Hesitation when speeding up
 

I have a 1999 Malibu 3.1.
Car starts fine either warm or cold but when it is warmed up, it has a bad hesitation. It will hesitate cold but gets much worse when warm. Most of the time the engine will have a very difficult time reaching 60mph. Idle quality is good. I get a P0300 missfire code or a P0410.
Exhaust back pressure was checked w/ vaccum guage and reving the engine to 2500 rpm. All OK
New TPS (return spring was broke)
New IAT switch (fixed loose wire on plug)
New fuel pump (gas guage wasnt working)
MAP sensor was checked at Auto Zone an it runs the same unplugged.
New MAF sensor ( There was a code 2 weeks ago) I tried to clean the old one and didnt change anything.
EGR valve checked and not sticking.
New coolant temp switch and thermostat
New secondary air injection pump and control valve. The old valve had the top blown out of it and tried to burn through the hood insulation. The exhaust gases went through the rubber hose and fried the pump. Replaced the 30 amp fuse as well. (blown)
Throttle body cleaned and inspected.
New head gaskets and intake gaskets.
New NGK plugs and wires.
Coils have been checked and passed.
Triple checked vacuum hoses. No cracks or splits.
New O2 sensor (left bank due to code)
IAC valve checked and cleaned.
New PCV valve.
Needless to say, I`m at my wits end. Every once in a while the car will start and run good but hasnt run good in a few weeks.
It seems that the less throttle applied (when its running poor) it will hesitate less but when you push the throttle a lot, the car will downshift, rpm`s come up, and the car will slow down. Fully loaded 18 wheelers could beat me in a race from stoplight to stoplight.
Any ideas would be gratefull.

I have a 2002 Malibu that is having the same problem as described in your post. Did you ever find the answer to your hesitation? What was the problem?

CHange the CAT or empty out the cat.
it worked for me

I did a cat test and it was clogged. Thanks for the reply.

Crankshaft positioning sensor

how do you do a CAT test?

I don't mean to jump in but, based on the age of this thread ... The test being referred to is to see if the catalytic converter is the cause of the restrictive exhaust. There are various ways to go about it like temporarily removing a test plug (if there's one) or the O2 sensor before the catalytic converter to give the exhaust gases someplace to go. The exhaust shops will actually inspect the catalytic converter itself.

I just had the same issues with my 2002 malibu and the garage changed one of my spark plug wires and also replaced the camshaft sensor. everything runs perfect now. so it may be your camshaft sensor that measures how much fuel your ECU gets.