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
August, 24, 2009 AT 6:55 PM
There could be many reasons for this condition.
Start with the basics, check plugs, plug wires, cap & rotor, fuel and air filters.
There could be another problem but these are the simplest and the usual suspects.
Check theres and let me know what you find and we can go from there.
August, 25, 2009 AT 10:05 AM
I had the same problem. With my blazer turned out it was a bad catalitic converter!
August, 25, 2009 AT 11:48 AM
Rasmataz is correct on all that information, we were posting at the same time, didn't see that untill I had posted mine.
Rubyblazer is also correct, a catalitic converter that is begining to plug will also give you those symtoms.
Catalitic converters do not as a rule wear out though,
they are damaged/plugged by engine oil or raw fuel getting into the converter. Raw fuel continues to burn in the converter damaging the filter and as it heats up the " honey comb" expands and being in a confined space blocks exhaust flow. Engine oil from worn valve seals or pison rings gets into the converter and plugs the honey comb.
Plugged converters can be checked by testing tempature into and out of the converter, should be higher exiting the converter. This can be done with a lazer temp sensor. Lacking the high tech toy, you can dissconnect the exhaust system before the converter and drive the vehicle. It of course will be loud, but if your drivability problem goes away the converter is damaged and will require replacement.
Before replacing the converter though I would strongly recommend diagnosing what damaged the converter in the first place, otherwise you will only be treating the symptom and the root cause will eventually damage your new converter.
The above reason is why the suggestion for checking the basics first. Not saying that you need to replace any or all of them, just make sure they are in good conditiion first then continue on with your diagnosis.
You have good answers from three different sources.
Thanks to all that contributed and I hope these ideas help you.
Please let us know what you find or if you have any more questions.