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

  1. #111
    To resort to a conventional ignition you will also have to install a condenser in the circuit, and no, that coil's windings are not the correct ratio for a non-CD system. I still want to check stuff like rotor resistance when the problem occurs, when the rotor is hot. I would install a temporary tee in the return side from the filter, where you measured the check valve pressure of 12 or so psi. Run a fuel line into the car so you can read the fuel pressure WHEN the problem starts. I'd also run a timing light into the car so you can see if it is ignition or fuel that quits with time and temperature. As I recall you had once run it from a temporary fuel can source so you have ruled out fuel tank and filter/line problems at least back to the fuel pump. Correct? Have you also checked for ground circuit from CD box to the ground stud on the fender behind the fuel filter? I don't think I've really checked the wiring to the coil for temperature so can't say whats normal, but heat tells me there is a ground issue some place. Did you once say you have used ohm meter to check resistance of the transmission ground strap to the body stud?
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  2. #112
    Serial old car rescuer Arne's Avatar
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    Yeah, I figured I couldn't use the current coil if I did that. Oh well...

    Will re-measure the resistance of the rotor after some judicious use of my heat gun tomorrow. Pretty certain I've checked the ground behind the CDI box, but will confirm, and check the ground strap on the transmission as well.

    Not sure if my timing light leads are long enough to reach inside the car while driving, but if I remove the engine grill and go through the rear quarter window it might reach. Ditto for the fuel line for pressure test while it fails.

    You are recommending I check the pressure with a tee in the return line that goes from the injection pump to the filter console? Ahead of the return check valve in the fuel return path?
    - Arne

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  3. #113
    Senior Member tcsracing1's Avatar
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    good to see you have not given up!
    i have same issue and been putting my car off..... but i will be back to it soon.
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  4. #114
    Quote Originally Posted by Arne View Post
    Yeah, I figured I couldn't use the current coil if I did that. Oh well...

    Will re-measure the resistance of the rotor after some judicious use of my heat gun tomorrow. Pretty certain I've checked the ground behind the CDI box, but will confirm, and check the ground strap on the transmission as well.

    Not sure if my timing light leads are long enough to reach inside the car while driving, but if I remove the engine grill and go through the rear quarter window it might reach. Ditto for the fuel line for pressure test while it fails.

    You are recommending I check the pressure with a tee in the return line that goes from the injection pump to the filter console? Ahead of the return check valve in the fuel return path?
    Yes, we need to know fuel pressure when it quits on you, same for timing light, should reach thru quarter glass to back seat, may need helper to hold light trigger in back seat while you drive. We need to know what is failing, If there was access to a chassis dyno that would be even better!
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  5. #115
    Serial old car rescuer Arne's Avatar
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    Timing light shows no loss of spark when this happens, not even when it dies completely. Moving back to fuel, which was where I started.

    I don't yet have fuel pressure gauges back in my possession, but again, today when it died completely and wouldn't restart, there was lots of pressure at the outlet on the bottom of the fuel filter can. I will arange to borrow the gauge again, but I really do not think this is a delivery issue.

    Back to wondering about the fuel shutoff solenoid. While the symptoms are unchanged whether the wire is connected or not, I still wonder if it is somehow triggering spontaneously and shutting down the fuel when something gets warm. Not sure how to test that, though. I suspect I can't simply remove it from the outside and drive it.
    Last edited by Arne; 05-13-2019 at 10:52 AM.
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  6. #116
    Serial old car rescuer Arne's Avatar
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    Update - the car is now parked about 5-6 blocks downhill from home. Will not start. Cranks fast (dual Miata batteries), timing light still shows spark, but no fire. There is plenty (too much?) fuel pressure at the inlet port of the injection pump on the right side (looking forward). But no fire, and no smell of gas at the exhaust.

    While there is lots of pressure at the MFI pump inlet (fuel sprays everywhere when the hose is pulled, even when the pump is not running), there is nothing at the return line from the left side of the pump to the filter console. Seems wrong to me, if the fuel pressure exceeds what is needed by the pump, shouldn't it flow through back to the filter console and then back to the tank?

    Going to let it cool off for a couple hours and hope that it will then start up (as it always has when cold).
    - Arne

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  7. #117
    At this point something has been overlooked. I would drain tank and remove the filter sock and fuel return fitting. You should not have pressure when removing those lines. I would visually verify clean sock by holding up to light, visually seeing the tube inside. I would verify return with compressed air from engine bay inside of tank. I want absolutely verify open circuits from start to finish. I'd also want verify clean ground studs. (You said engine bay was painted).It will be something simple.
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  8. #118
    Serial old car rescuer Arne's Avatar
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    Got the car started and back home later this afternoon. While frustrating, the experience was still valuable as I was able to verify that I have spark, and I can ignore the ignition from here out.

    Called Mark (MFI Werks) to discuss what I'm seeing, and he - of course - agrees with Ed that the supply line to the MFI pump should not be under pressure like that. He also explained how the fuel supply and return at the MFI pump and filter console is supposed to work, which helps me to better understand what I should be looking for. Will post again when I know more.
    Last edited by Arne; 05-13-2019 at 06:58 PM.
    - Arne

    In Progress
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  9. #119
    Senior Member 2.5MFI's Avatar
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    Arne, You most likely have double checked this but, fuel filter console fuel lines corrected in early service manual. Should be like this....
    IMG_2248.jpg
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    Mark Jung
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  10. #120
    Serial old car rescuer Arne's Avatar
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    Quote Originally Posted by 2.5MFI View Post
    Arne, You most likely have double checked this but, fuel filter console fuel lines corrected in early service manual. Should be like this....
    Oh. My. Word.

    No Mark, my manuals all showed the inlets from the two pumps reversed. Oh, crud. Really?

    I was checking the output from the fuel pump at the console when I saw this. Was pondering the difference in volume from the line from the fuel pump compared to the output from the bottom of the filter. Lots of fuel from the pump, not much more than a dribble from the bottom nipple on the filter canister. Didn't make sense, since I knew the filter cartridge was new.

    Then I saw the post above.

    I had "verified" early in the process that the various hoses were connected "correctly", but I was apparently basing that on a faulty diagram. Those two hoses have been reversed the whole time I've owned the car.

    I corrected them according to the picture above, then re-checked the output at the bottom of the canister, which now flowed fuel beautifully.

    Hooked everything back up and took it for a drive. Wonderful! For the first time, the car will pull nicely up a grade, in any gear.

    Not certain why it ran OK when cold when the fuel flow was that low. Or why the flow test results were sometimes inconsistent. But at this point, it runs good cold or warm. I could stop here and I'd be thrilled.

    But I will get the AFR and do some final tuning. The way it is running now, I don't expect to have to go more than one, maybe two clicks at most.

    Thanks to all here, especially to Ed and Mark for their patience and help!
    Last edited by Arne; 05-13-2019 at 08:13 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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