Odd misfire.

OhNoA2.0 · Post by **OhNoA2.0** » Fri Nov 30, 2012 5:57 pm

I'm having a weird misfire in my RT. It only happens at low rpm, idle, or when free-revving. As soon as the engine gets put under load, it runs great. I swapped the coil pack, the issue went away for about an hour,. I changed the plugs, it was good for another hour. One thing I notice is that when it happens, I spike lean on my wideband. Maybe my injectors are sticking or not operating? I am running 1680cc injectors with high impedance injector drivers, could that cause the issue.

The car is fully built and boosted, running on MS2V3

gilly02le · Post by **gilly02le** » Sat Dec 01, 2012 12:44 am

check your intake manifold for vacuum leaks, Squirting around the flanges(upper and lower) with a spray bottle filled with water will drop the idle if you have a leak there. My magnum intake leaked on the end near the thermostat housing because the gasket shifted a bit during install, but only leaked after a few months of driving. had the same weird misfire as you mentioned.

OhNoA2.0 · Post by **OhNoA2.0** » Sat Dec 01, 2012 2:00 am

Are you on MS as well?

Post by **occasional demons** » Sat Dec 01, 2012 9:59 am

If it is a vacuum leak, the engine management won't make any difference.

It will happen more at idle/no load, because that is when manifold vacuum is highest. Under load, the vacuum drops off and the pressure difference between the atmosphere and manifold vacuum is less.

Under boost, there will be pressure leaking out of the gap/hole. But it won't affect the fuel getting shoved into the cylinder like a vacuum leak at the flange gasket would.

Since the injector is before the flange, a vacuum leak will suck the air in from the flange, and the fuel from the injector won't make it into the port nearly as well. Thus the misfire.

It would be like an injector not operating properly until the duty cycle was high enough. but in reality, the correct amount of fuel is puddling in the runner.

While it isn't a 100% guarantee of what your problem is, it is a good place to start. and checking the flange for leaks is free, or darn close.

If the injectors were in the ports, a flange leak would primarily give you a high idle, like any other vacuum leak.

The other thing you could check, is the MAP sensor for being gummed/gooped up.

OhNoA2.0 · Post by **OhNoA2.0** » Sat Dec 01, 2012 10:58 am

Great explanation! I'll be checking that out today.

trojmn · Post by **trojmn** » Sat Dec 01, 2012 11:42 am

OhNoA2.0 wrote: One thing I notice is that when it happens, I spike lean on my wideband.

correct. a misfire WILL show a lean spike.

OhNoA2.0 · Post by **OhNoA2.0** » Sun Dec 02, 2012 1:38 pm

So I checked around the manifold, all seemed well there but I snugged up the bolts anyways. It was still misfiring. I tried replacing the injector o-rings and it seems to have fixed the issue

Thanks a million!

And it figures, after a few hard pulls, misfire free, my trans decides to crap out on me. Oh well, I've been wanting to toss in a 3.55HD

2GN.ORG

Odd misfire.

Odd misfire.

Re: Odd misfire.