I have a 2002 GMC Envoy, it starts missing when gas gets to 1/4

SOURCE: adjust timing on 1998 GMC K2500 pick up 5.7 liter

It has to be done with a scanner. You can adjust this distributor, but there are no timing marks to watch as you move it. There is a timing mark on the balancer and on the timing cover for referencing Top Dead Center #1 cylinder only, but it is only used for dropping the distributor in to get the engine up and running. A scanner is needed to properly adjust the timing. Connect the scanner and look for a data parameter labeled ‘cam offset’ or possibly ‘cam ******.’ It will be listed in degrees. Start the engine, and bring it up to 1500 rpm. Note the reading. If the timing specification is not within plus or minus two degrees of zero, you need to move the distributor. You can move the distributor with the engine running but the timing will not change. After you move the distributor, the ignition has to be cycled off to let the computer learn the new timing spec. Restart the engine, bring it up to 1500 rpm and see what the new spec is. You must repeat this procedure until you get the timing into the specification as mentioned above. Once you have it set to spec, the code should clear.

SOURCE: 2002 GMC Envoy

I just had this problem on my Envoy on July 14, 2008, I called the guaranty company and they sent my truck to a chevy dealer, after 4 days they fixed. It was the ignition switch.

SOURCE: 2002 GMC Envoy

If your 2002 gmc envoy or sierra will not even turn over with a kown good battery try starting it in nuetral then go back to park and try starting again. There is a park nuetral safety switch that can malfunction. Turn the key on then shift into nuetral, then turn fully to start.
 

SOURCE: Replace the thermostat on 2002 GMC Envoy

4.2L Engine the thermostat and the neck where the hose goes on is one unit $25 at autozone

1. Remove the necessary coolant from the radiator.
2. Remove the alternator, Loosen the outlet hose clamp at the thermostat housing. Remove the outlet hose from the thermostat housing.
3. Remove the thermostat housing bolts.
4. Remove the thermostat housing from the engine block.
5. Clean all of the surfaces of the thermostat housing.
6. Clean the sealing surface of the engine block.

SOURCE: Need firing order diagram for 2002 gmc envoy

one, five, three, six, two, four

The plastic intake should have this sequence molded into it somewhere, and even the corresponding cylinder numbers in front of their respective holes. Have fun, and thanks for posting on Fixya.com