What can cause engine to missfire? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

2badgn · 03-17-2011, 10:13 PM

1984 GMC 3500 2wd 350 engine

Last year, I had the timing chain changed along with cap, rotor, plugs, wires. truck was back firing after I turned off engine. A timing adjustment fixed that.

Truck is now at mechanic.
When driving, truck will misfire for like 2 seconds and then back to normal.

What things can cause misfiring?

Thanks

**old Rusty C10** · 03-17-2011, 10:22 PM

there are many things.. timing.. carbuerator off.. distributor.. crossed ignition wire...

James McClure · 03-18-2011, 10:48 AM

As Bob said, these things will cause a misfire or skip. There are other things to be aware of. The FIRST question you should ask yourself is "what was done to the eng (if anything) before this started. You covered the basics, BUT, GM made the screws that hold the coil down just a pinch to long to work well with aftermarket caps. The screw makes a small crack in the plastic and the spark will jump to that screw. Also, if you have vac advance, the leads on the pick-up coil tend to break insidee the insulation. When you mash the gas, the advance plate rotates to raise timing and the pick-up coil lead(s) loose connection. An easy way to check this is to remove the vac hose and drive it and see if it skips. Do this check a few times to make sure. If the skip goes away, the pick-up coil is toast. Next test would be a spray bottle filled with water. With the eng running, starting from the plugs, work you way up to the dist and see if it skips at all. Next, check under the coil cover and make sure the ground tab is installed from the coil frame to the connection block on the side of the cap. If all that is good, make sure of good connections to ign + (pink wire at cap) and 3 wire connector that goes back to the ign module and all the connections inside the dist. jim

motornut · 03-18-2011, 09:56 PM

check that the advance weights move freely/or worn

03-17-2011, 10:13 PM	#1
2badgn Registered User Join Date: Nov 2009 Location: Yonkers, NY Posts: 277	What can cause engine to missfire? 1984 GMC 3500 2wd 350 engine Last year, I had the timing chain changed along with cap, rotor, plugs, wires. truck was back firing after I turned off engine. A timing adjustment fixed that. Truck is now at mechanic. When driving, truck will misfire for like 2 seconds and then back to normal. What things can cause misfiring? Thanks

03-17-2011, 10:22 PM	#2
old Rusty C10 Robert Olson Transport Join Date: Oct 2002 Location: recent transplant to NC USA Posts: 20,310	Re: What can cause engine to missfire? there are many things.. timing.. carbuerator off.. distributor.. crossed ignition wire... __________________ Bob 1951 International running on a squarebody chassis "If a man's worth is judged by the people he associates himself with, then i am the richest man in the world knowing some of the fine people of this board" http://67-72chevytrucks.com/vboard/a...t.php?f=25&a=9 (you can review the site rules here!) PM Me for your vehicle/parts hauling needs in the North East US or see my Facebook page Robert Olson Transport Live each day to the fullest.. you never know when fate is going to pull the rug out from under you... I hate cancer!!

03-18-2011, 10:48 AM	#3
James McClure One shot, one kill. Join Date: Dec 2007 Location: Saratoga Springs NY Posts: 859	Re: What can cause engine to missfire? As Bob said, these things will cause a misfire or skip. There are other things to be aware of. The FIRST question you should ask yourself is "what was done to the eng (if anything) before this started. You covered the basics, BUT, GM made the screws that hold the coil down just a pinch to long to work well with aftermarket caps. The screw makes a small crack in the plastic and the spark will jump to that screw. Also, if you have vac advance, the leads on the pick-up coil tend to break insidee the insulation. When you mash the gas, the advance plate rotates to raise timing and the pick-up coil lead(s) loose connection. An easy way to check this is to remove the vac hose and drive it and see if it skips. Do this check a few times to make sure. If the skip goes away, the pick-up coil is toast. Next test would be a spray bottle filled with water. With the eng running, starting from the plugs, work you way up to the dist and see if it skips at all. Next, check under the coil cover and make sure the ground tab is installed from the coil frame to the connection block on the side of the cap. If all that is good, make sure of good connections to ign + (pink wire at cap) and 3 wire connector that goes back to the ign module and all the connections inside the dist. jim __________________ 1984 K20 350M engine with 465,000 miles. Well, it's finally done!! Almost 2 years of work, but it was really worth the effort. Little stuff left to do is mount winch, wet sand & buff out and build belly pan\running boards. Body work takes lots and LOTS of beer!!!! God, Guts and Guns made America and God, Guts and Guns will preserve it! The worst thing you could do is get into my sights, but that don't matter, you'll never know it, cause you'll never hear the shotJim or Paladin whichever you choose.

03-18-2011, 09:56 PM	#4
motornut 78K & 79C Jimmys Join Date: Nov 2009 Location: Ottawa Ont CANADA Posts: 7,901	Re: What can cause engine to missfire? check that the advance weights move freely/or worn __________________ John 1978 GMCJimmy4X4-350/203 1979 GMCJimmy4X2-305/350



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