Corolla, 2001 hesitates on acceleration, hard to start sometimes, runs "lean", check engine light has been on, has 138,000 miles, I already changed fuel pump & plugs

ENGINE HESITATES WHILE ACCELERATION

LOSS OF ACCELERATION

It can be fuel pressure issue or it's the problem with accelerator pedal or its clogged filters causing there.

There are many other possibilities. I suggest you to go through the list of help links mentioned below, these will help you to confirm where the actual fault lies: -------

Jerks and Hesitates During Acceleration?

http://technoanswers.blogspot.in/2012/07/2004-dodge-town-country-jerks-and.html

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Loss of acceleration?

http://technoanswers.blogspot.in/2012/02/loss-of-acceleration.html

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Headlights flickering and car hesitates to start?

http://howtobyme.blogspot.in/2012/02/headlights-flickering-and-car-hesitates.html

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How exactly the Vehicles Accelerator Cable works?

http://repairhelpcenter.blogspot.com/2011/11/how-exactly-vehicles-accelerator-cable.html

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How exactly Vehicles Accelerator Pump Diaphragm Works?

http://repairhelpcenter.blogspot.com/2011/11/how-exactly-vehicles-accelerator-pump.html

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2000 Toyota Avalon: Engine Lean Codes and Knock Sound on Acceleration?

http://whatisbyme.blogspot.in/2012/05/2000-toyota-avalon-engine-lean-codes.html

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2004 Toyota Avensis: Acceleration And Deceleration makes noise on Front End?

http://whatisbyme.blogspot.in/2012/05/2004-toyota-avensis-acceleration-and.html

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Wheel hop under hard acceleration?

http://technoanswers.blogspot.in/2012/04/wheel-hop-under-hard-acceleration.html

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2009 Chevy impala accelerator pedal sensor?

http://technoanswers.blogspot.in/2012/02/2009-chevy-impala-accelerator-pedal.html

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Nissan Pathfinder stalls and bucks on Acceleration?

http://technoanswers.blogspot.in/2012/02/nissan-pathfinder-stalls-and-bucks-on.html

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Acceleration lag problem?

http://repairhelpcenter.blogspot.com/2012/01/acceleration-lag-problem.html

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These details will help.

Thanks.

mine did the same thing and it was oxygen sensor
since engine light came on there should be an error code
on computer.Get a diagnostic checker or take to a garage and have them hook uo to the diag connector. I bought a diagnostic checker few years ago and it has come in very handy
sure hope this helps!!

Sounds like a misfire (bad ignition coils)
You need to get it scanned for trouble codes.

MOST TIME CAR STUMBLE OR HESITATION ON ACCELERATION YOU.HAVE A SLIGHT LEAN FUEL MIXTURE.CAUSED BY STOPPED UP FUEL INJECTORS OR FUEL FILTER SLIGHTLY PLUGGED.

If you own a 2004, 2005, 2006 or 2007 Toyota Corolla and your car hesitates when you accelerate or it hesitates when driving, you should try the solution described here. Below is a list of the behavior your car may show that tells that you should try this procedure before going to a mechanic or repair shop.

• The accelerator is slightly pressed while the car is running and it slightly jitters or hesitates
• After completely stopping the car (a red light for example), it hesitates when you accelerate to continue driving
• Engine power is below the usual level
• Sometimes check engine light turns on

You should know that while those problems can be caused by several issues, including low quality or altered fuel or even a loosely tighten fuel cap, the most common cause is a dirty MAF flow meter sensor.
The MAF sensor (Mass Air Flow Meter Sensor) is in charge of detecting how much air (in mass, not volume) is coming in to the engine, so the ecm computer can calculate the amount of fuel to be injected.
Some times this sensor gets contaminated by lots of microscopic debris that scape the air filter and accumulate on its sensing parts over time. The resulting effect is that the MAF flow meter sensor gets less "sensible" and detects less air than it is currently entering the engine. The reaction of the ecm computer will be to inject less fuel to keep the air fuel balance, taking as reference the false reading of the MAF flow meter sensor. This will produce a too-lean mixture, which is responsible of the hesitation, the loss in power and the check engine light.
If your car engine, besides the hesitation, is also producing knocking or engine pinging at high loads, like when going up through a hill, it is attributed to this too-lean condition.
Before going to an auto parts store or to a mechanic to fix the problem, you should try fixing it your self. If the problem is a dirty MAF flow meter sensor, the solution is to clean it, plain and simple. That may save you some money that a mechanic will charge for cleaning it (you could do the same) or for replacing the part when most of the times it is not damaged, just dirty or contaminated

The DAMN THING has 200,000 MILES on it, give it a break, it is just waring out, drive it easy. (most likely burned valve)

air leak at air duct to throttle body.

Basically this means that an oxygen sensor in bank 1 detected a lean condition (too much oxygen in the exhaust).

change the oil pressure switch or the wrong oil

No need to replace the MAF. If it was bad, the check engine light would have come right back on as soon as you started the engine. The MAF not only measures the air going into the engine, but also measures the air temperature. Those 2 circuits are constantly sending info the the ECU. If either would fail, the ECU would light the check engine light. P0171 is a common code for incorrect readings (dirty) in the MAF. Cleaning it always helps, and cuts off the light. Your acceleration problem may be either a wrong Air-Fuel sensor (the one before the cat), or an "aftermarket" AF sensor. "Aftermarket" AF sensors have been shown to not work properly in Toyota vehicles. Hope this helps