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Thread: Fuel delivery issue - Only new injectors not firing

  1. #1
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    Fuel delivery issue - Only new injectors not firing

    I'm trying to hunt down a not firing on all cylinders issue. When I removed the injectors to run a mason jar/spray pattern test I found that 3 had good pattern, 2 were weak, and 1 (cylinder 5) had no spray. I replaced 1 weak and 1 good with fresh injectors and neither new injector fired. I moved them around the distributor and no matter which of the ports I attached them to, the new injectors won't fire. I also switch fuel lines to confirm the lines aren't blocked. I can tell that the port for cylinder 5 is weak, so I'm guessing distributor? My question is, if my fuel pressure is weak, would that cause the new injectors not to fire with their fresh springs? When a distributor fails, is there a general lack of pressure or is it variable by port?

    Within the last year I have replaced: 9 Fuel lines, fuel pump, CPR, removed the Cat (was melted inside), and dropped and cleaned out the tank.

  2. #2
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    No way to know what pressure the injectors are opening at unless you test them. It is possible the distributor is plugged up. At this point you should send the distributor/mixture unit out along with the injectors to be cleaned and tested. Send the old ones too and maybe they can clean you up a good set. Look up Special T. If the tank was dirty (you said you cleaned it) you probably have all that dirt throughout the fuel system so you should flush the lines and reclean the tank before you put the clean parts back in. If the cat was melted inside that means your mixture was way too rich and your cat was turning red from the fuel burning inside it.
    David Teitelbaum

  3. #3
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    That makes sense, thank you. I'll call John. I wasn't sure how wide the pressure difference would be. I should clarify that when I cleaned the tank there was almost no contamination or dirt in it. I had removed it to do some frame cleanup and figured I'd clean what little there was while I was in there. The car has been driven regularly for the last 10 years, but I think a year or so ago one full tank lasted 9 months. I hoped that wouldn't be enough time for the fuel to go bad, but I guess it may have been.

  4. #4
    Senior Member DMC-81's Avatar
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    Because all 6 injectors fire at the same time, I think it is good practice to replace all 6, just like replacing all spark plugs or doing brakes at least in pairs. The other parts are worn and if you change them out on a piecemeal basis, I think you will always be chasing poor performance. The spring rate will be different on these injectors because some are worn (with an unknown life) and 2 are new.

    Just like a lawn irrigation circuit, a blown sprinkler head causes the other ones not to pop up and spray as the water pressure is wasted on the on the blown head (an extreme example, but hopefully one gets the point). That's why your new injectors with strong springs don't open even if you change them around.

    My advice is to treat the car to 6 new injectors before potentially wasting money on a fuel distributor rebuild, especially since you said that the fuel system was cleaned well. I bought new injectors when I got my car, and the engine runs smoothly... highly recommend.
    Last edited by DMC-81; 11-12-2018 at 01:23 PM.
    Dana

    1981 DeLorean DMC-12 (5 Speed, Gas Flap, Black Interior, Windshield Antenna, Dark Gray)
    Restored as "mostly correct, but with flaws corrected". Pictures and comments of my restoration are in the albums section on my profile.
    1985 Chevrolet Corvette, Z51, 4+3 manual
    2006 Dodge Magnum R/T (D/D)
    2010 Camaro SS (Transformers Edition)

  5. #5
    Senior Member DMC-81's Avatar
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    Quote Originally Posted by DMC-81 View Post
    Because all 6 injectors fire at the same time, I think it is good practice to replace all 6, just like replacing all spark plugs or doing brakes at least in pairs. The other parts are worn and if you change them out on a piecemeal basis, I think you will always be chasing poor performance. The spring rate will be different on these injectors because some are worn (with an unknown life) and 2 are new.

    Just like a lawn irrigation circuit, a blown sprinkler head causes the other ones not to pop up and spray as the water pressure is wasted on the on the blown head (an extreme example, but hopefully one gets the point). That's why your new injectors with strong springs don't open even if you change them around.

