QuestionVehicle: 2000 Chev Express w/ vortec engine. 146,000 miles. Misfires @ low throttle only when moving down road. Idles fine. High throttle fine. No misfire @ standstill with brake on. Mechanic did full tune up with new wires, cleaned injectors and added GM's cleaner to gas tank. No codes except for misfire. Misfires at all rpms and in all gears. Check engine light NOT on. Mechanic checked all sensors and fuel flow and fuel pressure.
AnswerMechanic checked all sensors and fuel flow and fuel pressure.
WHAT was the pressure readings and amount of fuel flow ?. was it within factory specifications or just a GUESS ?.
Misfires at low speeds can be caused by (if applicable) one of the 2 coil packs has an intermittent fault. Did you pull ALL the spark plugs and examine the tips to see if they are ALL burining correctly and appear to have the same redish color ?.
Also an intermittent fault in the ignition module, crank sensor and or wiring to and from the ignition module to the crank sensor can cause this fault.
An intermittent fault in the EGR system can cause this fault.
An intermittent fault in the TPS (Throttle Positioning Sensor) can also cause this fault.
Any faulty vacuum line also.
Did you examin the throttle plate area for a blackish residue buildup around the throttle plate area.
Intermittent faults will usually NOT illuminate the CHECK ENGINE light on the dash.
Without actually having physical access to the vehicle, Your problem is most likely one of the above and ALL need to be manually tested as alot of times a scanner will NOT pickup an intermittent fault.
an exhaust restriction can also be the cause. Did you do a back pressure test on the exhaust system ?.
The MAYBE'S go ON and ON and everything must be checked properly of which most of the newer techs will not take the time or effort to do.
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