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  1. #1
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    1984 911 - Cold Idle Issues

    I have a cold idle issue with my 84 911 and I'm wondering if anyone has any suggestions. It started last year and has the following symptoms: the car starts properly and transitions to high idle, but when it settles to normal idle the rpms gradually drop until the car stalls. This will continue until the car warms up but occasionally when the car is at full operating temperature it will stall when the clutch is disengaged when I stop in traffic.

    I cannot find any vacuum leaks, I replaced the ICV which has no effect on the problem, I have replaced the DME relay, verified the throttle position switch and micro switch are working correctly, verified the cylinder head temp sensor is working correctly, cleaned and inspected the throttle body, replaced the fuel pressure regulator, replaced the ignition coil, rotor, and cap all to no avail. I suspect its an air fuel mixture issue. Very frustrating as the car drives perfectly when warm but problematic when cold. Does anyone have any suggestions?

  2. #2
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    Bad gas? The shelf life is only3 months.

    Go for a drive and burn it off.


    Sent from my iPad using Tapatalk
    Perci

    PCA DE Instructor

    2014 Cayman S
    2000 Boxster S
    2000 Carrera
    2010 Panamera 4S
    1989 928GT

  3. #3
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    Quote Originally Posted by Perci View Post
    Bad gas? The shelf life is only3 months.

    Go for a drive and burn it off.


    Sent from my iPad using Tapatalk

    Thanks, but unfortunately that's not the issue.

  4. #4
    Member malcolm's Avatar
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    start with all the basics; compression? spark? fuel?

    If compression is good Id replace the spark plugs and check the leads.........all good? what colour are the plugs?

    other than cylinder head sensor what other sensors are there to control fuel/air?

    sight unseen Id suspect, given all the basics are good, theres a sensor issue thats leaning the mixture to quickly or the throttle body needs a rebuild.
    '93 928 GTS 5 speed
    Polar Silver/Linen, 192k kms

    '88 928 S4 A/T
    Black/Ivory, 190k kms

    '09 Cayenne GTS Tiptronic
    Nordic Gold/Black, +300k kms

    '88 928 S4 A/T
    Red/Champagne, 110k miles (US car)

  5. #5
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    Thanks Malcolm, you may be correct on it being an instrumentation issue.

    Here's a summary of everything I've done including a more detailed description of the problem.

    Problem: when cold, the car starts, goes to high idle, and then as it drops to normal idle the RPMs slowly drop until the car stalls. It is often difficult to restart. After about 10 minutes of driving, when the engine reaches normal operating temperature, the idle is normal (around 850 RPM). Driveability is good/normal, however occasionally when the car is at normal operating temperature, the car will stall when the clutch is disengaged after a long period of sustained driving. The car restarts without issue on these occasions.

    The following actions have been taken which have had no effect on the issue:

    Air System
    • Idle Stabilizing Valve replaced
    • Oxygen sensor replaced
    • Throttle body inspected and cleaned – butterfly clean and closes fully, general condition very clean
    • Air Flow Meter removed and inspected – gate clean and moves freely, general condition very clean
    • All vacuum lines checked, no cracks or leaks found
    • Idle speed adjustment screw removed and inspected – clean condition
    • Rubber boot between AFM and throttle body removed and inspected – clean condition, no cracks
    • Air-to-fuel ratio and idle speed adjusted

    Fuel System
    • Fuel filter replaced
    • Fuel pressure regulator replaced
    • DME relay replaced
    • Fuel pump relay replaced
    • DME inspected and rebuilt

    Ignition System
    • New spark plugs 8k km ago
    • Distributor rotor and cap replaced
    • Spark plug wires replaced
    • Ignition coil replaced

    Instrumentation
    • Cylinder head temperature sensor tested – OK
    • Throttle position switch tested – OK
    • Throttle microswitch tested – OK
    • AFM temperature sensor checked - OK

  6. #6
    Member malcolm's Avatar
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    extensive list, thanks for sharing.

    what sensor triggers the high idle, AFM? is there another sensor? are the sensors costly?
    '93 928 GTS 5 speed
    Polar Silver/Linen, 192k kms

    '88 928 S4 A/T
    Black/Ivory, 190k kms

    '09 Cayenne GTS Tiptronic
    Nordic Gold/Black, +300k kms

    '88 928 S4 A/T
    Red/Champagne, 110k miles (US car)

  7. #7
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    I just checked all the sensors and they all test fine. I have a new spare cylinder heat temperature sensor (CHTS) and I tested it against the one installed in the car and they agree within 200 ohms, so the CHTS is fine. I also checked all the electrical connections to the fuel injectors as well as all the engine ground connections and the engine to transmission ground strap. Didn't make any difference. I hate to admit it, but I'm totally stumped by this one! On the positive side, my engine is full of new components and sparkling clean!

  8. #8
    Member malcolm's Avatar
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    Quote Originally Posted by jdoering View Post
    I just checked all the sensors and they all test fine. I have a new spare cylinder heat temperature sensor (CHTS) and I tested it against the one installed in the car and they agree within 200 ohms, so the CHTS is fine. I also checked all the electrical connections to the fuel injectors as well as all the engine ground connections and the engine to transmission ground strap. Didn't make any difference. I hate to admit it, but I'm totally stumped by this one!
    Was the O2 sensor checked? Cycle the DME? Pelican has an interesting write up on setting the idle "In order to properly adjust the idle on the Motronic system, you need to disable the idle volumetric control"

    Quote Originally Posted by jdoering View Post
    On the positive side, my engine is full of new components and sparkling clean!
    ......and you know your car so much better
    '93 928 GTS 5 speed
    Polar Silver/Linen, 192k kms

    '88 928 S4 A/T
    Black/Ivory, 190k kms

    '09 Cayenne GTS Tiptronic
    Nordic Gold/Black, +300k kms

    '88 928 S4 A/T
    Red/Champagne, 110k miles (US car)

  9. #9
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    Feb 2016
    Location
    Calgary
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    Yes, I know that engine intimately now! I replaced the O2 sensor and tested it, all good. I have adjusted the idle according to the Motronic procedure to disable the volumetric control and it responds properly, allowing me to increase and decrease idle speed, but this does not affect the issue.

    Really an odd problem as I thought I would have surely found the smoking gun after all this. I took a video of the start up that shows the problem that clearly demonstrates the behaviour. It behaves like a vacuum leak but I cannot find a leak anywhere!



    Thanks for your suggestions - I appreciate you taking the time to try and help :-)
    Last edited by jdoering; 08-26-2020 at 09:29 PM.

  10. #10
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    Update - bad air flow meter connector!

    So as I was inspecting the instrumentation for the umpteenth time, I noticed that one of the female connectors on the air flow meter connector is damaged! However, when I hunted for a 4 pin Bosch type connector online, all I seem to be able to find are 3 pin type.

    Does anyone know where I may be able to find one in Calgary? Here are pictures of the connector:

    Name:  911 AFM Connector 1.jpg
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    Name:  911 AFM Connector 2.jpg
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Size:  112.4 KB


 

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