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Thread: Surging idle only for first minute of cold start... before and after major rebuild

  1. #1
    Join Date
    Jul 2013
    Location
    CT
    Posts
    434
    My Cars
    1989 325i 4 Door 5sp

    Surging idle only for first minute of cold start... before and after major rebuild

    Hey all

    89 325i 5sp 120k

    At cold starts I'll get a stumble and surging idle only when the car is entirely cold (sat more than 6hour say) and then only until I load it up some with some revs etc. For instance it'll happen while reversing in the driveway, pulling to the first stop then go away. So its not at all about the temperature of the motor. Nor is it really about exterior temp. Once it "clears its throat" it runs 10000000% perfect from idle to redline. Car was doing this before AND after a major rebuild which included the vast majority of culprits:

    every single head and block seal/gasket replaced
    valve adjustment
    plugs, rotor, cap, throttle switch, 02 sensor, vacuum lines
    fuel filter, fuel lines, fuel pressure regulator, fuel injectors cleaned with new seals.

    My first thought was fuel pressure regulator since its responsible for keeping pressure 'overnight' but its a new name brand unit and behavior was unchanged before and after.

    I gave the idle air valve a light cleaning, it move smoothly, cleaned elec contacts.

    Not really sure what else....
    Last edited by somecarguy; 05-14-2019 at 09:04 PM.

  2. #2
    Join Date
    Jul 2013
    Location
    CT
    Posts
    434
    My Cars
    1989 325i 4 Door 5sp
    I did not clarify that when I first start the car on these cold mornings the idle is perfect at 800. It’s only when I open the throttle that the surging begins which to me suggests a rich or lean condition. This morning I did a test and let the car idle for two minutes before moving off. This resulted in almost no surging or stalling which leads me to believe this is a time based issue I am struggling to think which component needs two minutes or so to ‘wake up’. Surely there is not that significant an amount of heat added to the motor in that two minutes.

  3. #3
    Join Date
    Dec 2005
    Location
    corpus christi, texas
    Posts
    6,738
    My Cars
    No e30s, again :(
    O2 sensor? Temp sensor to take car off fuel maps when warmed? No idea but just throwing stuff out
    No e30s again.

  4. #4
    Join Date
    Mar 2007
    Location
    Joliet,IL
    Posts
    1,121
    My Cars
    87 325is 96 GMC Suburban
    Did the condition occur before you replaced the throttle switch? Have you tried adjusting the new one?

  5. #5
    Join Date
    Jul 2013
    Location
    CT
    Posts
    434
    My Cars
    1989 325i 4 Door 5sp
    Yes and yes to throttle switch, adjusted per Bentley.

    O2 and its heater relay a good candidate will look this wknd.

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