misfiring in cylinders 3 & 4 (4.0 OHV) - months apart

[RammaR]

Last spring I had a misfire code in cylinder 3 -- I pulled the plug out and sure enough it was fouled. The other plugs all looked ok, so I did a compression check and found that #3 and #4 were down just about 5% from the others - but not too bad for nearly 160k miles. I changed all of the plugs at the time and it ran fine again.

Last week I got a misfire code in cylinder 4 -- pulled the plug and it is wet and fouled again. It idles just a little rough but doesn't start to misfire until I'm a few miles down the road and accelerating on the interstate. I was suprised it was in #4 this time so I cleared the code. The next morning it drove fine, but on the way home it did it again. It also seemed to happen when it was colder or more humid out. I replaced the plug and its stopped the misfire but still idles a bit rough on start-up. Its up to 170k miles now but the plugs are all pretty new.

I traced the plug wires back and both cylinders #3 and #4 are connected to the same pair on the ignition coil. So I'm thinking that it might be a bad coil causing weak firing that then fouls the plugs. The haynes manual says that the resistance on the secondary coils should be between 6.5k and 11.5k -- mine were all between 13-14k yesterday. I couldn't tell much of a difference between the two in question and the others either. But since it appears to be higher than normal and the real misfiring didn't happen until it was already warmed up a little bit and accelerating to interstate speeds maybe is just a crack that opens up a little with use?

Before I replace the ignition coils, wires and plugs - is there anything else you would recommend checking to verify the source?

I read that Aldive replaced his stock coil with a Scream'n Demon aftermarket - but am not sure what was wrong with the original one? Its a mostly stock 4.0 OHV, and it sounds like the Ford unit is pretty strong to start with - but if its worth upgrading while I have to replace it then it might not be a bad idea.

Thanks in advance!

 

[jam239]

there's a few thing that could cause this, it could be the plugs and wires and those are the things you want to check first then the coil pack, this happened in my buddies 99 ranger and he thought it was the timing chain since he had 190K on it so when we changed it and plugs wires and the coil pack and the problem was still there, i did a compression test and the pressure was low in the 6th cylinder so i figured it had to be a ring on the piston or a bad valve. I took the head off the motor and he had an exhaust valve stuck open. He replaced the valve and the problem was solved. a bad intake manifold gasket also causes that problem sometimes. good luck

 

[dogfriend]

The haynes manual says that the resistance on the secondary coils should be between 6.5k and 11.5k -- mine were all between 13-14k yesterday. I couldn't tell much of a difference between the two in question and the others either. But since it appears to be higher than normal and the real misfiring didn't happen until it was already warmed up a little bit and accelerating to interstate speeds maybe is just a crack that opens up a little with use?

I was getting a random misfire when pulling hills and towing. I measured 12.5k, so I replaced the coil pack and it solved the problem. So in my opinion, your resistance is too high.