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Stumbles on heavy acceleration
Posted to Technical Discussion Forum on 4/15/2014 9 Replies

Sorry I know this isnt the best (correct) place to post this but im pulling my hair out

Stated vehicle has an intermittent stumble on heavy acceleration. It will "cut out" for a brief second around 5-10mph and then sometimes do it again around 30mph. Drive it around some more and it may be fine for 100 start/stop sequences and then do it again for 3 times in a row and then be fine again (kinda makes it hard to diagnose). To me it feels as if it has high exhaust back pressure for a brief second and then is fine or like pinching off a fuel line where your pressure remains good on a gauge but volume is being starved for whatever reason. Also kinda feels like a traction control event kicking in to keep the rear wheels from spinning but truck does not have trac control and i monitored wheel spin and none show during symptom. Only DTC's were a P0300 im memory but that was all.

I have monitored fuel pressure while problem occurs and it stays steady @ 58-60psi. Measured exhaust back pressure and it is below 1.25 @ 2500 rpm in stall. I kinda thought the GM test for back pressure seemed crappy so just to be sure, I removed the upstream O2 and drove it and problem still occurred. I also checked the brick in the cat to see if it was loose or melted and all looks fine. I have monitored app and tp while problem is occurring and they correlate with each other but for good measure, i cleaned throttle body. Truck recently had new ac/delco plugs put in it by me per customer request and they are gapped properly. During the stumble, misfire counters will sometimes randomly show misfires on random cylinders but only in the single digits and it doesnt show misfires everytime. No DTC's reset after making the symptom happen a dozen times or so.

I have checked engine and body grounds and all ok. I have removed each COP and checked cranking spark and they jump fairly well to my grounded test light and are bluish/white. I have removed the exhaust cam solenoid and checked for debris but solenoid and screen were spotless. Oil is clean and full. Fresh tank of fuel. Swapped MAF sensors with a known good and symptom still there.Fuel injector balance test shows all injectors even flow.

I have no real good direction of where to go with the diag on this. I am primarily a ford tech but I have a modis ultra and autoenginuity and try and use them to the best of my ability on this truck. Can someone please point me in the right direction?

Matt Barnes
Technician/Shop Foreman
Davis Ford
Canton, Illinois, USA

9 Replies Received (View Replies)


car Vehicle Data

2006 Chevrolet Colorado LT 3.5L

Engine3.5 L
Trans4-speed Automatic (Electronic)