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Steve
Steve, Mechanic Wide Ranging
Category: Car
Satisfied Customers: 303
Experience:  22yrs Auto,Truck,Tractor Trailer,Heavy Equipment, Industrial Truck,Forklift,Motor Coach,Tour Bus
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1993 GMC Sierra 3500: dually..acceleration and the idle..rough..tach

Customer Question

I have a 1993 GMC Sierra 3500 dually with a 454. It has started "bucking" or hesitating on acceleration and the idle is alittle rough. The tach will bounce. I've recently changed the plugs, cap & rotor, air filter and fuel filter. Any ideas?
Submitted: 5 years ago.
Category: Car
Expert:  Steve replied 5 years ago.
Hi,
If you have chance to do this at night
or in a good dark area, while the truck
is running listen for a snapping noise,
that will tell you if you have something
arcing(sparks).

You may or may not here it.

Look all around the distributor cap and
look for any arcing,this may be small and hard
to see.

Look at each spark plug wire and see if you see
any arcing.

Look at the spark plugs and see if you see any
arcing,look for arcing down the side of the plug
to the bottom,look where the plug screws in for
arcing in there.

Look through the wheel wells of the truck to get
a good look at the plugs.

Take your time and look at all of that carefully.

If you see Arcing then there is your problam.

If you don't see anything then take the dist cap
off and I do know that it is new,inspect it for and
cracks,burn marks,a hair like crack or burn between
two or more terminals.

Make sure the rotor is on good and looks good.

If all of that is ok,you can do your own power drop
test.

Fit all the plug wires on loosely,you can do this at the
distributor or the plugs,which ever is easier for you.

With the truck running start to pull one plug wire off
slowly,use insulated plug wire pullers and some gloves
when doing it(I don't know if you have ever done this type
of test before,but you can get SHOCKED badly,so if you don't
feel confident don't do it.)You should hear the rpms go way
down when the wire is fully off the plug and the rough idle
get Worse,and when you put the plug wire back on the rpms rise
back up and the rough idle goes back to the way it was then that
is a GOOD cylinder.

If you pull the wire fully off the plug and the rpms don't go down
or go down very little and the Rough Idle either goes away or stays the same then that is a Bad cylinder.

Do that for every plug wire.

Now if you don't want to do it that way you can pull one
wire off a plug,move it out of the way so it don't get burnt,
then drive it and see if the problem has gone away, if not put the wire back on and go to the next cylinder and do the same thing,repeat
until you've done them all, and you will have found the bad cylinder
when the problem goes away or lessons.

When you find the bad cylinder:
Swap its plug wire with the one next to it(That means at Both Ends! Do Not Cross the wires.)
Now when you do that and test those two cylinders like you tested
all the others.
If the Bad cylinder has moved to the one next to it then you know
you have a bad Plug Wire.

If it hasn't moved then swap the spark plugs and test again, same thing,if the bad cylinder moves to the next one you have a bad Spark Plug.

Now if you didn't find a bad plug or wire there is a possibility that
an rocker arm,intake or exaust valve,camshaft and lifter,could be the
problem so you would have to be sure.

Try the tests first and see what happens.

Steve

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