Backfire on cold start up blows vacume plug off of intake

Mass Air Flow sensor just measures incoming air flow at a predetermined rpm/load. The computer is saying the flow measurement is not correct. This can be caused by mechanical faults within the engine like a bad valve seat or timing chain but would generally be accompanied by backfires through the intake. Usually its caused by a vacume leak between the sensor and the intake valve,like the intake manifold,loose intake pipe clamps, broken vacume lines,a leaking vacume brake booster [it will hiss loud when you apply the brake],bad spark plugs/wires, a clogged catalytic converter, stuck open egr valve or pcv valve,bad oxygen sensor or even a very clogged air filter. I know there is a lot listed but the MAF or MAP sensors show general flow through the engine that can be affected by several things. A vacume tester from your local auto parts store is about 30bucks and usually has instructions how to test for vac. leaks within the engine also to determine a if thers is bad valve seat or clogged exhaust.

Back fire is only caused by intake valve being open when a spark plug fires on a cold engine.
Recheck timing marks of cams where the drive sprocket notch of each cam shaft aligns with the cam cap mark at the rear of the head.

It has been known for the crank sprocket to come loose on the crank nose and timing will be off if this is so.
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YOU HAVE INTAKE MANIFOLD LEAK ALSO COULD HAVE BAD OR STICKING INTAKE VALVES. CODE PO 226 IDM SIGNAL NOT RECEIVED AND CODE PO 542 FUEL PUMP SECONDARY CIRCUIT.

Do a compression test on all of your cylinders when it is cold. Backfiring is a sign of a worn valve seat allowing mixed fuel and air (lit by spark) to be forced back into the intake. You will probably see compression loss on one or more cylinders.

Also... there is a chance that the EGR valve is not totally seated when the engine is cold with just a small blow by. The EGR valve seats coke up over time with black gunk.

you have a leaking intake mainifold gasket that is common on the model

you seem to have a stripped tooth on the camm gear

theres alot of 2002 chevys of different kinds but they all can get a vacume leak the 3.1 and 3.4 even the 4.3 and the 5.7 blow intake gaskets. un plug the pcv valve and the vent hose from the valve cover to the air cleaner hose. plug the holes good with tape and rags till sealed if it runs smooth ar idle then the intake is leaking. Coming into cold weather I did then as fast as I could as there was always another one in the parking lot. Leak coolant too. about half and half, some both. a few were driven too far they were blown. Plug wires and injectores dropping out will do it too. but check engine light on with codes P0300 or any code P0301-308. Vacume leak can set all kinds of stuff from map senser, cam senser Egr back pressure, ramdom cylinder miss to certain cylinder miss. and the leak will be the problem not the sensers or wires

it soubds like you have a vacum leak problem listen for a "his" with the engine running. you may want to take apiece of hose @2-3ft long put one end at you ear and probe arround the top of the engine arrount the intake manafold and any hoses when you find the his find wher it goes and replace repair the hose. it may also be your pcvvalve thst is stuck it willcome out of the top of your valve cover and plug right the intake hose right before the throttle body

a common problem with these engines is a leaky intake gasket which sucks in outside air spray carb cleaner around intake base see if engine idle increases .most likely this is your problem

engine must be running to check for vacum if u dont have vacum then the hose has come of the intake look for hoses that have come off if the car has backfired in the past this has caused it and if mechanical work been done in the past it may have been left off or accendently knocked off hope it helps

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