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Thread: MFI troubleshooting - high idle, pops and bucks at part throttle

  1. #101
    Serial old car rescuer Arne's Avatar
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    Going through the basic non-MFI checks as time permits. Unfortunately, my family believes that I need to do other things at times, rather than spending all my time working on the Porsche.

    Ohm'd all the plug leads, as well as the coil lead. Each plug wire was between 960-1020 Ω. The coil wire was just over 2000 Ω. I can't recall whether resistance is cumulative or if only the highest value rules. If you add them up, the effective resistance is ~3000 Ω. If you don't add them, they are only ~2000 Ω.

    Also checking valve adjustment. I did 4, 5 & 6 today (I do them one side at a time, more turning of the crank but less mess and back and forth). 5 of the 6 were just the tiniest touch loose. Nothing I'd normally get worried about.

    Going to be on the road for a couple days, so I won't get to checking 1, 2 & 3 until probably Sunday.
    - Arne

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  2. #102
    Check the cam timing while you're in there. You're fine on resistance. Be sure to check the rotor resistance. Since the problem occurs with temperature heat the rotor a little and check it. To duplicate the conditions you describe.
    Early S Registry member #90
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  3. #103
    Serial old car rescuer Arne's Avatar
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    What's the simplest way to verify cam timing? Dial indicator on a valve to check where it opens or closes?
    - Arne

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    - 1972 911T coupe, born (and someday will be again) Silver
    Sold - 1984 911 Carrera coupe, Chiffon white; 1973 914 2.0, Saturn Yellow; 1984 944, Silver Metallic

  4. #104
    Arne sorry you're having so much trouble getting it to run right. Keep digging, you'll find the problem soon.

  5. #105
    911 cam timing is done by measuring the intake valve lift at T.D.C. You use #1 intake valve for the 1-2-3 side and #4 intake for the 4-5-6 side. You'll need a 'Z' block and metric dial indicator. These are available from many sources, Stomski Tools is a good one. When I suggested that you do this since it only take a few minutes I guess I assumed you had the tools and had done this before. But none the less this is one spec that must be tested and either corrected or checked off as okay.
    Early S Registry member #90
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  6. #106
    Serial old car rescuer Arne's Avatar
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    Metric dial indicator I can borrow locally. Will look into the 'Z' block, but probably won't get to this for a while. But really, pulling the upper valve covers is not hard.
    - Arne

    In Progress
    - 1972 911T coupe, born (and someday will be again) Silver
    Sold - 1984 911 Carrera coupe, Chiffon white; 1973 914 2.0, Saturn Yellow; 1984 944, Silver Metallic

  7. #107
    Senior Member jt912's Avatar
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    I’ve got a pair of blocks and digital gauges. It was super convenient to use both. I can bring them Saturday if you need them.
    John Thompson

    1966 912-first car 31 years ago and still my driver.

  8. #108
    Serial old car rescuer Arne's Avatar
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    John, I'd love to borrow them, but won't be in town Saturday, leaving tomorrow for a couple days. I'll text you later to see if we can match schedules.
    - Arne

    In Progress
    - 1972 911T coupe, born (and someday will be again) Silver
    Sold - 1984 911 Carrera coupe, Chiffon white; 1973 914 2.0, Saturn Yellow; 1984 944, Silver Metallic

  9. #109
    Serial old car rescuer Arne's Avatar
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    Found a possible oddity in the ignition (CDI) wiring. Separate thread here - CDI wiring question
    - Arne

    In Progress
    - 1972 911T coupe, born (and someday will be again) Silver
    Sold - 1984 911 Carrera coupe, Chiffon white; 1973 914 2.0, Saturn Yellow; 1984 944, Silver Metallic

  10. #110
    Serial old car rescuer Arne's Avatar
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    OK, here's what has transpired today.

    Rotor ohms out at 4.85k Ω. I think it should be ~5k, so I'm going to call it OK. Checked, cleaned and dressed the points, they looked great. Re-installed and gapped them.

    Finished checking valve lash. Found several that were the slightest amount loose, none that were too tight. Corrected them all, but since there were none that were way out of spec, I did not expect any change there.

    Have not verified cam timing yet, as I haven't been able to meet up with @jt912 to borrows his blocks and dial indicator. Will do that before long, but as with the valve lash, I don't expect that to be the cause of this issue.

    Corrected the CDI ground connections as noted in this other thread - CDI wiring question. As with the above, I did not expect any change from this.

    Took the car out for a drive, and as I expected there was no improvement. Once it got good and warm, it began to stumble, sputter, and pop. Occasionally it would simply die, and generally was hard to restart.

    When it happens, it still feels like it is running out of fuel, and the tach does not bounce or otherwise indicate that it has lost signal. However...

    On the way back home, during two separate instances when it died and refused to restart, I checked for fuel supply by pulling the hose from the bottom of the fuel filter. Plenty of fuel pressure there, but the engine just cranked away like crazy with nary a cough.

    That would tend to confirm that fuel delivery is not the problem. From that point to the MFI pump is just that 50cm length of fuel hose, which in my case is new, and not kinked.

    Since the tach continues to show RPM while the engine is not firing and I'm under compression, it would seem to me that the points are still breaking (signal sent to the tach), but for some reason the coil is not firing.

    Remember that earlier I had swapped in a known good CDI box and coil from a '73.5 T (CIS) that runs perfectly, with no change, my car still experienced the issue once it got warm with the known good parts installed.

    The problem first shows up as a bad stutter, and rapidly progresses to complete no fire. Because of this, I don't think the problem is the plug wires, as having all 6 short out at once seems unlikely. The coil wire is still suspect, even though its resistance is OK.

    I'm also wondering about the CDI wiring harness. I noticed that the harness itself was noticably quite warm to the touch, even near the distributor. The CDI box generates a fair amount of heat as well. What if the CDI harness shorts or breaks the circuit when warm?

    I'm thinking of bypassing the CDI system (and it's wiring), doing a temporary re-wire to a pure points/coil ignition. Will the Bosch brown-top coil tolerate normal non-CDI ignition?
    Last edited by Arne; 05-12-2019 at 07:16 PM.
    - Arne

    In Progress
    - 1972 911T coupe, born (and someday will be again) Silver
    Sold - 1984 911 Carrera coupe, Chiffon white; 1973 914 2.0, Saturn Yellow; 1984 944, Silver Metallic

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