1999 GMC Safari MISFIRING

SPEEDKLL

MEMBER
1999 GMC SAFARI
6 CYL
2WD
AUTOMATIC
56,000 MILES

MY SAFARI HAS BEEN MISFIRING AND I CAN'T FIGURE OUT WHAT IS WRONG WITH IT. THE FOLLOWING HAVE BEEN CHANGED: CAP & ROTOR, IGNITION COIL & MODULE, WIRES, PLUGS, FUEL PUMP, FUEL FILTER, AIR TEMP. SENSOR, COOLANT TEMP. SENSOR, THROTTLE POSITION SENSOR, BATTERY, STARTER, CAM POSITION SENSOR, & AIR FILTER. IT MISSES WORSE IN COLD WINTER WEATHER (RUNS A LITTLE BETTER IN DRY SUMMER WEATHER). IF YOU TRY TO SHIFT THROUGH THE GEARS MANUALLY 1ST- 2ND- 3RD YOU REALLY CAN NOTICE THE STUMBLING AND MISFIRING. SOMEONE PLEASE RESPOND WITH HELP. NO ONE HAS BEEN ABLE TO FIGURE OUT WHAT IS WRONG. EVERY ONCE IN A WHILE I'LL GET A CODE U1041 SCP (J1850) Invalid or Missing Data for Vehicle Speed & P0420 Catalyst System Efficency Below Threshold (Bank 1). I REALLY WOULD APPRECIATE ANY HELP THAT ANYONE CAN OFFER.

1 Reply

2CARPRO JACK

MECHANIC
11,533 POSTS

The cat efficiency code may be pointing you toward a partially plugged converter, this will cause a misfire. Those CPI's also had big issues with injectors. Need to put it on a real scanner to watch the misfire data to see which cylinder is missing. If it has an injector that is stuck open, it can ruin the cat by dumping too much fuel into it, which in turn makes the PCM adjust the mixture to try to compensate for the raw fuel in the exhaust