i've had problems with the 4.o v6 #3 cylinder misfiring on startup after engine cooled down (either overnight or after work). the problem starts slowly. for about a month, the symptoms disappear after letting the engine idle for between 15 and 30 seconds. after a month or more of this...it suddenly won't clear up any longer and i'm running on 5 cyl. the code id'd the #3...but no diagnosis as to root cause. so, i had the plugs, wires and distr. cap replaced for an atrocious $400+ bill from the dealer. that fixed it. but...4 months later...same thing all over again. same cylinder. this time...plugs only fixed the problem, but still no root cause. ran fine for another 3 months. now the problem is back. WTF is this...anyone know??? once i'm in this situation, it idles, runs, accelerates, etc...all on 5 outta 6.

any comments???

 

I'd be willing to bet your injector is crap.

 

injector? yes quite possible here. im kinda leaning towards a ignition(spark) problem, or leaking head gasket. when they changed the plugs, did they mention if the plug was extremlly fouled or had build up on it? if so, i'd say take it to a trust worthy place and get it checked for a head gasket. it might also be spark related. i know that this sounds odd but if one of the 3 wires(it think its 3) going to the coil has anywhere to arc, then the same cylinder might be misfiring during peroids of heavy moisture or long periods of rest(over night). sounds odd i know, but that is how the cookie crumbles, if in the way the wiring is. also, if it is an injector, MAKE SURE U LOOK AT THE GASKET BETWEEN THE INJECTOR RAIL/PLATE. those have a history of deteirating and causing a vaccum leak, serious enough to cause a miss.

 

Pull the plug out after running awhile. I am thinking head gasket is leaking.

You see signs of anti-freeze on the plug that is the problem.

tk

 

I had a similar problem on my 98 4.6l. The cap on the flushing tee that I had installed wasn't sealing properly and leaked coolant down into the #3 sparkplug well.

Replaced the cap, cleaned out the well and checked plug - all was fine for a while, then more mis-firing. Switched the plug from 3 and 1, then 1 started to mis-fire, so plug was bad. No mis-fires in several months since.

 

My #3 plug blew.. When they put the helicoil in it there was still a misfire.

The plug wire had a nick in it and was shorting on the cylinder head.. dunno if this helps since you said you alreayd replaced wire.