I think its the catalac converter .. Kinda jerks and hesitates and seems like it's wanting to back fire but doesnt..

Did you clean the Throttle Body when you replaced the MAF?

A clogged Catalytic Converter will always throw a P0420 code.

The new Crank Position Sensor may be faulty. They come in grades and varying prices. Always buy the one that the maker specifies, OEM spec.

Hello. It seems your Pontiac's catalytic converter is clogged; can't breathe. Some vehicles do not give a 'warning' their catalytic converter is clogging until its too late. Clogging catalytic converters causes missing, hesitation and stalling. Good luck and keep me informed; if you want to.

Sounds like a fuel injector is clogged up or is not firing due to the computer not firing it. Check by putting a noid light on each injector and checking for pulse by viewing a blinking light from it. Also, one can check to see if a stethoscope can be used to listen for a clicking from the injector indicating it's at least pulsing that way.

Hmmmm...are you sure when you changed the timing belt that all the timing marks were ligned up correctly?

Did you check to make sure you had no damaged valves?

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This has me thinking because its an older car and iam thinking of our europeon models.But if this is a californian model ?? then i would be totally wrong.But to start with i would check the any pipes connected with the inlet manifold,for air leaks ,like the brake servo feed pipe and any small ones that work the fuel vapour thingy or the gearbox.Then if that doesnt work then get a local garage to have a look because a mechanic would have come across this before and would have a good idea what to change without a meter reading .my one suspect is the air flow meter

Yes it can.

just wondering,, when you accelerated you say the needle went up ,,but did the car kinda splutter, or not pull away at all when you accelerated,,,if the needle goes up but the car doesnt seen to pull away then that sounds like a clutch slipping,,.. but if it just hesitates it could be lots of things ,,maybe a dodgy airflow meter..that can also make the car hesitate,,hope that helps you,,,,,

The jerking and shuddering at 45-50 mph seems like it could be a transmission/torque converter problem. Check your transmission fluid. Make sure it is at the right level, and that it is not dark or burnt. It should be a bright red or pink.

Frank
Frank's Friendly Cars Maui Car Rental
www.MauiCarRental.biz

check your ignition timing, looks like firing is advance.

when was the last time u changed the fuel filter??
if it is not slipping gears then its not the clutch.
this also could be the beginning of a bad fuel pump.they also start that way