MSPNP backfire when turning key to on
 

I've been getting the odd backfire when turning the key on lately but today I got the mother of all backfires! It blew the charge pipe off my blower and caused a cloud of smoke to rise out around the bonnet!

Do you know if I can prevent these at all?

Cheers

sounds like maybe you have a leaky injector, the pressure from the fuel system causes the fuel to lead down into the cylinders while the car is parked, and then it ignites on key on.

Have had it happen on other vehicles, with stock ecus..

That sounds like a good possibility, time for these 330cc injectors to go in I guess. I had assumed it was the MegaSquirt's fault as it started happening around the time I installed it.

It's not a leaky injector. In the cranking settings set the priming to after 2secs.

MS sparks the coils at key on, if you prime straight away and the cylinder that has open valves also gets a spark, boom, backfire.

Mine did it loads till I swopped that setting.

Awesome, thanks!

Cool.

this answers one of my problems as well, thank you.

The new setting is working well, but I had a look in my engine bay and when the charge pipe got blown off from the old setting it broke my radiator thermo fan from the force! Pretty crazy!

Thank you, I was wondering about that too. Very helpful information

I was switching between my stock ECU and MS wondering why this was happening; that makes perfect sense. Thank you.

I've been thinking about this issue lately. Yes, with a Megasquirt installed the plugs do spark right at powerup. In looking at the schematic, I've decided this isn't by design, and it's our fault.

Have a look at this image, which is snipped from the 3.0 schematic. This is one of the two LED outputs that we typically hijack and use to drive our ignition outputs. I've added in green the pullup resistor and DB-37 jumper which constitute that mod that we all do, based on DIY's writeup.

http://img238.imageshack.us/img238/3222/image1yq1.gif

First off, understand how the ignition system works. When the igniter sees +5v, it turns on and provides a path to ground for the coil primary to begin charging. When the igniter sees 0v, it turns off and opens the path between the coil primary and ground. It is at this moment that the plug fires. (important point- the plug fires when the coil turns off, not when it turns on.)

Ok, this circuit is an open-collector, with pullup. That means that the voltage at the output pin is normally +5, because voltage is flowing through the 1k resistor (and the LED / 330 resistor) and has nowhere to go. When voltage is applied to the base (pin 2) of Q6 by the CPU, then the transistor turns on, providing a low impedance path for the voltage coming through the 1k and the LED, and pulls the output pin to ground (0v).

So the logic of the "spark output inverted" setting is sorta backwards. When set to "no" then the output of the pin on the CPU is indeed inverted relative to the output on the DB37, at least the way we do things.

When you first turn on the MS, the CPU comes up in an inactive state- all the pins are off. During this period, Q6 is off, and therefore the ignition output is +5. It takes a few milliseconds for the CPU to load its code and boot, after which it initializes all the pins, bringing the ignition pins high, turning on Q6, and pulling the output low. It's at this moment that the plugs fire. FWIW, this is also why igniters and coils fry when you do a code update- while the CPU is being flashed all the output pins are inactive, and therefore the ignition outputs see +5 continuously.

So why the heck do we wire it this way? It's convenient, granted. It requires minimal modification of the original design to use a usable output.

But wouldn't something like this eliminate the spark event at startup? The output will be high when the CPU pin driving it is high, so set Spark Inverted to Yes, and away you go.
http://img98.imageshack.us/img98/4406/image3ej8.gif

Joe, when we used to all run inverted, we simply ran it like this:

http://www.boostedmiata.com/MS/invertedspark.jpg

just FYI.

No kidding? I wasn't sure how much current it required to drive the igniter.

And the reason we stopped doing it that way is... ?

cause it was unstable; missed, and burnt igniters :)

Fuck, I just killed my second thermo fan today due to the charge piping blowing off and smashing it when the backfire occurs. This is even with the revised "after 2 seconds" setting.

Pretty damn big backfires compared to what I see. If you have it set to after two seconds, also set it to prime>0. How rich is your idle. At one point I idled at like 10:1 just because i hadn't messed with it. That rich idle left enough gas in the cylinders that it would backfire when i came back to the car and turned the key. Lean it out if you can.

Thanks I'll give it a go tomorrow. My idle is around 13.5:1.

That shouldn't be a problem then.

That shouldn't be a your problem then. Does it happen every time you start it after it's sat a little while?

Those settings didn't seem to fix the problem unfortunately. It happens when I turn the key to on and leave it for a few seconds but don't start it. If I jump in and turn the key to "start" straight away, it won't do it.

I have managed to avoid it for a few weeks but my wife called this morning saying it had happened and I had to talk her through reconnecting the charge pipe. Pretty dangerous as car runs totally unthrottled when this happens. Killed another fan, too! :vash:

Cheers