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Thread: how to accurately test ECM?

  1. #1
    Fuel Injected!
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    how to accurately test ECM?

    I put did a TBI conversion on a crate motor and put it in my 72c10.. it was running great for about a year and then starting to have phantom intermittent problems during acceleration. Went through every wire, rebuilt TB, every vacuums tube, consistent fuel
    pressure, two fuel pumps and redid intake manifold twice.. No error codes are present. I cannot find the problem? Now I have given up and took it to a local shop.. they are hesitant to look at it because of the custom wire harness for tbi.. they are now telling me that they suspect the ECM to be faulty. I appreciate their effort but I feel that hey are guessing the the problem and not providing evidence as to the cause? I do not have a spare ECM lying around to do a test swap. Does anybody have a known method to test tbi ECM? It is a 1227747. Or possibly any other info or ideas. I have given up. Thanks, Tyree

  2. #2
    Super Moderator Six_Shooter's Avatar
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    Easiest way is to swap the ECM for a known good one.

    You can also buy engine simulators to connect to the ECM to bench test it and observe the diagnostoc data. I use a JimStim for this, that is intended for bench testing of Megasquirt ECUs, but with the right harness it can be used to test just about any ECM/PCM/ECU.

    I have only ever had 1 ECM die on me, and that was partially my own fault, FWIW.

    What mechanical tests have you done?

    What's the fuel pressure, both engine off and running, also under load?

    Have you done a leak down and/or compression test?

    When's the last time a full tune up was done?
    The man who says something is impossible, is usually interrupted by the man doing it.

  3. #3
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    I used a new distributor, cap, rotor, new wires, and plugs. Originally thought it was low fuel issue...new gas tank and fuel
    Sender as well.. I installed new high flow fuel pump with new relay and measured 11 pounds with a tee fitting behind the tbi at idle.. I also did TB rebuild and o-ring kit and cleaned out injectors. I do not have a gauge to measure during load..I have not done leak down. I have 5500 miles on this new engine. The truck was running great for a while and then this symptom came up.. it is random and intermittent... it does not send any new error codes with the poor acceleration and bogging out? I can connect to tuner pro and watch diagnostics while trucknis running and I am also able to get a 12 when I switch attempt to read codes... this is why I am skeptical of the ECM being faulty.. I do not have a second ECM to swap.

  4. #4
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    Two years and 5500miles on new engine

  5. #5
    Fuel Injected! uncabob's Avatar
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    Had heavy acceleration problem with one of my Explorers. Turned out to be the inline fuel filter. Could yours be that simple?
    Bob
    Don't force it, get a bigger hammer!

  6. #6
    LT1 specialist steveo's Avatar
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    Quote Originally Posted by Tyree View Post
    I put did a TBI conversion on a crate motor and put it in my 72c10.. it was running great for about a year and then starting to have phantom intermittent problems during acceleration. Went through every wire, rebuilt TB, every vacuums tube, consistent fuel
    pressure, two fuel pumps and redid intake manifold twice.. No error codes are present. I cannot find the problem? Now I have given up and took it to a local shop.. they are hesitant to look at it because of the custom wire harness for tbi.. they are now telling me that they suspect the ECM to be faulty. I appreciate their effort but I feel that hey are guessing the the problem and not providing evidence as to the cause? I do not have a spare ECM lying around to do a test swap. Does anybody have a known method to test tbi ECM? It is a 1227747. Or possibly any other info or ideas. I have given up. Thanks, Tyree
    in service shop diagnosis, the technician will usually read the factory service manual.

    most service manuals have this diagnostic flow chart, where you check a whole bunch of stuff..sensors, voltages, pressure..

    at the end of the flow chart is always 'replace ECM'. why is it the last step?

    the ECM is really the only thing that can't be reliably tested by your average shop. if everything else is good, ECM must be bad

    problem is if any of your tests PASS because of improper testing procedure (or worse, a failure that the author of the service manual didn't anticipate as a possible cause) the rest of the chart will probably PASS too and you'll be throwing an ECM at it according to factory service procedures.

    every dead ECM i've dealt with either stops being able to datalog, throws weird errors, or has a solid output failure (like it stops actuating the fan relay or can't hit a certain injector anymore)

    i would definitely tend to focus on looking for a wiring issue but you really need to narrow it down

    first you need to reproduce the issue while datalogging. if it doesn't show up in the datalog we can at least rule out a large percentage of possible causes. ecm datalogs are good at finding input problems even if it doesn't meet the conditions to set an error code.

