1998 GMC Envoy P0300 Code

Engine Performance problem
1998 GMC Envoy 6 cyl Four Wheel Drive Automatic 159K miles

The engine misfirines underload. It idles smooth. The odometer shows 159K miles.

Disconnected the MAP sensor while troublshooting. This resolved the misfiring and made the vehicle drivable. Obviously this is now masking the real problem.

When reving the engine in neutral gear, the misfiring occurs in the mid-range rpms. When this happens, an incomplete combustion smell eminates from the exhaust. (Must not be lacking fuel- maybe ignition timing problem?)

Checked for vacuum leaks, none found. Measured vacuum at idle- 19".

While resolving a previous problem the following components have been replaced within the past 6 months: plugs, wires, distributor, rotor, fuel filter, fuel pump, fuel pressure regulator, new fuel injectors/spider, MAP sensor, EGR valve, new vacuum hoses, new ignition module, cleaned PCV valve.

OK, I'm stumped. Any further suggestions will be greatly appreciated!

Steve
Mocksville NC