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RPM Bounce @ 1.5k

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    RPM Bounce @ 1.5k

    Hello R3V. I have an issue I’m looking to get to the bottom of. I recently purchased an 88’ 325i. The owner told me there was nothing to worry about mechanically. A week in to owning it I found it leaking coolant and power steering. I’ve since fixed both with the addition of a new water pump, thermostat housing, timing/accessory belts, radiator hoses and throttle body. After putting everything together as I started the car I noticed the check engine light was on. After trying the “5 step” trick to initiate the code sequence I was unsuccessful and I’m curious why. However, that’s not my main issue. I went on to start the car and it was fine. Idle was fine as well but when I stepped on the gas ever so slightly reaching 1.3k-1.5k rpms. It began to bounce or pulsate as seen on the video I’ve attached via Reddit. I haven’t preformed a smoke test on the vacuum system however I’m not aware of any leak that could cause that type of issue. My other thoughts are leading me to the idle control valve or the throttle position sensor. If anyone is familiar with this issue or knows how to resolve it I’d greatly appreciate the advise. Thank you.
    Last edited by MarioAndretti78; 09-27-2020, 07:46 PM.

    Is the engine turbo charged? If not, I would ask the mods to move this to the M20 section. You'll get a lot more views there, the forced induction forum isn't frequented very often.

    As for your check engine light issue, from the video the "CHECK" flashing on the dash isn't a check engine light. That's referring to the overhead check panel that checks your fluid levels and things. The check panel could very well be pointing to a problem, but more than likely you have some bad level sensors or some wiring that needs to be looked over. To turn off the "CHECK" you can just press the button on your overhead check panel and it will stop until the next time you start the engine.

    For the rpm bounce, that doesn't seem to be an ICV issue because the car is idling fine and I wouldn't expect a bad TPS to cause an issue like that either because the E30's TPS acts like a simple on/off switch. It may be that the engine isn't getting enough fuel, it almost seems like you're hitting an artificial limiter when you put your foot on the gas. Is there a restriction in the fuel system? Bad filter or rusty gas tank maybe?


      Thanks for the recommendation on posting to a different forum, I’ll be sure to do that. The M20 is completely stock. There are no modifications. And I’m aware of the flashing “check” light and it’s significance. I know it’s not a check engine light. My main issue is the bouncing of the rpms. I plan on addressing that issue by taking the throttle position sensor off of the throttle body and cleaning it up a bit. Would you recommend preforming anything else while I have the tps off and taken apart?


        Since you're pulling the throttle body I would replace the rubber lines running to the charcoal canister, usually they're cracked and rock hard at this point and they're easy to ignore.

        Like I said I don't think you'll find the TPS is the issue, it essentially functions as an on/off switch for wide open throttle. The symptoms of a bad TPS in an E30 would be poor full throttle performance, but the engine would still be able to pull to redline no problem.

        I recommend pulling the backseat and checking out the internals of the gas tank by removing the fuel pump. It's common for them to get rusty if the car has sat in the past and the rust would clog your fuel filter. I suspect your issue is related to fuel pressure, the engine has enough fuel to idle but not enough to move past 1.5k rpms or so.


          Usually when TPS is failing, it will cause high idle. For your issue, I would suspect a failing AFM. See my thread on the AFM rebuilding so understand why it will cause this issue.
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