - Split Second Loss Of Power When Driving

    So this has happened once every several miles. I'm cruising along and all of a sudden the car jerks. When it does this the CEL flashes and more recently I noticed the Fuel Economy Gauge spikes to reading high MPGs. The tach and speedo are not affected. This all happens in less than a second, the car shudders, and it's done and the car continues on as if nothing happened.


    All new ignition parts: plugs, plug wires, cap, and rotor. Low mileage, known working CPS.


    Based on what I've read, I'm thinking the TPS might be bad. It's adjusted properly, but I'm thinking that the ECU is getting a faulty reading that the TPS is closed (or less than what it's actually at) and reducing the injector duty and the car is leaning out, causing the economy gauge to spike and the car to jerk.


    Any other ideas or input would be appreciated.


    Thanks

    #2
    https://www.r3vlimited.com/board/sho...d.php?t=372569

    Go through this. Good luck.
    AWD > RWD

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      #3
      Originally posted by Kershaw View Post
      Well, that was a sad story. Basically he replaced everything possible multiple times and nothing helped. Lol

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        #4
        There is a bus bar/connector underneath the fuse box which can come lose, C100. It is held in place by a hex socket bolt on the top of the fuse box. It's underneath the low beam and fog light relays in the fusebox.

        I didn't have your issue exactly, instead things would cut out if I turned my high beams on, but I did occasionally encounter similar issues that you are having. The car would stutter, and I would lose electronics for a second or so.

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          #5
          Now I'm not losing any electronics and the tach isn't affected. If the CPS was bad and it was cutting out, would the tach lose signal for the brief second?

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            #6
            Originally posted by gilber33 View Post
            Now I'm not losing any electronics and the tach isn't affected. If the CPS was bad and it was cutting out, would the tach lose signal for the brief second?



            Engine speed is sent to the tach by the ECU, the SI board processes it and generates the pulses to move the motor in the tach.
            There is some buffering in the tach, so a brief 100 ms dip in RPM may not show up.


            What RPM does this happen? Is it sub 2,000 RPM?

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              #7
              It has happened at all rpm. While taking off at a stop light in 1st gear, cruising at 45 mph in 5th gear and anywhere in between.

              I'm starting with the TPS. It appears to be original to the car.

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                #8
                TPS is easy enough to test. Based on your description id almost guarantee it wont be it. Unplug it and see if it makes it better or worse.

                Sounds like something fundamental like a loose wire or plug.

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                  #9
                  I had talked to the previous owner and he said he experienced this once in a while, too.

                  When I bought the car I pulled the engine, resealed it, and replaced almost everything. It has all new ignition stuff: cap, rotor, plugs, wires. Low mileage known working CPS. Coolant temp sensors. New fuel filter. The fuel pump is not new. I replaced the pan to frame ground wire.

                  I should clean the grounds on the fender and the passenger side.

                  I wish I knew if it was spark or fuel cutting out. At least that would narrow it down a bit.

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                    #10
                    Just some follow up - I replaced the TPS, but still had this intermittent hiccup. I tested the ignition coil and I guess it's supposed to read .5 ohms between the + and - terminals, mine was reading 1.2+, so I think I'll replace that. If that doesn't take care of it, that's pretty much all the spark related items.

                    Is it possibly the fuel pump? I don't see how if the pump cut out that the car would jolt like it does. I would assume it would keep cruising but it would run extremely lean for a second.

                    Again, this is not RPM dependent, it's happened while accelerating 1st gear and cruising in 5th gear at 2500 rpms.

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                      #11
                      I had an issue like this and there were 2 things I did. First, I disconnected the idle valve. That remedied the issue for a few weeks. Got a new idle valve, plugged it in and the issue happened again. hm so maybe its the ECU? plugged in a new ECU and the issue went away completely. Then a few weeks later it came back...turns out it was the CPS and the ICV/ECU that were intermingled. My issue was like someone turned the key off for a moment and there was a completely loss of power.
                      325IX Build Thread
                      RX7 Build Thread

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                        #12
                        Originally posted by driftxsequence View Post
                        I had an issue like this and there were 2 things I did. First, I disconnected the idle valve. That remedied the issue for a few weeks. Got a new idle valve, plugged it in and the issue happened again. hm so maybe its the ECU? plugged in a new ECU and the issue went away completely. Then a few weeks later it came back...turns out it was the CPS and the ICV/ECU that were intermingled. My issue was like someone turned the key off for a moment and there was a completely loss of power.


                        Out of curiosity, how would the ICV failing affect this?


                        The previous owner experienced this once in a while and when I did all the work on the car, I put in a low mileage CPS and the issue persists. I don't think the sensor is my issue, but I guess the only way to fully rule that out is putting in a new one.


                        And I have heard a few people say mention the ECU when it comes to weird engine stutters like this. Definitely something to look into as well.

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                          #13
                          Just out of curiosity, is the CPS supposed to be dead center on the gear? I noticed mine is not. Seems to be it is definitely sticks out towards the front of the car, like the bracket it sits in is an 1/8" too long.

                          Hard to find pics of the CPS installed to tell what its supposed to look like.

                          EDIT: Actually the bracket must be tweaked a little bit. As I tighten down the screw the sensor doesn't sit flush but points out a little bit. This causes the sensor to read the other edge of the gear, with a real little bit of the sensor over the edge of the gear.

                          I dont know how picky these are, but could they cause maybe some random misreadings?
                          Last edited by gilber33; 10-25-2018, 02:09 PM.

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                            #14
                            IIRC the issue was when the resistance of the idle valve got too high it basically took out the circuit on the ECU. its supposed to be 20/40/20 and it was something like 2k/10k/2k
                            325IX Build Thread
                            RX7 Build Thread

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                              #15
                              I noticed something similar with an E30 parts car that I purchased. It drove great and shifted smooth, however I noticed this at least twice in the three weeks or so that I drove it. I never bothered looking into it because I was parting the car anyways. Good luck with this, it freaked me out when it first happened to me even though it was for a split second.
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