Injector Balance Testing

[thefixer]

Hey all, I am trying to do an injector balance test on 99 silverado 4.3 when i hit test the pressure drops to 40 and vehicle cuts off. I remember this happening on previous vehicle to. Fuel system is the spider type csfi. Am i doing something wrong? Problem is hard miss on 3&4 under load at 55mph or powerbraking and always at 1850 rpm and up. Pressure holds steady at 60 under load. But ignition pattern shows upward slope spark line meaning lean on all cylinders but 3&4 are the worse. So i guess this is a two part question. Any input would be much appreciated.

[greasybob]

As far as I know all the GM injector balance tests for gas engines is done key on engine off. If this was a lean miss you should see the fuel trims go way positive well before the misfire and the 02 voltage drop out to near zero.

[sbebenelli]

Yes this test is done KOEO. I'm surprised the test would even activate with it running.

[greasybob]

I thought the same thing, I didn't know the test would run with the engine running, anytime some one says misfire and 4.3 in the same sentence my thoughts immediately go to distributor cap.

[greasybob]

Here is an ignition wave form from a 4.3 with a bad distributor cap, had misfires mostly showing on cylinders 4 and 3

[crackerclicker]

as greasybob and benelli have said, don't do the injector balance with the engine running. that test is meant for static pressure drops.

when you mention upward spikes in secondary that usually means a lean density misfire, but be cautious with that assumption because iatn member scott hager has discussed this at long length and determined alternate issues.

if you have access to iatn, i recommend reading these threads regardless of rather they pertain to your current issue or not.

http://members.iatn.net/forums/search/search.aspx?action=viewthread&m=236670&t=236670&forum=forum2&ft=1&fv=4&qv=1

http://members.iatn.net/forums/read/msg.aspx?f=forum2&m=237617&fv=4&ar=2152

http://members.iatn.net/forums/search/search.aspx?action=viewthread&m=243928&t=243928&forum=forum2&ft=1&fv=4&qv=1

[thefixer]

Just wanted to say thanks for the info,it turned out to be clogged injectors and pinhole in #4spider tube. I am not a member of IATN so could not view that info.What was the just of it pertaining to upward slope? I would like to know so i dont tunnel vision on thinking lean next time i see this pattern.

[Crusty]

Just wanted to say thanks for the info,it turned out to be clogged injectors and pinhole in #4spider tube. I am not a member of IATN so could not view that info.What was the just of it pertaining to upward slope? I would like to know so i dont tunnel vision on thinking lean next time i see this pattern.

On an ignition scope, years ago with very rich carburetors, the firing line of the plug would be aproximately 1.5 to 2.5 KV and last for 2 or 3 milliseconds and be fairly straight across as the plug fired.
With today's leaner burning engines, the plug firing line is usually around 2-kv and tends to last for 1.75ms to 2.25ms and will rise slightly, and I mean slightly as the plug fires and uses up the fuel, just a little leaner as it burns, thus increasing the voltage as it finishes firing by 0.1 or 0.2kv.

EG, if you plug firing line starts at 2.0-kv and runs across for 1.5ms and then finishes up at 2.2kv in the last 0.5ms then it's burning properly.
If the plug starts at 2.0kv and by the time it has burned for the full 2.0milliseconds it is up to 4 or 5 or 6 (or even more) KV then that cylinder is suffering from a lean condition (not enough fuel in the air/fuel mixture)
At least that's what I was taught 30 years ago and hasn't been proven wrong (yet LOL)

[crackerclicker]

I am not a member of IATN

basic membership is free. you can then click on those links to read the archives. scott's problem ended up being mechanical.