2003 Chevrolet Malibu • 107,000 miles

I have a 2003 Chevy Malibu. Here it goes.

Car is having trouble on the highway, and strange enough, ONLY on the highway. Doing city driving the car drives just fantastic. Once I get on the highway the car feels sluggish to accelerate to get up to 70 mph. If I need to pass someone the engine seems to hesitate when I press the gas pedal and for some reason the RMPs will sometimes seem to refuse to go above 2000. Unless I really lay on the gas pedal, the RMP will seemingly freeze at exactly 2000 while I >slowly< accelerate 1 mph at a time. I have: a brand new fuel filter, new air filter, cleaned MAF (that helped initially but now it is going back to the same bad performance), professionally cleaned throttle body of carbon deposits, new muffler and tailpipe along with a very recent oil change, new PCV valve and radiator flush.

Not once have I had a check engine light come on during this whole ordeal (which is frustrating) I've also had a trusted mechanic drive the car with the computer gauge plugged in and as far as he can tell the car is running and reading perfectly fine. Any idea's?

October 14, 2011.

Check your fuel pressure and put vacuum gauge on intake and if 15" suspect a plugged or partially plugged converter

Oct 14, 2011.
Try scanning the computer for code/s.

Could be caused by one of the following below

Oxygen sensor.
Catalytic converter.
Fuel injectors dirty/sticking.
Mass airflow sensor/Airflow meter.
Throttle position sensor.
Crankshaft position sensor
Knock sensor
Manifold absolute pressure sensor.
EGR Valve
Fuel pressure regulator leaking or defective fuel pump.
Fuel contamination.
Foul/defective spark plugs.
Open spark plug wires.
Ignition coil/Coil packs defective.
Incorrect ignition timing.
Cap and rotor.

Note:If it doesn't apply disregard it and keep testing.

Oct 14, 2011.
*Forgot to mention that I just got all brand new spark plugs as well.

Hmac - I appreciate your advice, what do you mean by 15" exactly?

Oct 14, 2011.
Its the vacuum gauge reading of 15 inch or lower indicates a plugged Cat

Oct 14, 2011.
What is the best way to go about having all of those things on the list tested? I would imagine just testing one of those things would cost a little bit of money, so to test all of them would be pretty pricey would it not?

Oct 15, 2011.