1996 sonoma 440

Make a gasket up from a coca cola can and block off the EGR valve and see what happens

If the check engine light is on I would have that checked first. The trouble codes in the computer will help point you in the right direction for troubleshooting.
You could have a sticking fuel injector, or an ignition problem, or both.

Unplug negative battery for few minutes; then reconnected and, if the check light tuno on again, I suggest scanning for knoew what code fault have there.

Hope help (remember comment and rated this).

I'm pretty sure 440 is an evaporation malfunction code. The cheapest cause is usually the fuel cap. However, there are evap solenoids that go bad over time. I believe there are three of them at around $50.00 each. Also check the connections where they plug in as they may have become loose or corroded.

You can raise your hood and look for any signs of an obvious rubber hose that has come off of anything. The Vacuum situation is sometimes hard for someone to diagnose and we use a smoke machine to find leaks a lot of the time as sometimes it is hard to find a vacuum line that has a small hole or is disconnected. So the answer is look for anything you can that might be obvious...other then that you will probably have to take it to the pro's.

Do you get black exhaust smoke when first started? If so, then there is too much fuel at first which probably fouls several spark plugs, but as the engine warms up, the plugs start firing again. If no black smoke--is the check engine light ON? Read the code(s) if so and follow the clues.

have you tried changing the wires and fuel pump, exhaust wont make it not go over 40 just back fire from pressure build up is there a engine light on

Which code number?

I have a 96 cavalier and if i forget to turn the gas cap until it clicks air gets in the tank and the ck. engine light comes on. So, don't forget that could be part of the problem

there could be several possibilities, first did you change the fuel filter? most often times a partially plugged fuel filter will cause the sputtering. plugged fuel filters generally lead to the failure of the fuel pump. if you have a fuel injected vehicle the injector themselves can fail, this happens often especially if regular maintenance doesn't happen. i always use an injector cleaner additive when i fill up each time. a common symptom of sputtering is a plugged catalayic converter. the engine needs to breath to run right. also check the airfilter this is important for the engine to breath as well as proper fuel air mixture. if the engine senses a loss of power due to low fuel air mixture it will try to pump more fuel then it should therefore it's basically flooding itself. try these basic fixes first. if this doesn't solve the problem then post back with more detail and i will try a different approach.. please rate this thanks.