  7. #7
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    I installed a brand ne filter as well.. I was hoping it would be something simple but it is turning out to be a phantom

  8. #8
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    Thank to all for thier replies.. I will continue looking for a solution. Truck is at shop and waiting to hear their opinion.? When I get it back, if they can’t find the phantom problem, I will do a data log while driving and try to recreate the issue. Although shit happens and anything is possible, I am
    Confident in wiring as I specifically soldered and heat shrink all connections and wrapped with wire loom.

  9. #9
    LT1 specialist steveo's Avatar
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    tons of solder in a wiring harness can be problematic, especially if too much solder is applied to a particular joint.

    copper is flexible, solder isn't. soldered stranded wire isn't really supposed to move at the solder joint, when it does, especially in an engine harness where the motion is repeated many times back and forth on the same joint, it can eventually fracture the wire or just break the solder and create an intermittent connection

    notice almost everything in a factory harness is crimped instead of soldered, except maybe major splice branches, and those are held rigid by insanely thick hunks of shrinktube

  10. #10
    Super Moderator Six_Shooter's Avatar
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    My bad, I read it originally as you had a second ECM.

    I would still go over the harness, since it seems to be the last thing in question. I'm not saying that you did anything wrong with it, but after 2 years and 5500 miles something can become a problem.

    I mean I've wired many vehicles, some from head light to tail light, to just adding a simple accessory, and everything in between. I sometimes find things that I could have done differently or better later.

    Best to check everything over and be sure than to assume something is good.
    The man who says something is impossible, is usually interrupted by the man doing it.

  11. #11
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    When installing the donor harness, I had to lengthen a few wires.. i soldered the necessary joint and properly shrink tubed all of the connections. In addition, I rewrapped everything in a new loom. Which I then wrapped.. also carefully fastened the loom away from heat and sheet metal. The truck ran fine for a year and I wa able to put 5500 miles on it.. At this stage I am
    Confident in the wiring.. I have triple checked a hundred times everything. I recently did a new cap and rotor and wire set.. and triple checked the base timing. I checked the
    Vacume and it is fine..I also put in a new TPS today..I am still getting the same symptoms.. I am
    At this point I am wondering if it is possible for the transmission to be failing and putting some kind of load on the engine during acceleration? I am open to any possibilities?.. I am close to giving up on the TBI and going carberated... carberated is a last resort but I have no clue what is wrong my truck.. I am lost and it’s costing me money every time I look at it.

  12. #12
    LT1 specialist steveo's Avatar
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    Quote Originally Posted by Tyree View Post
    I am close to giving up on the TBI and going carberated... carberated is a last resort but I have no clue what is wrong my truck.. I am lost and it’s costing me money every time I look at it.
    i don't even think you've narrowed it down between spark or fuel and you even have theories about parasitic drag from the transmission.
    throwing a carb on it might be as helpful as the TPS you just changed

    once more, get a datalog of the problem occuring

  13. #13
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    Are you confident there isn't a mechanical issue in the engine? Messing with the fuel injection forever won't fix a mechanical issue inside the engine.

    The '7747 is a rather basic setup. You can watch the TPS, O2 and MAP sensors to see if they are working correctly. You can watch the logging tables and see where in the fuel and spark tables it is operating and if that location matches the expected rpm and vacuum. If stuff checks out, then look further into it simply having a fuel/injector problem making it rich or lean, or an ignition system problem not providing the right spark to all 8 cylinders.

    The diagnostics of the 7747 are also rather basic. Not throwing a code doesn't mean the EFI system is working correctly.

  14. #14
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    TBI poor accelleration/bogging log

    I am confident that nothing is mechanically wrong. I again tested my fuel pressure and it is a consistent 12 psi and I also removed my new in line filter to quadruple check that fuel was not being restricted. I got 12 psi at idle and also at Wide open throttle. I took the truck a few blocks away and went down the main straight away to stomp on it. I was not able to get above 2000 rpms without the truck wanting to die on me and it was bogging. I took a log. If anybody can review the log and see for anything that would be great. I do not know too much about the log to interpret for myself. Thanks
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    Last edited by Tyree; 04-30-2018 at 10:04 PM.

  15. #15
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    Does anybody have a known log of a good running factory TBI 5.7 engine. I want to try and compare and contrast against my own log to see if I can learn something.
    Thanks again to anybody on this forum who has offered their suggestions.
    Tyree

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