[FIXED] S10 Blazer misfire "Not stuck injectors"

Well when I look this problem up on IATN all I see it problems with stuck poppet valves and bad injectors. My problem is not even close.
I am trying to diagnose a 99 S10 blazer with a #6 misfire. The shop replaced the spider before calling me and that did not fix the problem. At idle the engine misfires on #6 cyl and and fills the shop up with raw fuel exhaust. Verus confirms that its a steady misfire on cyl 6 and I confirmed that by doing a cyl drop test, so my first thought was a stuck open injector. I pulled the injector hose out of the intake runners and there is a big difference in the amount of fuel that number 6 is spraying compared to the rest of cyl, but the injector is not stuck open.
I hooked up the Pico to #6 injector and #4 injector and the ecm is commanding the #6 injector to fire 5 times to 1 time for the number 4. The waveform looked great on number 4 and the #5 injector looked good to but had some odd spikes occasionally. So my first thought was it has to be the cam or crank sensor messed up when it gets to the #6 injector. So I scoped the crank the cam and the #6 injector. The crank sensor looked great but the cam had some hash "very little" so I told garage that it needs a cam sensor.
Well the garage called me today after they put a new sensor on and its still doing the same thing.
They had a ecm out back and gave that a try and no difference.
Anyone ever have this problem or have any ideas that I could check?
I thought I saved the files for my waveform but I didn't.

Have you tried doing the injector balance scan tool test with your scope hooked up?

That is a good one...

I plan on doing that when I go there tonight. I have a feeling its going to look good, Not sure what to check next if it does
Thanks

Wow :ohmy: That's a new one on me, never even heard of that before. Great catch with the scope on that one - who knows how many spider assemblies this could have had thrown at it!

For my info, how are the five different injection events spaced in relation to each other? Like, one big cluster, or spread out over 720 crank degrees? The #6 control circuit somehow shorted to the other injector drivers? Wild idea, just speculating.

any update

Blazer is fixed .
I didnt pay attention to my voltages on waveform pattern the first time. I went back the next day with a clear head and noticed that the number 6 injector only had power on one side with KOEO. So I thought what the hell I had power on both sides last night, I figured either a bad injector or it was being turned on. Hooked up the amp clamp and it was pulling 650 ma. Told customer he got a short from the ecm to the injector or a bad ecu. Within 30 seconds he noticed the wire harness laying on the exhaust manifold. Pulled the harness away and amperage was at zero. Problem fixed
With the engine running it was vibrating causing the short to open and close and fire the injector so many times, the crazy thing is that the injector firings were pretty evenly spaced with the engine running.
Thanks guys and have a merry Christmas and happy new year!!
John

Good catch............... it had to be something like that but it is never obvious in the heat of battle.

Sweet! Great move with the amp probe. Crazy that the engine vibration was causing the short to come and go at such as specific frequency :blink:

Good find, I was expecting some kind of crossed control circuit fault mainly because of the regularity of the injector firing events you observed (5 to 1) . An intermittent short to ground on the control wire makes much more sense in hindsight, thanks for getting back to us with the results!
Glad it wasn't anything too crazy.

Thanks guys for the help, the funny thing is the shop owner kept saying it was a short , I just blew him off just because he knows about electricity as I do about a space ship. I told him if it was a short the injector would be firing all the time,
The lesson learned here is I wont dismiss the probably the most common problems first.
Thanks again and I appreciate everything you guys do
Merry Christmas

johnphufnagel wrote: Thanks guys for the help, the funny thing is the shop owner kept saying it was a short , I just blew him off just because he knows about electricity as I do about a space ship. I told him if it was a short the injector would be firing all the time

:lol:

Honestly, though, I probably would have said the same thing :blush:

Merry Christmas, sir!