Ok, so it's been about 3 years since I did spark plugs on my '94 cherokee 4.0 so I figured it was due. No problems with the truck, idles and starts fine, gets up and goes when you step on it, just figured I'd change 'em while I was in the mood.
This time instead of the cheap Champions I went with AC Delco plugs.
So I pull the old ones out noting that they are toasty brown (perfect!), toss 'em away, check the gap on the Delcos (adjusting as needed), screw them down and snug them up, put the wires back on, and fire it up.
Starts right up. Take it around the block. Misfires!
Misfires under load, like if I coast in 2nd to like 10-15 mph and step on it then I get a lot of misfires. At normal speed and free-rev it seemed fine.
Get it home, pull the plugs out, double check gap, put back in, double check tightness, put wires back on. Now it misfires at idle randomly and at random times when I'm driving around.
It's getting late so I park it for the night, and today I got new wires for it (and I opted for the more expensive ones). I figured when the engine shakes/vibrates a bunch and a wire isn't making good contact I get an intermittent spark.
Hook them all up one by one so I don't mess up the firing order, start it up, seems ok.
Take it around the block, seems good.
Take it on the local 45mph streets and it seems fine. Oh wait, no, it just misfired again...
Seems like if I keep the gears, speed, and RPM all in the correct range it's fine, but if I just stomp on it to speed up I can hear and feel it missing. Not every time, though, sometimes it does and sometimes it doesn't.
I can't make it happen. It just decides on it's own.
I can drive for 20 minutes without a misfire, then I'll take a turn, go under a bridge, the sun will come out, then I'll get a whole bunch in a row...
Not sure where to go from this, especially since it didn't have any issues before changing the plugs and wires.
Is it possible I got a bad plug? Will part stores even exchange spark-plugs (especially after they've been used)?
I'm waiting for the truck to cool down right now as I just got back from work some 20+ miles away on the expressway.
I'll pull the plugs and wires again in a few hours and have a look at everything. Triple check gaps, re-grease the electrode ends on the wires, make sure they seat fine...
Anything I should look for?