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Thread: Rough Idle at Cold Start then Dies

  1. #1
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    Rough Idle at Cold Start then Dies

    1981 733i - daily driver and properly maintained. Put into storage four months ago. Tried starting her up this weekend, and she was not happy. Put two gallons of super premium in the tank. She cranks strong, will start and go into a rough idle between 500-800, and then after about 10 seconds shut down.

    Recommendations of what to verify would be appreciated - thanks

  2. #2
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    Hey Darko, does it run rich during startup idle?
    my 79 does the same thing, but if I nurse the throttle for just a few seconds at aprox 1000RPM it starts to idle better then settles into a steady idle- On mine it's the aux air valve, I was able to clean it some and it's sorta working.
    I just parted an 81 733i, it started and ran great, I kept the aux air valve (goes on the valve cover) and you're welcome to try it- also, I'd check that coolant temp sensor, causes all kinds of issues on these cars.

  3. #3
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    Quote Originally Posted by kojo96 View Post
    Hey Darko, does it run rich during startup idle?
    my 79 does the same thing, but if I nurse the throttle for just a few seconds at aprox 1000RPM it starts to idle better then settles into a steady idle- On mine it's the aux air valve, I was able to clean it some and it's sorta working.
    I just parted an 81 733i, it started and ran great, I kept the aux air valve (goes on the valve cover) and you're welcome to try it- also, I'd check that coolant temp sensor, causes all kinds of issues on these cars.
    Kojo96, what did you use to clean the Auxiliary idle valve? Did you clean it on the car? Off the car? Thanks

    Sent from my HTC6545LVW using Tapatalk

  4. #4
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    Hey Dave - thanks for the reply. I try to feather the throttle but she seems to chock on the effort. Once this last weekend I was able to feather the throttle and get her to idle and warm up. But when I came back the next day, same issues and she won't start and hold an idle. I'll take a look at both the aux air valve and coolant temp sensor. Cheers

  5. #5
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    Quote Originally Posted by bigperm206 View Post
    Kojo96, what did you use to clean the Auxiliary idle valve? Did you clean it on the car? Off the car? Thanks

    Sent from my HTC6545LVW using Tapatalk
    On mine I took it off, pass side of block, and very hard to find even used, new is $$$$$ if even still available- Seafoam, let it soak for a cple of days. It was stuck and didn't open, got a little movement out of it after letting it soak. Depending on outside temps, its either fine on first start or needs a little help.

  6. #6
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    Quote Originally Posted by kojo96 View Post
    On mine I took it off, pass side of block, and very hard to find even used, new is $$$$$ if even still available- Seafoam, let it soak for a cple of days. It was stuck and didn't open, got a little movement out of it after letting it soak. Depending on outside temps, its either fine on first start or needs a little help.
    Kojo96, thanks for the info. Time to get out the Seafoam.

    Sent from my HTC6545LVW using Tapatalk

  7. #7
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    Quote Originally Posted by bigperm206 View Post
    Kojo96, thanks for the info. Time to get out the Seafoam.

    Sent from my HTC6545LVW using Tapatalk
    Don't submerge the entire unit, just pour into chamber, where the build up is and prop it so it doesn't fall over.

  8. #8
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    Quote Originally Posted by kojo96 View Post
    Don't submerge the entire unit, just pour into chamber, where the build up is and prop it so it doesn't fall over.
    Good to know. I was going to submerge the AAV. Thanks for the heads up.

    Sent from my HTC6545LVW using Tapatalk

  9. #9
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    Thumbs up

    Are the injectors pulsing or is the thing just running from the CSV?

    My knowledge is limited on the early N/A EFIs...so if I am off base, let me know. I do know both the mid gen Ljet and Motronic 745i all would run on the CSV for a few seconds on cold start with the six injectors disconnected.
    I make E23 parts.
    09/1983 745i (stolen spring '13 around Houston, TX Achatgruen on nutria buffalo. 8481080)
    10/1984 745i
    11/1984 745i
    11/1984 735i (10:1-265/6)
    Ford, MB, and GM round out the pack.



  10. #10
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    Replaced the CSV with a new unit. Inspected aux idle valve and looked perfect - no build up. With the new CSV, same symptom - cold start, she would have a strong start and rough idle between 500-700, but as soon as I feather the throttle to get her to 1000+, she dies - and won't restart.

    Eric, at this point I do think she is running only from the CSV, based on description above. I changed out the DME about a year ago with unit bought from eBay, and I'm now suspecting the DME. Not saying the aux idle valve is not the issue.

    Any additional recommendations welcome - cheers

  11. #11
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    When I had my E23 (83 7331) I had this problem and I seem to remember replacing the cold start injector after going through everything above. It did amazing things to straighten out my idle. Just a thought.
    1990 735I (Auto) E32 Feb 1990 to Present
    1996 Z3 Roadster (5 Speed) E36/7 Jan 1999 to Dec 1999
    1983 733I (5 Speed) E23 April 1983 to Feb 1990
    https://www.youtube.com/watch?v=2d4tHoHGaJo
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  12. #12
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    It does sound like an ECU issue, that aux air valve only works when cold, you could actually jump the two wires and take it out of the ECU programming, see what that does.
    If you need an ECU, I have a cple, known working.

  13. #13
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    Thanks Dave - I have a spare and will report back - have a few things that i will work to get her started again - cheers

  14. #14
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    Hello all - reporting an update - diagnosed the problem to a corroded mass air flow meter. When the 7-pin DME plug on the side is disconnected, she starts and idles as expected. As soon as the DME plug in inserted back into the AFM, she would die. Opened up the AFM and found lots of reasons why the AFM was no longer working. Sourcing a new AFM now and she will be back on the road shortly - cheers

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