Engine Performance problem
2002 Lexus IS 300 6 cyl Two Wheel Drive Automatic 92694 miles
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I have changed my spark plugs twice in the last 2 months and everytimne I do, the car runs great for a few weeks and then slowly begins to hesitate more and more. During my last plug change I replaced the center ignition coil because the dealer said it was missfiring on the 3rd cylendar. But still it hesitates. I don't really trust the dealer to go in and "look" to see what's wrong for $400.00 and then fix whatever they find for about $900.00. But I haven't been able to find anyone who has ever heard of this sort of thing before. I hope you can stear me in the right direction.
Thank you.

Mark

Possible Causes of Engine Hesitation or Stumble:

Dirty fuel injectors (cleaning the injectors often fixes this).
Bad MAP (manifold absolute pressure) sensor
Bad TPS (throttle position) sensor
Bad or dirty MAF (mass airflow) sensor
Low fuel pressure (leaky fuel pressure regulator or weak fuel pump)
Vacuum leaks (intake manifold, vacuum hoses, throttle body, EGR valve)
Bad gasoline (fuel contaminated with water or too much alcohol)

Sometimes, what feels like a hesitation is actually ignition misfire rather than lean misfire. The causes of ignition misfire may include:

Dirty or worn spark plugs
Bad plug wires
Weak ignition coil
Wet plug wires