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Hilarity with Megasquirt

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  • Hilarity with Megasquirt

    Scott, you'll really appreciate this. OK, there's a friend of mine who somehow has conned me into letting me keep his car at my shop while he builds it. It's a generic 89ish nissan 240sx that we built a little engine for and he's bolting on a t3/t4 turbo and running it with MSv3. It's been slow coming along, especially since he moved to Knoxville a few months ago and just comes out about every other weekend to work on it. Yesterday was going to be the day of truth when he fired it.

    key on.

    fan turns on

    fuel pump turns on

    a few seconds later gasoline starts dripping out of the throttle body (we had the turbo inlet hose disconnected for the moment)

    key off

    open the throttle blade and gas POURS out of the intake.

    We look inside and the intake is COMPLETELY full of gas. I mean solid gas all the way down and filling up the cylinders!!!

    "how do I get it out?" he asks

    I tell him there are two options; he can pull the manifolds to drain them and siphon the gas from the cylinders or just pull the spark plugs and crank it over.

    The spark plugs go in from the top of the engine on this particular unit. Has anyone seen the fountain show at the Bellagio? Picture that but with gasoline now. I swear to God that gas shot up more than 25 feet in the air in four perfect streams! It was dripping from the roof of my shop! Totally hilarious. And a mess to clean up.

    He's trying to figure out what happened and why the injectors were held open. I can't figure out why. At a quick glance he has keyed 12v constant to one side of all the injectors and two injectors hooked to green and two to brown on the MS wiring harness from DIYautotune which is supposed to ground it. I guess it must be issuing a constant ground instead of cycling it. I let him use my stimulator to check things out and hopefully it will be running (and out of my shop) today.

  • #2
    Re: Hilarity with Megasquirt

    damn, it's a good thing gas isn't flammable!

    I think you're supposed to make a "dry run" with homebrew electronics first.
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    • #3
      Re: Hilarity with Megasquirt

      Just another reason I tell my buddies that there is no-smoking when firing up an engine for the first time. Everyone laughs at me when I bring out the fire extinguisher when we fire up a new engine.

      I am going to heed scotts advice with the new ECU and crank with the fuel pump off to check a few things first.


      • #4
        Re: Hilarity with Megasquirt


        To sort out what is going on try the following using either a 12v test lamp or better a DVM.

        1) disconnect all the injectors.

        2) with the key off - connect one lead to ground and probe each injector plug on both contacts- you should not get any voltage or the lamp on on either contact. If you do then the +12 needs to switched via a relay when the key is on - another way is to feed the injectors from the load side of the fuel pump relay.

        3) with the key on - connect one lead to ground and probe each injector plug - you should get voltage or the lamp on one contact only (the one coming from the +12 source).

        4)with the key off - connect one lead to the positive battery terminal and probe each injector plug on both contacts- you should not get any voltage reading( or very low - less than 1 volt) or the lamp on. If you do unplug the MS at the 37 pin harness connector and retest. If the results are the same you have a harness problem - a short to ground on the wires going from the injectors to the MS unit. If the problem clears you have a problem in the MS unit itself.


        • #5
          Re: Hilarity with Megasquirt

          I sort of went through that test procedure with the guy this afternoon. We found key on 12v on both sides of the the injectors. We also saw that there was just a single pulse on the MS box LED light. I checked the box with my Stim and it was fine. Also plugged in my extra MS box and had the same results.

          He is using a factory Nissan style optical sensor with 12v keyed to the blue/white wire, ground to blk/white and yel/white to the grey IGN wire (pin 24) on the 37 pin connector. I didn't know if it was an issue with the 12v on both sides of the injector (I double checked his grounds and they are fine) or if there was a prob with the dist signal. Using a DVOM I couldn't see any kind of signal coming from the dist. He has a wiring diagram someone made for wiring the nissan dist but the colors don't match right.

          Any ideas? And BTW, thanks for the reply.


          • #6
            Re: Hilarity with Megasquirt

            Key on - both sides of the injector +12 << sounds like a wiring error.

            I'd unplug the MS unit and then with the battery disconnected use a ohm meter to beep all the wiring going to the 37 pin connector 1 pin at a time. Most likely the only way your going to track this one down. I use a paper clip to stick into the DB37 connector and then clip the meter test lead to that. Unplug all the sensors and injectors then start tracing.

            I haven't used this wiring harness so the wire colors are meaningless to me - if you had the DB37 pin number then I can help more.



            • #7
              Re: Hilarity with Megasquirt


              If it looks fine on your Stim then the MS box is fine.

              I've had it lock the injectors open during wiring mishaps. If you short 12v directly to the injector output, they fry and lock open. I've also seen this on other EMS's so it's not a "homebrew" thing. It's a "BE MORE CAREFUL WIRING" thing!

              but I dig the geyser story. glad it wasn't a "megasquirt burnt down my shop" tale!!!

              his injectors might be cooked, and make sure he's got the right settings of they're low impedance.!

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              • #8
                Re: Hilarity with Megasquirt

                having 12v on both sides of the injector, when the system is powered up and the injector should be closed (not energised), sounds to me like the way it is supposed to be. The MS output circuit is open, the injector has relatively low resistance, so you'll see battery voltage on both sides of it.

                or am I missing something? I never did install that MS I built a few years ago.

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                "If it don't go, chrome it!" --Stroker McGurk


                • #9
                  Re: Hilarity with Megasquirt

                  Sure does sound like wiring. I've tried to stay away from the project to let him learn as he goes as this is his first car build. I'm guessing next weekend I'l be tracing down all of his wiring per your suggestions.

                  And yes, I did have two fire extinguishers handy when performing the geyser procedure!


                  • #10
                    Re: Hilarity with Megasquirt


                    don't you wish you ALWAYS had a camera around to catch such things!

                    dang, that's even more funny than the day I fired up the old van behind my shop to move it (it's turned into a somewhat mobile semi permanent storage facility) the old 302 fired right up (pulling fresh fuel from a gas can sitting on the floor next to me)

                    then I see this brown streak and a shriek like I've never heard in my life.....

                    apparently I disturbed the squirrel that was living in the van - he freaked out and ran through the fan, over the engine through the open doghouse - and right out under the hood - didn't even get caught up in the fan belts! lucky little bugger!
                    There's always something new to learn.


                    • #11
                      Re: Hilarity with Megasquirt