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Old 01-03-2008, 04:01 PM   #1
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Default Bouncy Tach Needle

Ok, this is really starting to irritate me. I'm using a Megasquirt V3.0 and built it using the format that Braineack has displayed on the "how to build your own Megasquirt" sticky. I bought a harness from him also and am using that (nice harness by the way). I have AC therefore i must use the OEM ecu to control my idle. I'm using the OEM coils right now. I have a brand new ignitor in there since i cooked my old one.

The problem is that I still get this misfire around 2-3k rpm and whenever that happens, the tach needle drops like it turns off for a split second. I've been told to check out the TACH WIRE AND THE SHEILDING as they might be shorting. Done that and made sure they won't touch by wrapping each in electrical wire. Add more grounds and i've added 2 more grounds, but that didn't help at all. Checked all engine grounds, the one on the block and PPF, all seems good.

Just to make sure, how do i check the megasquirt unit itself to see what version software i have on it? I wanna make sure that i'm using the 29v and not some other firmware. I've heard that the other firmware have misfiring issues or something like that?
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Old 01-03-2008, 05:33 PM   #2
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Might want to take a data log - see if the Megasquirt actually "thinks" the RPM has dropped and adjusted the tach output accordingly.

If you run the PortCheck.exe file included with MegaTune, it will tell you what firmware you have. Note that 029r through 029v all have the same signature and identify themselves as 029r.
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Old 01-04-2008, 05:37 PM   #3
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When I took datalogs, it doesn't show the RPM dropping and returning to normal. Everything looks good. I just get this irritating misfire. Usually happens with some load on the engine. Maybe I should check my plug wires, they're about a year old but I hear they go pretty fast? Could be bad wires?
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Old 01-18-2008, 05:08 PM   #4
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I'm having a similar issue, but I believe mine is caused by my coil pack going bad. I unfortunately left the output inverted with a bad MSQ and now I get frequent misfires especially when below 13.5AFR. Coil pack is something you might want to check if you burned your firmware while the coils were connected or had output inverted at any point. Check spark angle and AFR in that area as well.
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Old 01-18-2008, 05:12 PM   #5
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matt you think the .1uf capacitor from js8 to ground would help?
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Old 01-19-2008, 04:22 AM   #6
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Hmmm, I did leave the coil packs hooked up when I fried my ignitor. Wonder if my coil packs are goin bad?

Matt, I'm also curious if that capacitor might help if the coil pack isn't bad. I think I've read that somewhere in the msefi forums.
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Old 01-20-2008, 11:54 AM   #7
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I've seen grounds correct this in some cases (if they're inadequate and the source of the issue, otherwise they won't help)... but the low pass filter cap will likely do the trick. In any case it's a good idea... I bet it fixes you right up.
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Old 01-20-2008, 12:09 PM   #8
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Could it be resets?
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Old 01-20-2008, 12:28 PM   #9
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That could be seen in a log pretty easily-- log it and see if you see the red line at the bottom of megaLogViewer indicating there is a reset. The symptom sounds like a 2nd trigger sync issue though, which the low pass filter cap will most likely fix...
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Old 01-20-2008, 04:56 PM   #10
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In my MS install, there's two main places on the harness where it connects to the factory ecu ground. In each place, three wires connect to 1 larger gauge wire. Can I just connect up the two places and add 1 large gauge wire and run it to the chassis ground where the factory ecu grounds itself? Or would I have to run 2 sep grounds from the harness. Right now I have it set up so that the two grounds on the DB37 harness are connected and one large gauge wire is run to the engine block ground in addition to the factory ground. I'm gonna try and install that cap soon to try and get rid of this crazy misfire.
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Old 01-20-2008, 05:17 PM   #11
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Individual smaller guage wires for grounds, all run back to the same place on the block (or the brackets the stock ECU used) will be better than coming together to a single wire and going back on that.

cap! cap!
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Old 01-22-2008, 12:15 AM   #12
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For the 0.1 uf capacitor from JS8 to GND, does it matter which ground point on the DB37 connector I connect to?
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Old 01-22-2008, 12:16 AM   #13
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Why dont you just connect it to the ground point on the proto?
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Old 01-22-2008, 12:19 AM   #14
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That's what I'd recommend too-- but you could go to one of the grounds at the DB37 if you wanted.... anywhere that ties directly to the ground plane on the board.
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Old 01-22-2008, 12:32 AM   #15
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Ok, was just making sure which ground i had to get it onto. I'm gonna do it tonight if the crazy rain lets up here.
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Old 01-23-2008, 03:06 PM   #16
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Ok, thought i'd give you guys an update. The rain finally let off so I could remove the megasquirt from my car and install the 0.1uf cap. So far its been running even smoother. No misfires so for my drive to work this morning. We'll see if this cap really fixed it during my ride back home when its hot. Thats when it misfires a lot. I have my fingers crossed!!
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Old 01-24-2008, 03:00 PM   #17
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Car has been running perfect eversince I put in the Cap. Thanks Guys.
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