1999 Chevy G-30 utility van code p-0174 , cleaned then replaced MAFcleared code, back on 50 miles later, found vacum leak hose off vac. ball, code cleard 50 miles light back on,

Sounds like you have a serious vacum leak.; DTC 0174 and 0174 are lean conditions on bank 1 and bank 2, 0300 is random misfire. The only thing I can think of is a big vacum leak like the brake booster / vac line leaking, or maybe the intake manifold is cracked ... even a stuck-open EGR doesn't normally lean a motor that much. Start by checking for a vacum leak, if the motor will idle you can spray a little WD-40 around ports and lines >> if the idle speeds up or becomes smooth you on the right track in the area you judt sprayed.

Has your Check Engine or Service Engine Soon light come on? If so, there is a code stored in your van's onboard computer. Have that code read and interpreted at any large auto parts store, and either arrange to have the problem repaired or repost with the code number. If no code is present, we have a whole new ball game.

Engine light lit ? Have it checked for DTC'S - diagnostic trouble code's ! Do you know what DTC'S are for ?

Your scanner probably said I/M rediness tests not complete. It takes most OBD II systems about 50 miles to complete all the self tests. Best way to find a vacum leak is to start the car and then using a spray can of carb cleaner, spray around any hoses and the intake manifold gasket. If there is a leak the engine will change speed. What code are you getting?

there is a vacum maudual that screw's into the transmission were those three vacume lines connect,a little ball inside there moves up and down to controle the vacume ingageing and disengageing the 4 wheel, i had the same problem,replaced it and wooks great, you will have to go through a chevy dealer to get the part tho, auto-stores don't carry it. good luck

have u had your shokes check

This normally indicates that one of the pressure hoses has either popped off, or has a leak..the hoses run from the vac pump to the doors, with several T connections in between..Start at the pump and trace from there..Check for popped hoses at the T connections, the pump, and at each door lock actuator (inside each door)..Listen for hissing sounds when the pump is running, this indicates a leak.

Do you have a Vac leak? easy way to find is to let run and open hood.. buy a can of starting fluid.. sprey around the Vac hoses and when the engine RPM picks up you have found your Vac leak.. hope it helps

check all vacum lines starting with the pcv valve vacum line. check while engine is running ,lisen for air leak.if u have a vacum leak then this will set these codes.

check the pcv valve vacum lines for a leak a vacum leak will set 171 174 codes. lift the hood with engine running and listen for a air leak , check all vacum lines if no vacum leaks can be found then clean the mass air flow sensor by spraying carb cleaner directly on it for about 5 seconds. in other words take the air filter cover up and spray down the tube that leads to engine. shine a light down hose first and see if u can see the mass air flow sensor so u can get a direct hit