Hyundai Repair: 2001 sonata bogging out, air flow sensor, vacuum leaks

Question
QUESTION: hi  i have a 2001 sonata that starts and idles fine. if i put in gear and give it gas it hesitates and lacks power. once i get up to speed i can floor it and it will move right along.  if im in gear and hold brake and give it gas the car has no power. i changed the fuel filter and plugs but no change. i unplugged the maf sensor with car running and it did run worse.  im out of ideas

ANSWER: I'm wondering if your transmission is in failsafe (third gear only).  This will cause poor acceleration at slow speeds.

Is your check engine lamp on or has it been on recently?  Even if the answer is no, it may still be a good idea to check the trouble codes in the engine and transmission control modules.  There may be information that will help us know where the problem lies.

Can you feel the transmission shift gears as you accelerate from a stop?

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QUESTION: it did read that it was running lean. the car does shift through all the gears.  it just has no power from a stop. it feels like the car is out of timing

Answer
Usually, if the engine is out of time, you'll have a poor idle when hot.

Most frequently, on four cylinder Sonatas, P0171 (lean condition) is caused by a problem with the air flow sensor, but can also be caused by a vacuum leak.  Vehicles with this code can display the symptoms you describe, so I'd recommend addressing things that could be causing this code.  First, check to be sure that there are no vacuum leaks-- these could occur at the PCV hose, the brake booster hose, or, to a lesser degree, at any of the vacuum hoses.  Also check the bellows between the air flow sensor and the throttle body.  If it has a break in it, a significant portion of the air going into the engine won't be going through the sensor.  

Just to be safe, you should also check for leaks at the manifold or a problem with the timing.  Again, I doubt either of these is the cause since you say it idles normally.

If you find no air leaks and the timing is okay, then the problem is most likely the air flow sensor-- could also be caused by a faulty oxygen sensor or fuel with excessive methanol, but these are much less frequent.  Unfortunately, there is no way to test the air flow sensor without a scan tool.  To do this, you'd need to monitor the voltage values from the air flow sensor at different engine rpm.  The sensor is expensive, so at this point you're probably stuck with making an expensive guess or taking it to a shop very competent in driveability diagnosis.