Firing / skipping problem
02-23-2015, 08:02 PM
#1
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Firing / skipping problem
Hey guys so I need your help here. I have a 1980 f150 ranger xlt with a 351 w, four barrel carb, rwd, and its developed a new problem. At idle the truck sounds like it occasionally is skipping a piston firing, also when I'm driving it sounds like it is backfiring however it isn't a loud backfire its rather quiet actually, also it happens a lot while moving. I cant figure out the issue, but I have new plugs, new icm, yet the coil is old. I'm not sure what the issue is. Please help.
02-24-2015, 08:53 PM
#2
Sounds like possibly a bit of a misfire. Try hooking an inductive timing light (the kind that clip around the spark plug wire) on each spark plug wire in turn and pull the trigger. If the light flashes, power's going through. If it doesn't flash on one, there's your problem cylinder.
After that it's inspect the cap, check the wire with an ohmeter (and compare to another wire), try moving the plug to a different cylinder and test with the light to see if it moved.
More in depth would be a dry and wet compression check to see if any of the cylinders have a problem.
It could be electrical - coil or distributor pickup but they usually cause the motor to die completely.
You can't rule out a new plug being bad - it happens.
And there's always crossfire - that's when you line up all your wires nice and neat in order. That's a bad thing, as good as it looks.The wires to the cylinders that are beside each other that fire one after the other have to be kept completely separate from each other (302, 302HO, 351 - they all have that issue).
And make sure you didn't mess up a vacuum line while changing the plugs.
After that it's inspect the cap, check the wire with an ohmeter (and compare to another wire), try moving the plug to a different cylinder and test with the light to see if it moved.
More in depth would be a dry and wet compression check to see if any of the cylinders have a problem.
It could be electrical - coil or distributor pickup but they usually cause the motor to die completely.
You can't rule out a new plug being bad - it happens.
And there's always crossfire - that's when you line up all your wires nice and neat in order. That's a bad thing, as good as it looks.The wires to the cylinders that are beside each other that fire one after the other have to be kept completely separate from each other (302, 302HO, 351 - they all have that issue).
And make sure you didn't mess up a vacuum line while changing the plugs.
02-24-2015, 09:00 PM
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Update
Thanks chris I will check all of those when I get a chance, also update: I just bought a new coil today and it sounded at first like it helped a bit however upon further driving the problem still occurred.
02-25-2015, 04:25 AM
#4
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After the test of the light, I would get a new rotor and new wires as well. Ive had bad wires where they still passed the timing light test but caused issues. The resistance goes up and the wire acts up.
