Rough idle/arcing at plug - Ford Forum

91F150XL · #1 10-12-2009, 10:01 PM

I have a 91 F150 with the 300. I've been having trouble with the truck idling rough and shutting off when I hit the clutch to come to a stop. I started it tonight and checked for arcing and I can see at least 3 of the wires arcing where they meet the plug. Would that be a bad plug or wire? Possibly a coil causing problems? Cap and rotor were replaced approx. 6 months ago and so were plugs.

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Bullitt94 · #2 10-13-2009, 09:55 AM

If there is arcing, chances are the wires are shot. Plug wires need to be replaced or put on all the way if there is arcing.

91F150XL · #3 10-13-2009, 08:52 PM

I'm gonna bump this up, I replaced the wires, coil, and plugs and still have the same missing, arcing and low idle. Again, cap and rotor were replaced this past march. I'm not sure as to what else it would be.

nickt892003 · #4 10-20-2009, 11:22 AM

the only thing i can think of is you may have a short somewhere causing the wires to prematurely arch....good luck

Bullitt94 · #5 10-20-2009, 05:12 PM

Make sure the wires aren't touching anything en route to the plugs that could cause the wires to fail and ground out (i.e. exhaust manifold).

Also, when it's dark out you can opend the hood and watch for the spark to see where it is happening.

91F150XL · #6 10-21-2009, 10:16 PM

Yeah, that's how i found out it's arcing at the plug. I'm just gonna have to start tracing wires and try to find a short. I know it's not touching the manifold or anything seeing as it's a 6 cylinder.

Bullitt94 · #7 10-21-2009, 10:40 PM

If it's arching at the plug, then your plug wire probably isn't on all the way. If it has like that for a while, then the wire is probably fried from the arch and will need to be replaced.

Use Common Sense · #8 10-22-2009, 05:17 AM

Did you put dielectrical grease on the plug end and inside the wire boot(s)? This will help eliminate spark leakeage at the plug. And like Bullitt94 stated, ensure that the plug wire is seated and locked onto the plug.

91F150XL · #9 10-22-2009, 08:32 PM

Yeah, I greased the caps and made sure the wires are seated all the way. The only thing I can think of now is like NickT said, a short or bad ground. Are there any other grounds on the 6 cylinder besides the one running from the intake manifold to the firewall? I haven't got a chance to check yet if there is one from the block to the frame or not.

Bullitt94 · #10 10-23-2009, 10:00 AM

I am under the impression that you saw the arching and it was at or near one of the spark plugs. Is that correct? If you can/have seen the spark jump, that is where the problem is.