How JustAnswer Works:
  • Ask an Expert
    Experts are full of valuable knowledge and are ready to help with any question. Credentials confirmed by a Fortune 500 verification firm.
  • Get a Professional Answer
    Via email, text message, or notification as you wait on our site.
    Ask follow up questions if you need to.
  • 100% Satisfaction Guarantee
    Rate the answer you receive.
Ask Steve Your Own Question
Steve, Mechanic Wide Ranging
Category: Car
Satisfied Customers: 303
Experience:  22yrs Auto,Truck,Tractor Trailer,Heavy Equipment, Industrial Truck,Forklift,Motor Coach,Tour Bus
Type Your Car Question Here...
Steve is online now
A new question is answered every 9 seconds

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: 7 years ago.
Category: Car
Expert:  Steve replied 7 years ago.
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

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

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.


Please press the Accept Button if you are satisfied with the answer.

JustAnswer in the News:

Ask-a-doc Web sites: If you've got a quick question, you can try to get an answer from sites that say they have various specialists on hand to give quick answers... seen a spike since October in legal questions from readers about layoffs, unemployment and severance.
Web sites like
...leave nothing to chance.
Traffic on JustAnswer rose 14 percent...and had nearly 400,000 page views in 30 days...inquiries related to stress, high blood pressure, drinking and heart pain jumped 33 percent.
Tory Johnson, GMA Workplace Contributor, discusses work-from-home jobs, such as JustAnswer in which verified Experts answer people’s questions.
I will tell you that...the things you have to go through to be an Expert are quite rigorous.

What Customers are Saying:

  • I would (and have) recommend your site to others I was quite satisfied with the quality of the information received, the professional with whom I interacted, and the quick response time. Thanks, and be sure that I'll be back whenever I need a question answered in a hurry. Stephanie P Elm City, NC
< Previous | Next >
  • I would (and have) recommend your site to others I was quite satisfied with the quality of the information received, the professional with whom I interacted, and the quick response time. Thanks, and be sure that I'll be back whenever I need a question answered in a hurry. Stephanie P Elm City, NC
  • used your service this weekend with "Trecers" help. thank you ,thank you, thank you. replaced an A/C fan motor. Local Auto Zone had part. $15.00 "tracer" fee and $40.00 for parts, I saved several hundreds of dollers at a shop. i will recommend you and use you in the future. David L. Richmond, TX
  • 9 dollars, 2 hours of my time, and I drove away. Your diagnosis was right on the mark. Thank you so much. Phil Marysville, CA
  • Lurch. Thank you very much. I had real doubts about this website but your promptness of response, quick followup and to the point answer with picture was incredible. Charles Walnut Creek, CA
  • As a single woman, I really appreciate an excellent and affordable opinion.
    Thank you Geordie, I will not hesitate to contact justanswer in the future!
    Sue Charleston, WV
  • Another great insight to what may be the problem. I will have my mechanic take a look at it tomorrow. Thanks again, do indeed know your stuff. Jim Castleberry, FL
  • Excellent reply, and also very quick. Really sounds like the Expert knows what he is talking about. I will be back to use your service when I need more help with my RV. Dutch USA

Meet The Experts:

  • Chris (aka-Moose)

    Chris (aka-Moose)


    Satisfied Customers:

    16 years of experience
< Last | Next >
  • Chris (aka-Moose)'s Avatar

    Chris (aka-Moose)


    Satisfied Customers:

    16 years of experience
  • Ted G.'s Avatar

    Ted G.

    ASE Certified Technician

    Satisfied Customers:

    20 years auto repair experience, ASE Master Tech, Mechanical Failure consultant, Expert Witness
  • George H.'s Avatar

    George H.

    ASE Certified Technician

    Satisfied Customers:

    ASE Master Tech 15+ yrs, AAS Automotive Technology, Factory trained Asian specialist
  • GM Tech (Cam)'s Avatar

    GM Tech (Cam)

    Auto Service Tech

    Satisfied Customers:

    GM Grand Master Technician 2007. 14 years experience.
  • Amedee's Avatar


    Former ASE Master Tech

    Satisfied Customers:

    ASE certified tech advanced level specialist. Wisconsin certified emissions state inspector
  • Jerry's Avatar


    Master Mechanic

    Satisfied Customers:

    ASE master, 30+ years. All makes and models. Trouble shooter, shop forman, service manager
  • Richard's Avatar


    ASE Certified Technician

    Satisfied Customers:

    12 years Ford Lincoln/Mercury Jaguar dealership as a technician and shop foreman reparing all makes