    My advice is to treat the car to 6 new injectors before potentially wasting money on a fuel distributor rebuild, especially since you said that the fuel system was cleaned well. I bought new injectors when I got my car, and the engine runs smoothly... highly recommend.
    Sorry, one more thing: Along with new injectors, I would advise to start with a fresh tank of gas (i.e. pump out the 9 month old gas), and then have your fuel pressure tested. You may need some PPR washers to bring the pressure up to spec.
    Dana

    1981 DeLorean DMC-12 (5 Speed, Gas Flap, Black Interior, Windshield Antenna, Dark Gray)
    Restored as "mostly correct, but with flaws corrected". Pictures and comments of my restoration are in the albums section on my profile.
    1985 Chevrolet Corvette, Z51, 4+3 manual
    2006 Dodge Magnum R/T (D/D)
    2010 Camaro SS (Transformers Edition)

  6. #6
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    I appreciate the analogy, that makes a lot of sense. I went ahead and replaced the full set of injectors and swapped out a functional distributor. I also put in another fresh fuel filter just to be safe and the fuel is fresh. Now I get no spray at all from the injectors. If it is a fuel pressure issue now, would I be adding or subtracting shims from the PPR?

  7. #7
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    You would add or subtract shims on the PPR based on your fuel pressure reading (should be between 72 and 75 PSI).

    Also, every Fuel Distributor has to be reset every time it is installed on a new (or different) Mixer-control Unit. Essentially with a fuel line disconnected at the fuel distributor, jump the RPM relay and turn the CO screw clockwise (richen) until gas starts coming out - at which point you turn counter clockwise just to the point where gasoline stops flowing out the fuel line connection. Once reset, (assuming all your injectors are good and you don't have ignition issues) you can turn the car on, wait for it to warm up and adjust CO once your O2 sensor kicks in.


    Quote Originally Posted by RobotMartian View Post
    I appreciate the analogy, that makes a lot of sense. I went ahead and replaced the full set of injectors and swapped out a functional distributor. I also put in another fresh fuel filter just to be safe and the fuel is fresh. Now I get no spray at all from the injectors. If it is a fuel pressure issue now, would I be adding or subtracting shims from the PPR?

  8. #8
    Senior Member DMC-81's Avatar
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    Quote Originally Posted by RobotMartian View Post
    I appreciate the analogy, that makes a lot of sense. I went ahead and replaced the full set of injectors and swapped out a functional distributor. I also put in another fresh fuel filter just to be safe and the fuel is fresh. Now I get no spray at all from the injectors. If it is a fuel pressure issue now, would I be adding or subtracting shims from the PPR?
    Cool. You would be adding shims to increase the pressure.

    Here is a good thread on the topic: http://dmctalk.org/showthread.php?12...egulator-Shims
    Dana

    1981 DeLorean DMC-12 (5 Speed, Gas Flap, Black Interior, Windshield Antenna, Dark Gray)
    Restored as "mostly correct, but with flaws corrected". Pictures and comments of my restoration are in the albums section on my profile.
    1985 Chevrolet Corvette, Z51, 4+3 manual
    2006 Dodge Magnum R/T (D/D)
    2010 Camaro SS (Transformers Edition)

  9. #9
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    You should not have to adjust the PPR but you will definitely have to set the mixture screw. Since you should be checking primary pressure anyway you might have to make a small adjustment with shims. Make sure to verify you have the correct mixture unit. They all look similar but the numbers tell what the specs are and the specific application. The internals are set up differently for each car. There is no exact cross/over to a Delorean. Same for the CPR and the fuel injectors. You MUST have the Delorean specific parts.
    David Teitelbaum

  10. #10
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    I'm curious, and a bit confused, with the chronology of your car and what happened up until now. While nothing you said jumps out as being the culprit, I'm wondering if maybe one or more of those things you mentioned or changed is the issue?

    Could you lay out a timeline of what you worked on or replaced, when and why? Going back to when the car was running fine. Have you owned the car going back long enough to when it was fine?

    What makes me think there's something else to this is reading how you had to replace your cat because it was melted inside. Sounds like the car ran very rich for some time to do that and maybe there's something else needing attention we aren't thinking of?

    Again, list out what the original problem was and then the steps you took to try and correct it, and I think someone on here will be able to spot where the real problem is or was.


    Sept. 81, auto, black interior

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