Car gets great gas mileage but the idling is erratic when I first start it and when I hit about 45 mph Have replaced the map sensor the throttle position sensor and no engine light is on

have you replaced the egr valve

You vehicle is equipt with a EGR (exhaust gas recirculation) and a MAP (manifold air pressure) When the EGR is stuck open you affect the performance of the MAP sensor,Take out the EGR clean the carbon that has build in there or replace the EGR

Check the help links related to this problem.Click the link below to go through its troubleshooting procedure :-----Idles rough and quits when gas pedal is pressed less? http://technoanswers.blogspot.in/2012/01/1999-ford-f-150-idles-rough-and-quits.html---------
Car won't start without hitting the gas pedal? http://schematicsdiagram.blogspot.com/2011/12/car-wont-start-without-hitting-gas.html
-------Complete loss of brake pedal then all gets normal? http://repairhelpcenter.blogspot.com/2012/01/complete-loss-of-brake-pedal-then-all.html
------Car dies when gas pedal is pressed? http://schematicsdiagram.blogspot.com/2011/12/car-dies-when-gas-pedal-is-pressed.html
--------Pushing on the gas pedal, but the car slows down? Gas pedal to the floor and the car slows down?

http://technoanswers.blogspot.in/2012/02/pushing-on-gas-pedal-but-car-slows-down.html

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After replacing brake rotors and pads, brake pedal remains on floor? http://technoanswers.blogspot.in/2012/02/after-replacing-brake-rotors-and-pads.html

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

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For rough idle issues and problem troubleshooting,go through the help links provided below :----- van idles rough?

http://howtobyme.blogspot.in/2012/01/2004-ford-freestar-van-idles-rough.html

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will not idle when cold?

http://technoanswers.blogspot.in/2012/02/ford-explorer-40-will-not-idle-when.html

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runs lean and idles rough?

http://technoanswers.blogspot.in/2012/02/ford-runs-lean-and-idles-rough.html

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Engine stumbles coming off idle when warm?

http://repairhelpcenter.blogspot.com/2012/01/engine-stumbles-coming-off-idle-when.html

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Erratic idle on car?

http://schematicsdiagram.blogspot.com/2011/12/erratic-idle-on-car.html

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Low idle when cold out side?

http://repairhelpcenter.blogspot.com/2011/12/low-idle-when-cold-out-side.html

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Idle Air Control (IAC) Valve Replacing?

http://schematicsdiagram.blogspot.com/2011/12/gmc-idle-air-control-iac-valve.html

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Engine quits at idle?

http://repairhelpcenter.blogspot.com/2011/10/engine-quits-at-idle.html

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Car overheats and Air blows hot at idle?

http://howtobyme.blogspot.com/2011/11/car-overheats-and-air-blows-hot-at-idle.html

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While stop engine idles like cylinder missing?

http://schematicsdiagram.blogspot.com/2012/01/while-stop-engine-idles-like-cylinder.html

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This will help.

Thanks.

Hi and welcome to FixYa!

There are two things that can result to this issue. It's a bad MAF (mass air flow) sensor or a failing TPS (throttle position sensor). If one of these sensors are faulty, an erratic idle or unresponsive throttle will be expected. As a first course of action, have it looked at and replace if necessary.

Hope this helps and thank you for using FixYa! Have a good one!

Replace TPS (Throttle Positioning Sensor) It's a rheostat that controls throttle. After time and mileage it develops hot spots/wear in areas that get most use. In your case the 45-55 mph area of rheostat. Your description is spot on to symptom of bad TPS.

YOU HAVE A TORQUE CONVERTER LOCKUP ISSUE, HAVE SOMEONE LIKE THE DEALER,AUTO ZONE OR IF YOU ARE HANDY CHECK FOR CODES, NO CODES....THEN TAKE THE CAR OUT STEADY DRIVE AT AROUND 45 MPH, GENTLY HIT THE BRAKE PEDAL JUST ENOUGH TO MAKE THE REAR BRAKE LIGHTS GO ON BUT NOT HARD ENOUGH TO STOP THE CAR...DO YOU FEEL THE CONVERTER SHIFT IN AND OUT EACHJ TIME THE BRAKE PEDAL IS TOUCHED MAINTAINING A STEADY 45 MPH SPEED, IF IT THAT SAME SHUDDER YOU ARE FEELING THE TORQUE CONVERTER NEEDS TO BE REPLACED, NOT AN EASY JOB AS THE TRANSMISSION HAS TO BE REMOVED TO GET AT IT.SORRY ABOUT THE CAPS.......

i think if you checked the sparking plugs and map sensors then check your distributer its maybe not delivering the current nedded to perform the spark to some of the cylinders

Here are some of the causes of surges, stalls at stops, slow idle speed, erratic idle speed, rough idle and engine hesitation (and other problems), it is in most cases the idle speed control air-bypass valve and or throttle valve and upper intake, these area's get full of gunk and combustion residue over the miles and cause idle issues (stalls, low idle) like yours, Get a can of intake cleaner from any local parts store, not carb spray, intake cleaner, it is made by a company called CRC, remove the air intake hose to the engine, hold the idle high so the engine won't stall, then spray the can of cleaner into the intake while keeping the engine running, use at least 1/2 the can, shut down the engine and disconnect the battery for 5 minutes, then restart and complete a number of mixed driving cycles, town, freeway, stop and go etc., after a few days the problem will go away as the system will relearn to the clean intake.

Just trowing some suggestions out there, but, I would check the idle air control valve (solenoid) or the throttle position sensor. If they check out then my last guess would be the map sensor or the vaccuum to the map sensor. Hope I was able to help. Best wishes.

Check PCV valve and be sure it is not stuck open.  Check for vacuum leak at intake manifold.