These two conditions seem to be very common, and requires similar repair procedures (removal of the intake manifold). Gm has recognized these problems and has issued a parts revision and TSB (Technical Service Bulletin) to repair these symptoms.
First we will talk about a rough idle. Most vehicle owners complain about a rough idle or a misfire when the engine is cold, and as the engine warms up the problem goes away??? But, leaving your check engine light on, with codes PO300, PO171 and PO174. PO300 is a misfire code that is usually fixed by new ignition parts like spark plugs and wires. But, when accompanied by PO171 and PO174, this indicates a Fuel Trim problem. The engine may be running to lean, due to a intake vacuum leak. When your vehicles engine is cold, the PCM (power control module) is running in ‘closed loop’ a predetermined run program. As the engine reaches normal operating temperature the PCM switches to ‘open loop’ using the engine control sensors to read the data from the engine to refine the running condition. One of these sensors is a oxygen sensor, this monitors the air/fuel ratios in the exhaust to calculate the amount of fuel to deliver to the engine. With a vacuum leak the the computer controls can not calculate for the extra air in the engine, making a lean condition. The oxygen sensor sees the lean condition and the PCM increases the amount of fuel to the engine to compensate. This is called fuel trim, as the fuel trim enrichment reaches 25% or more adjustment the PCM sets a code for the problem.
GM has recognized two part problems with sealing of the plastic intake manifold, the intake gaskets and bolts. The original red/orange gasket has be revised to a teal green colored gasket were the material has been updated. The bolts are sleeved with a rubber crush washer and require a special torque sequence to insure a proper seal. The intake bolts should NEVER be reused!
GM has also issued a TSB (02-06-04-023A) for code PO332 (knock sensor), which may be heard as a mild to severe engine ping usually sounding worse on acceleration. This condition is a result of corrosion on the rear bank knock sensor due to water intrusion into the sensor cavity. This condition is more frequent on vehicles that the owner washes the engine compartment.
To correct the Knock sensor problem requires replacement of the rear knock sensor (their is also a front knock sensor, we also recommend replacing) and building a dam (wall) around the sensor using RTV to divert water away from the sensor.
Tighten the intake manifold bolts in sequence, tighten on first pass to 44Lb in. On the final pass torque to 89Lb in.