Buick Bucking Problem Solved by VXDIAG VCX NANO GM

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.

Tools:

1. 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.
2. Fuel pressure gauge.

Observations:

1. Car jump/buck/lunge forward while accelerating or climbing a hill. Worst at 2-3 gear.
2. 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.
3. Misfire data show occasional misfire in random cylinder, but not bad enough to set any code. During the “pause” there is no massive misfire.
4. Fuel pressure gauge shows normal ~50psi idle or under load — normal fuel system
5. 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
6. STFT and LTFT are within 5%

Parts thrown at it (without improvement):

1. Cleaned throttle, IAC, EGR. Not dirty at all. Tech2 tool is able to control the valve to expected position.
2. Fuel: replaced pump, filter, fuel pressure regulator, all fuel injectors (expensive!)
3. Ignition: replaced spark plugs, wire, coil, ICM
4. 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 Culprit:

1. 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:

1. 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.