Lil'John

At RPM, underload my engine conversion runs like hell. It has either a significant misfire or backfire. I'm not sure which.

Standard cookie cutter information:
LQ4 from 2004 2500HD
NV4500 transmission
Hooker shorty headers
rear o2s programmed off
air diverter programmed off
e8249 (similar to e2000) fuel pump
VSS installed
"Custom" tune... but issue was the same before custom tune

I've been using a log program put out by Roadman. For the data presented, I am running open loop due to exhaust shop screw up(badly placed o2 sensor)

During my tests, I got the following:
RPM 4k, throttle pos% 52.6, engine load 50.2, temp 200, timing 48.5
RPM 2k, throttle pos% 24.3, engine load 3.1 temp 200, timing 35.5

VSS seems to be very off BUT those timing numbers seem to be really friggen out there.

So, where should I start looking?

Old Geezer

iTrader: (3)

I'd start w/ someone that can actually look at the tune files...

Lil'John

I actually can look at the tune files. But given that the mostly stock tune and the custom tune BOTH have the misfire to me rules the tune as being the major issue.

Lil'John

Is this effect something that torque management would cause?

If not and I assume the tune is not the culprit, what other causes?

Spark plugs? The original ones are in the engine still(30k miles)

Spark plug wires? One boot is burned up... but I would think that a bad wire would cause a misfire at all RPMs and not just under load.

Knock? The PCM isn't showing a knock.

BLK02WS6

iTrader: (2)

Doubt it... that would pull timing - doesn't jive with your data.

Doubt it.

That'll do it - maybe enough spark to light the fire at low load... takes more under load.

Doubt that would do it either...