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