Misfire
 

I've been chasing this misfire problem for awhile now. I've gone back and forth between stock and MS controlled spark several times now, and it only happens with the squirt. No problems with stock ignition. The misfire is accompanied by a quick flutter down on the tach, usually a drop of 200-500 rpm. In an average 20 minute drive I'll get it maybe once or twice and usually more at the beginning when colder.

So far I double checked the wiring and looks like I have good connections although I only stripped the wires and twisted them together(1/2" of wire tighly twisted). Ignitiong wire is grounded on the MS side only. I added 3 more smaller ground wires to the DB37 that all go to the same stock spot on the block. I also cleaned up the grounds across the whole car to completely rule out a ground problem. I've tried a running dwell between 3.5 and 5 with no change. Took apart the MS today also to double check insides and all soldering looks good. Also checked the resistors that were added in the DIYautotune mods and again all check out.

Over the past few weeks I've cought it several times in datalogs but all that shows up is a small lean spike on the WB. Nothing else indicates anything happening and I've checked every other variable that gets logged.

Anyone have any ideas what else to check? I'd hate to not be able to use MS for ignition control but I might end up that way :td:

Could the ignitor be going out? And it is only showing on the system when under heavy laod obviusly you dont run the car as hard on stock ecu with boost etc as you do with MS right?

It usually happens during cruise under lighter load (probably because I just happen to be there most while driving), but has happend under boost once and occasional high load. Seems like it just misses whenever it feels like it. I also run just as hard on the stock ecu as I'm only at the wastegate 4.5 psi right now and it runs smooth as well.... stock :)

You really need to crimp your connections or solder them in order to guarantee that they are a good connection. Rule out all the variables you can.
Are you running the newer decoder settings that DIY autotune determined were better than what we were all using before? That seemed to help a few people.

RPM holds strong in the datalogs so I figured the connections were good. Hope my board is screwed up =/

And yea I'm running the updated settings. I check back on their site all the time to see if things are updated as they seem to always be trying to tweak the settings to get it just right.

I'll try soldering after work tomorrow if I have time just to make sure

I had the EXACT same problem. I added ground wires (7 total) from the DB37 to the block, and added a 6 ga ground from the battery directly to the block. This improved the situation, but it still missed occasionally. Finally I replaced the coils and the problem was solved. I found a coil from a Hyundai that fit, and I reused all of the stock coil wiring. That was a few months ago, and it has run great up to 10 psi.

I'd bet on the grounds myself. The situation you're describing is one I've seen to the letter, and corrected with additional grounding, never to return.

Individual grounds back to the same point on the block that the stock ECU grounds at (assuming you're using the stock ECUs grounds). Bottom line is make sure they all go to the same place, and the block is where you want them.

Could be dirty grounding point as well, a few swipes of emry cloth wouldn't hurt.

Do you think it makes much difference if I dont have the screws on the DB37 connector to hold it to MS? Does the extra pressure help?

They're really just they're to keep the harness from coming loose-- I'd recommend using them, but I wouldn't think it's related to your misfires...

So I think I am ahving the same problem. Very occasionly it misses and always seems to be right when i press the gas at a green light and am about to start going. Even more so if I am at a hill. I have atached a datalog and you can see at 31.523 seconds my rpms drop then go abck up quickly. Is that a misfire? I plan on redoing my grounds on this weekend.

mason.gmu.edu/~lsnead/drivehome.xls

In your log that mark is labled as "reset 1". Doesn't look like it comes back, so not sure about that. Other than that it looks pretty clean. It doesn't look like you are short on accel enrichment, but it would be an easy thing to increase and play with.

So that dosent look like a misfire? Any clue as to what it is or what causes it?

Yes sir-- that is a reset, as indicated by the red line in MegaLogViewer. Also if you highlight that instant in the log viewer, and look at the text near the bottom left, you'll see it marked as a reset as well.

Most often this is caused by either a noisy 12v power supply or inadequate grounds. It can also be caused by high current noise effecting the low-voltage signal lines. Check out this section of the MegaTune Manual for more info: http://www.megasquirt.info/megatune.htm#mt

It's a few lines down in this section ^^

You ever use a car audio power filter jerry?

No I haven't-- if the source is noisy 12v then I can see how it would help. If it's inadequate grounding it won't. Better grounding will help in either situation.

I understand, just curious if you'd used one. I'll use that as a last resort, still fabbing my boomslang.

Well I added 5 more wires to the block total of 8 plus stock two :eek:. I also took a wire brush to the ground strap on the passenger side of the motor, the stock ecu ground bolts and all parts they are connected to and the battery posts. Also added an extra 8ga wire from the battery to the power plant frame and cleaned that spot up as well.

Still get the misfire. :cry:

Are there any checks I can do to the MS board itself?

Where does your car misfire at? Idle? Light throttle/cruise? WOT? It's usually one of these three, and not all three, and each can have a different cause.

What are the indications of a misfire? Do you feel it? Do you see the tach drop out?

(Every question is here for a specific reason)

I've never noticed it happen at idle. Usually happens during cruise but has happened a couple times a WOT. Seems pretty random though.

Indications of the misfire are a small spike on the WB and a quick drop on the tach and I definatly feel it. Sometimes its small with a little flutter of the tach and barely feel it, sometimes bucks the car a little with a big drop on the tach of about 500 or so.

Checked over many datalogs and see no other sign on any other channels either