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m42 intermittent CEL and rough idle

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    m42 intermittent CEL and rough idle

    My 318 i has been giving me some idle issues these last couple of months. Driving home yesterday CEL came on at about 60mph but as soon as i let go of gas, the light turned off. When I went to accelerate back up to speed CEL back on only to shut off when I let go of the gas again. Managed to get home though i could feel a slight buck and dip in rpm occasionally. Suspected it was a dirty ICV that was causing the spring mechanism inside to stick so i cleaned it out and reinstalled. I had replaced a majority of the intake hoses as well as the rubber intake boot to try and remedy the issue of rough idle in the past, but now its running worse after the icv clean. On acceleration car bucks heavily and wont rev up to rpm smoothly. Starts and idles fine for 2 minutes and then surging happens. What should i be checking for at this point ?


    #2
    I would bet it is your CPS (crank position sensor). I would do the stomp test to see what fault code is stored. If the cam sensor is saved, still could be your CPS. I would then check resistance to each sensor. I just replaced mine as I highway speeds it would buckle as well as pulling out of driveway it would die. The CEL would flash as well randomly. If you have questions, let me know and do check out the m42 specific site, m42club.com.
    Alex 88 m5 | 91 318is | 19 Subaru Ascent
    BMW Tool Rentals & Fender Roller

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      #3
      Originally posted by monty23psk View Post
      I would bet it is your CPS (crank position sensor).
      while you're checking the CPS, check the harmonic balancer.

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        #4
        thank you boys! I don't have access to a scan tool but I can surely check the resistance to see if it is up to spec. Would bad CPS also cause long crank starts with occasional no start? Since im going to pulling the voltmeter out was thinking about checking the ohms for the coil packs as well since they are probably original. Will post back with an update tonight after work.

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          #5
          update:i believe i got the code 1222: Lambda Control System Bank 1:The ECM has been unable to maintain Lambda (fuel mixture or fuel trim) on Bank 1 (cylinders 1–3) of engine. Posted another youtube link for any other code breakers who want to double check my work.
          https://www.youtube.com/watch?v=P7XOnxULfPA

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            #6
            The code flashing is correct, as stated above start with the crank sensor and inspect/wiggle the trigger wheel. The best way to check for a clocked damper is to rotate the engine to see if it shows that timing is correct via the oil filter mark/hash on the trigger wheel and that a screwdriver or similar fits in the flywheel lock hole as well. If they don't agree and you can see both cyl 1 cam lobes pointed up and at each other it's a safe bet that the damper has rotated.

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              #7
              Aren’t codes 1243/1244 related to the sensor being faulty? suppose it wouldn’t hurt to try though. this user over on m42 club has the exact same symptoms as I do but yet the thread led to a dead end.

              http://www.m42club.com/forum/index.php?topic=14341.0

              gonna try and tackle some more in the morning.

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                #8
                Codes can also be a symptom and not a causation, so don't think of them as gospel, more as a guide.

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                  #9
                  Definitely check the crank trigger wheel and sensor. See post #5 for photos of what to look for.
                  https://www.r3vlimited.com/board/for...f-broken-stuff

                  The diagnostic capabilities of these ECUs is very limited, and often the codes that are given do not actually correspond to what is actually causing the issue. A crapped out O2 sensor usually just leads to poor fuel economy and (sometimes) an unstable warm idle. Bucking and whatnot is usually something else. However, if you are still running the original O2 sensor, it is pretty likely that you would benefit from replacing it.

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                    #10
                    oxygen sensor
                    1991 318is ---230K - DD
                    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?

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                      #11
                      1221 code usually means vacuum leak. If you have never replaced ALL the rubber, plastic and gaskets and hoses in your car, then suspect a vac leak. A failing AFM can also cause this.
                      Owner - Bavarian Restoration
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                        #12
                        1/21 update: went ahead and ordered both the crankshaft position sensor and the O2 sensor. Shipping took forever (thank u COVID) but after pulling the battery and running the replaced parts for couple days no codes/check lights have been illuminated so im hoping all is good! will report back if anything else happens but thanks to everyone who helped. :-)

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