95 jimmy 4.3 - 300 misfire
 

ok ill start from the top (and give some useless info)

this is a late model 1995 jimmy with a 4.3l vortec (with horizontal distributor).
it is running really rough and the check engine light is on.
so i started with a tune up, i changed the plugs, cap & rotor (was running low on cash so i skipped the wires).
it was still running rough, but had a lot more power.
so i got it scanned...bank2 o2, rich gas & random misfire.
so i changed bank2 and the fuel filter (was really clogged but running still)
it was still running rough, so i got a coil and changed that over.
still rough, didnt change anything.
i got it scanned once more....bank1 o2 & random misfire.
so i replaced the wires finally.
but guess what?...its still running rough!!
so for some stupid reason, i decided to take it on a road trip.
drove 450 miles and the dang thing broke down (go figure, huh).
i got the thing home and pulled the plug from cylinder 1 (cylinder 4 is the same...the others were just fouled)
http://img64.imageshack.us/img64/6534/downloadxi.jpg
not the best pic but you can see the negative terminal is nearly gone, all from 450 miles!!

i had hoped that it was the o2 sensor that was making it rough, and killed the plugs.
so now i replaced bank1 o2 and put new plugs (again)
guess what?...still running rough.
but at least now the only code is 300 (random misfire).
so at least im getting somewere (slowly).
but i am at a total loss now.
what else could it be?

i forgot to mention i put a few used sensors under the hood at some point here. changed maf, tps & aic.

it just seems funny that cylinders 1 & 4 are like that (they are both middle towers on the distributor)...coincidence?

please help, im ready to get rid of this thing and forget about it

-d

[edit]
i forgot to mention that the truck seems to run good while driving, its just if im not hitting the gas pedal that its rough
[/edit]

A MAF sensor wasn't used until '96 so you didn't replace that. Also, you only have 1 upstream sensor that the PCM uses to control fuel. Which sensor did you replace? Should have been the one on the exhaust pipe closest to the engine exhaust manifolds.

What brand of cap/rotor did you use? AC Delco has been found to work the best for these ignition systems. The same goes for the O2 sensor. I have tested many different sensors side by side in the same vehicle and AC Delco sensors were consistently faster than the rest with Bosch being the absolute worse.

map not maf, my mistake.
yeah only one upstream o2...but i changed both
it had a downstream code first, then after more fixing/driving the upstream code came up.
not sure what brand they were...to tell the truth they were used.
but those codes are not coming up now...could it still be the problem?
the rotor, cap and wires are duralast (i know im cheap) and the plugs are bosch (platinum plus 2)

Bosch +x plugs are junk IMO. I have seen them cause issues straight out of the box and have removed a enough of them.

wish i woulda known that before hand
i spent 50 on plugs alone, and what i have now are literally a few hours old with about 5 miles on them.
so i am hesitant in changing them again.
i think i may try to clean the egr tomorrow, if this rain lets up.
anything else (free :P) to try while im at it?

Bosch Plats!!!!!
:icon_shrug:

Ford people laugh at 'em. Chevy people laugh at 'em

My prefs: Motorcraft Autolite AC Delco
I've run all three in my Fords
You can flip that if you want. wont matter

I dont think that's your main problem though Those cyls are running too hot/lean

That means they arent getting enough fuel

So the egr makes sense...I'm going to get wet today, dang I hate the rain.

well i cleaned the egr, but didnt help
i tried another set of sensors and what not...map, aic, tps & imtv, but none helped.
think i going to move onto the fuel system, check the pump and the cpi/cmfi (unless someone thinks different).

man im learning too many acronyms, lol :P

I am not sure if this will help but i have ran into a couple of vehicles where is the plug wires cross each other in the routing (especailly some aftermarket replacements) they crossfeed. I fixed this by buying the plastic wire loom covering and isolating each wire in its own cover cost like 5 bucks for 20 feet . just an idea.

Just because I'm curious (and because you've replaced the normal ignition failure parts), have you run a bottle of Techron or GMs fuel system cleaner through it? I've had odd misfires in my '93 (same CPI system) cured by a bottle of Techron. The poppets get varnish stuck in them and either won't spray a full cone or get stuck open, dribbling fuel all the time.