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Thread: It all started with a dead battery

  1. #1

    It all started with a dead battery

    A few weeks ago my battery was stone dead even with battery tender on it. So i jumped started it to move it and it ran fine. I then purchased a new battery and started right up but misfiring. Drove it a few miles and wouldn't clear up. Seemed to be running on 5 cylinders but not. So i began searching and trial error with spare parts. Idle is poor and sluggish with heavy gas fumes and really down on power.

    72 911E
    Compression 160-170
    Leak down 2-5%
    Fuel pump flow 1000CC+ in 30 seconds
    valve timing spot on
    valve lash GOOD
    Timing and advance... Good with both Distributors
    MFI pump timing good
    Disassembled and checked fuel console
    Checked fuel pressure 13psi

    Replaced ignition coil
    Replaced cap/rotor
    Replaced with rebuilt CDI
    Replaced plugs/wires
    swapped MFI pump
    swapped distributor/points
    Swapped injectors
    Even swapped voltage regulator as last resort
    Still runs like crap no matter what is swapped. 0 improvement as when started.
    Last edited by kwikt; 01-29-2023 at 04:26 PM.

  2. #2
    Is the hot air hose hooked to the thermostat, and delivering hot air to it. Double check your firing order.
    Early S Registry member #90
    R Gruppe member #138
    Fort Worth Tx.

  3. #3
    Yes firing order correct. First item I checked was thermostat tube. All good. Ed this car finally stumped me in last 38 years of owning it. Got to be something simple like not enough voltage to cdi/distributor. Ideas!!!!

  4. #4
    It may go back to WHY was the battery that dead? So have you checked charging voltage and voltage to the CD box as you said? If the car was running perfectly before the battery died then there must have been something in that sequence causing this. Wth engine running use an infra red heat gun and aim it at each exhaust pipe near the head flange. Look for even temps, need to know if it is one cylinder or all of them.
    Early S Registry member #90
    R Gruppe member #138
    Fort Worth Tx.

  5. #5
    Ed I checked for dead cylinders already with my Temp gun close to cylinder head and all seemed uniform on temp but to me it seems like it's not firing hot. I haven't looked to see if there is a purple spark at the plugs yet which would indicate weak firing. I'm also going to check voltage at CDI center pin which I believe is 12v. I swapped voltage regulators thinking maybe voltage but that wasn't it. I checked voltage at battery and while running and all is fine. Really appreciate the input Ed!!!!!!

  6. #6
    Senior Member Scott A's Avatar
    Join Date
    May 2014
    Location
    South East USA
    Posts
    897
    Two things that caused rough running for me in the past.

    My distributor was off by a tooth. (Tho that was long ago, and I may have fabricated a new memory)

    My 16 pin plastic connector had melted, and a short developed.

    (The pin connector back in the engine area, on left, near the fuse box)
    I had to open the connector to see it.
    (It could be 14 pins, I forget the count)
    In this case, there was a poor ground at the rear, which caused the car to search itself for another ground.
    Last edited by Scott A; 01-31-2023 at 04:31 AM.

    Current long term ownership: 63 Cab, 71 911, 74 914

  7. #7
    Senior Member
    Join Date
    Aug 2020
    Location
    Shawnee, OK
    Posts
    179
    I would verify the voltages to coil, cdi, and mfi(if it has any electricity). I had problems with my ignition switch which cut the voltage to the engine-unplugging the wires at the ignition switch, then plugging them back in picked up 2 volts to the coil. My starter quit working and that problem was the ignition switch was coming apart, some metal clips spread and separated the sections of the switch. Copper films on old Fuses and relays can lower or stop voltage. My horns quit and unplugging and re-installing the same relay solved the problem. Good luck!

  8. #8
    Thanks for the reply.....I'm going to check grounds and connections this weekend. Hopefully I'll put an end to this madness lol

  9. #9
    While you're at it check for battery drain by removing ground strap and using an ammeter to check for battery drain. Should not be more than 25 milliamps. Still want to know why battery died with even with battery tender on it. Exactly how long ago was it that car ran perfectly. Week, month?
    Early S Registry member #90
    R Gruppe member #138
    Fort Worth Tx.

  10. #10
    The car ran great up until battery died about a month ago now. Like I said I jumped it with my F150 and started right up and ran great while i drove it around the block to park it back in the garage. Turned the key and NOTHING. That's when I purchased the new battery and problem started.....Today i checked voltage at center plug going to CDI 12.3 volts so I have voltage! I'm checking grounds to see if bad connections. I was just going to swap batteries from my truck just for the hell of it who knows.

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