Momentary Sag On Light Due To Moderate Accel When Cold - Valve Deposits - keywords acceleration additive black brief carbon cleaner decarbon DTC engine exhaust fuel gasoline hesitation intake no smoke

This PI was superseded to update Model Years and revise the bulletin numbers that are mentioned in this PI. Please discard PIP3644B.

The following diagnosis might be helpful if the vehicle exhibits the symptom(s) described in this PI.

On rare occasions, a momentary sag and black exhaust smoke may be encountered on light to moderate acceleration following a cold start. Typically, there will not be any DTCs stored and this will no longer occur after running the engine for 1 - 2 minutes. This concern may be a result of intake valve deposits.

If the SI diagnostics do not isolate a cause for this concern and the latest calibrations are in the PCM, remove the fuel rail and inspect the back side of the valves, using a pen light through the fuel injector ports.

If intake valve deposits are noted, it will be necessary to remove the cylinder head and intake valves. Since it is not suggested to clean these valves with a wire wheel, it will be necessary to try removing the deposits by hand after soaking them in GM Fuel System Treatment Plus overnight. In the U.S.A., use Part Number 88861011 or in Canada, use Part Number 88861012. If the deposits can not be removed by hand, replace the intake valves by following SI repair procedures.

If the above suggestion corrects the concern, the use of top tier gasoline's (see bulletins 04-06-04-047G and 05-06-04-022C) may prevent this from occurring in the future. It is also suggested to add one bottle of GM Fuel System Treatment Plus to the fuel tank by following the directions on bottle. In the U.S.A., use Part Number 88861011 or in Canada, use Part Number 88861012. This should keep the valves clean until the vehicle can be filled at a top tier gasoline station.

Please follow this diagnostic or repair process thoroughly and complete each step. If the condition exhibited is resolved without completing every step, the remaining steps do not need to be performed.