98 chevy 4.3 SMPI from a fullsize running on a 96 ECU and custom wiring harness built by speedscene. I've been running it for a couple months and have gotten a bunch of codes and been slowly repairing all the bugs. Its had a problem with running rich and backfiring so i've been fixing everything i can think of. i finally got the new fuel pump in and replaced the bad O2 and the only codes left on the computor are Vehicle Speed Sensor not working (dont have one) and a random multi cyl misfire. The thing runs strong as hell with the new pump, actually makes it downright scary it torques the body so hard when it shifts. Basically what i'm getting is a rough idle when stopped and when running at partial throttle you can feel the exhaust build up pressure and if you give it some gas it will blow out. Also when coasting down hills with the truck pushing the motor it will pop and bang a bit, and sometimes it will backfire so loud it sounds like i fired off a shotgun in the back seat. Anyone have any ideas what could cause this? I'm Baffled. Could it have anything to do with the evap system? i dont have the cannister in and have the lines plugged. Could the cat cause this? muffler? Thanks everyone.

Dallas

I love random multi-clyinder missfire codes, it's so helpful.

A tech-II scanner should be able to isolate the cylinder, unless it isn't really a cyldinder giving you the random misfire code.

Check grounds and disconnect the rear o2 sensors, leaving only the front two.

Use a Tech-II to reset the hard codes as disconnecting the battery will only clear the soft error codes. Await for the return of the random misfire code then read the scanner for the correct cylinder. If it doesn't give a cylinder it's either a ground or o2 giving false readings or switched bank 1 and bank 2 causing over rich/over lean from one side of the engine to the other.

Well there goes all my knowledge in that area.

Mine was switched o2 banks on the random misfire.

interesting, i'll have to check that. I'm using the scanner they sell at kragen for like $275, autoXray i believe. I'll double check my leads.

Dallas

BTT anyone else?

mine ran that way after i switched cams.....tured out to be a tight valve. i adjusted them and it runs fine.

Check your timing, vacume leaks, exhaust leaks around the O2 sensors, I dont think the $275 scanner is going to tell you much, I have not used them, but from what I know they just read code not sensor values. The tech II scanners are 4 or 5 thousand dollars and give you allot more information. You may need to track one down or take it to a good shop.

my scanner does give the O2 voltage values and about 20 other readings, temp, speed, intake air temp, air mass, and a buncha shit i dont understand. No exhaust leaks, new intake gaskets (just replaced all the injectors)Camo when you say tight valves what do you mean, when i got the motor all the rockers nuts were backed off and i tightened em to i think 15 ft-lbs, cant remember exactly. it was whatever the PBB Gurus or the chiltons told me was correct for that motor. One other thing, could a bad spark plug wire or 2 do this, i'm not sure how good my wires are since they came on the motor. thanks.

Dallas

i mean when i adjusted my valves i had one of them with to much pre load on it so the valve never closed all the way.

what year is your motor. would that be a problem for a motor with hydraulic lifters?

Dallas

i was pokin around last night and noticed that the center plug wire on the passenger side was sparking on the heat shield sometimes. I'm guessing this is what is giving me the random misfire code, but would this cause the backfiring too?

Dallas

if it is anything like a ford v-6 you can have the cam sensor turned 180* and it will still run(dis). that will cause a backfire. usually not at idle but when driving also a dirty mass air flow sensor will send bad readings to the computer.

BTT

If the plug wires are going bad, you could also have a crossfire between two wires which could definitely cause the backfiring. Put on a new set of wires and see what problems are left.