GMC Savana Condemned PCM

[just2damfast]

2006 GMC Savana van, the other technician had misfire problems and replaced the maf only to find no improvement. I connected the escan to find very negative trim on road test. However vehicle intermittently running rough. Performed volumetric efficiency test and also found numbers off. connected vp and first used transducer on fuel rail, back probed, cyl. 7 injector and the cam sensor. found fuel pressure to drop and rise while running rough, cam signal good. But injector appeared to be firing a second time during compression stroke. Back probed cam and crank to check to see corallation. Seemed good. Probed cyl. 1 injector, crank signal (24x), and trigger of cyl. plug wire, cyl. 1 injector fires twice - posted below. If my theory is right, I believe the pcm to be bad if both cam and crank signals are good. Still new to the lab scope and thought I would post my findings.

Brian

[crowbar]

I don't know if this helps, I work on motorcycles.
Some models fire the injectors twice, each time giving the cylinder a half load of fuel.
I knew once why they did this, but I can't remember why right now.
Could be because some motorcycles rev to 15,000 RPM.

[Joe Rappa]

I don't think I'd ever condemn a PCM on a GM vehicle for delivering an asynchronous pulse. It's common practice for them.
Joe

[just2damfast]

However, the extra injector pulse only shows while the vehicle is running bad. It's somewhat of an intermittent condition. When it runs well, the second pulse isn't there.

Brian

[just2damfast]

Joe Rappa, you were correct. replaced PCM only to find vehicle still running bad after about 3 hours of hunting, Low reference wire going to throttle position sensor was found broken inside the insulation. I wonder if this could have been the reason to the extra injector pulse. Perhaps for accelleration enrichment to keep the engine from bogging?

Brian

[Joe Rappa]

Originally Posted by just2damfast

Joe Rappa, you were correct. replaced PCM only to find vehicle still running bad after about 3 hours of hunting, Low reference wire going to throttle position sensor was found broken inside the insulation. I wonder if this could have been the reason to the extra injector pulse. Perhaps for accelleration enrichment to keep the engine from bogging?

Brian

I bet you're correct. Small increases in TPS lead to big increases in fuel enrichment. GM likes to add that extra fuel with Asnychronous pulses. Most everyone else increases injector on time.
Great find!

I was thinking you were on the right track with the cam/crank signals, but a bad TPS signal will definitely cause your problem

Thanks for letting everyone know.

Joe

[vasau]

What size engine or code?

[just2damfast]

It is the 4.8 liter v8

[eddie8]

By the way just2damfast, how did you get the escan screen shots saved?i have the escan and cant seem to save my screen shots Any hints?

[Joe Rappa]

Hi Ed! Hope you're doing well.
Just use a screen capture program. I use MWsnap. It's free and doesn't use much memory.

I'm not scheduled to be back down in the Newburgh area until November, but I'll be sure to look you up when I get there.
What a crazy schedule I've got this year! Flying to 5 different cities in 5 days this week.

Joe