For the last 2 weeks, my 2001 Mustang V6 147,000 miles has been intermittently hesitating in all but low gears. I may not keep it in the lower gears long enough for it to occur. The car doesn't slow down or anything, but I can feel the hesitation. My mechanic had previously diagnosed bearing troubles - humming in 5th gear was the problem, and I thought this was related. Now he says it's the engine. Thoughts? I am NOT a DIY owner but I like to know what I might be looking at. We have not tried anything to counter this. Engine has had, in last 6 months, machined head gasket, new intake, new IAC idle drops below 4k intermittently and re-training it did not work.
My best guess is from the info you provided that you have a vacuum leak somewhere. Can you hear a hissing if you listen closely while the engine is idling?
the idle speed of 4000 rpm is way high and should be somewhere around or slightly under 1000 rpm's max.
At higher rpm (lower gears) you wouldn't notice a "hesitation " as much (if you are properly describing the symptom") whereas under load or at lower rpm of the higher ratio gear it would be more noticeable.
If there is a vacuum leak the excessive air leaking into the intake will " fool" the ECM into thinking it needs to provide the engine with more fuel which will cause an excessive idle speed that you're describing.
An OBD scan is a good idea but may not be accurately read by non professionals blaming components (which may be needed also) rather than a problem that can cause the component (sensors) to read an out of range problem.
You neglected to say if you have a MIL (check engine or equivalent) light illuminated........... hope this helps
I would suggest that you take your car to O'rielly's or one of the larger auto parts chain stores and have them connect the diagnostic reader to your car. It sounds to me as though you are experiencing something to do with the cars computer and the way it is reading it's sensors. They will do this for free and then tell you what the error codes mean, if any. If you do receive error codes, chances are that is your problem and you will then be aware of what it is and what it will cost to clear it up and not have some mechanic charging you for his time doing exploratory diagnostics and charging you an arm and a leg just to find out whats wrong.
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The hesitation usually occurs in lower RPMs and when accelerating rather than driving at speed or slowing down.
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