It’s a 2003 Buick Century 3.1L bucking jumping when accelerating. No fault code. This problem haunted me for half year. Not a single engine/tranny code in the entire process including history or pending code. After throwing tons of parts at it, I finally fixed it.
- a PC-simulated GM Tech2 scanner (VXDiag VCX NANO GM from vxdiagshop.com) which has live data, misfire data, 2-way programming or sensor controls.
- Fuel pressure gauge.
- Car jump/buck/lunge forward while accelerating or climbing a hill. Worst at 2-3 gear.
- It appears to be engine problem, not transmission problem, because engine RPM drops from >2000rpm to <1000rpm like a “pause”. The engine quickly picks up RPM and the car lunge forward.
- Misfire data show occasional misfire in random cylinder, but not bad enough to set any code. During the “pause” there is no massive misfire.
- Fuel pressure gauge shows normal ~50psi idle or under load — normal fuel system
- MAP starts at 30kpa idle. Opened throttle to increase and maintain rpm to 2800, MAP increase to 45kpa and quickly restores to 28kpa — good vacuum, no CAT restriction
- STFT and LTFT are within 5%
Parts thrown at it (without improvement):
- Cleaned throttle, IAC, EGR. Not dirty at all. Tech2 tool is able to control the valve to expected position.
- Fuel: replaced pump, filter, fuel pressure regulator, all fuel injectors (expensive!)
- Ignition: replaced spark plugs, wire, coil, ICM
- Sensors: replaced TPS, IAC, MAF, MAP, 3x Crankshaft Position Sensor (facing firewall) Some articles call it 7x CPS but Tech2 tool shows 3x CPS.
- The wiring harness that connects 3x CPS to the ICM
It is a 2-wire cable, purple and yellow, with male plugs on both ends. After 18 years and 250000 miles, this cable deteriorated due to proximity to exhaust manifold. Outer sleeve disintegrated. A plastic retainer which plugs to a hole near thermostat is broken. The cable is now dangling near crossover exhaust pipe, and partially melted. From google search I learned the 3x CPS is “critical” and will halt ignition without setting any code. (The other 24x CPS behind harmonic balancer is non-critical)
The Replacement part:
- GM 15301408 or Dorman 645-514
I bought the Dorman. Dorman lists it as a “Knock sensor harness” but it is actually 3x CPS. It’s a exact fit with the 2 plastic retainers (hoops) in the middle. I added a fiberglass sleeve around it for extra protection. The bucking problem is totally gone.
Many people had bucking problem without reaching a solution. I hope my story will help someone.