I just want to cover the basics for this problem before we move on.
Did you know there are two TSB's for this problem?
Here they are.
Bulletin No.: 06-06-04-046
Date: September 12, 2006
Information on Engine Misfire MIL/SES Light Illuminated or Flashing DTC P0300, P0301, P0302, P0303, P0304, P0305, P0306, P0307, P0308, P0420 or P0430
1999-2007 Cadillac, Chevrolet, GMC Full-Size Pickup and/or Utility Trucks
with 4.8L, 5.3L, 5.7L, 6.0L or 6.2L VORTEC GEN III, GEN IV, V-8 Engine (VINs V, C, T, Z, B, 3, M, 0, J, R, U, N, Y, K, 8 - RPOs LR4, LY2, LM7, L59, L33, LC9, LH6, LMG, LY5, L31, LQ4, LQ9, L76, LY6, L92)
with Active Fuel Management(TM) and E85 Flex Fuel
If you encounter vehicles that exhibit the above conditions, refer to SI for the appropriate DTC(s) set. If no trouble is found, the cause may be due to an ECM ground terminal that has corroded with rust over time. Inspect the main engine wiring harness ground terminal (G103) for this condition.
The wire terminal (G103) attaches either to the front or to the rear of the right side cylinder head, depending on the model year of the Full Size Pickup and/or Utility Trucks. If the ECM ground terminal has been found to be corroded, then follow the service procedure outlined in this bulletin to correct the corrosion issue.
Remove either the nut or bolt securing the main engine wiring harness ground terminal (G103) to the right cylinder head. Refer to the above illustration to determine where the ground is located on the vehicle (1).
Remove all rust from the ground terminal, the cylinder head and the retaining nut or bolt.
Position the main engine wiring harness ground terminal and install the nut or bolt.
Tighten the retaining nut or bolt to 16 N.m (12 lb ft).
Apply some type of electrical moisture sealant to protect the harness terminal from further corrosion.
File In Section: 06 - Engine/Propulsion System
Bulletin No.: 00-06-04-024
Date: June, 2000
Engine Misfire, Water in Fuel, Service Engine Soon Lamp illuminates, DTC P0300 Set ( Replace EVAP Canister Vent Hose Asm/Replace EVAP Canister If Necessary)
1999-2000 Chevrolet and GMC C/K Pickup Trucks (Silverado and Sierra) Built Prior to August 31, 1999
Some drivers may comment about the Service Engine Soon lamp illuminating. The Powertrain Control Module (PCM) may have stored a Diagnostic Trouble Code (DTC) P0300, Engine Misfire Detected. The engine may be exhibiting a misfire, possibly in the # XXXXX and # XXXXX cylinders. This condition may also affect the six cylinder models. This condition may be more prevalent in areas with high amounts of rainfall.
Diagnosis of the fuel system may reveal the presence of water, possibly in the fuel rail near the # XXXXX and # XXXXX fuel injector. Water entering through the evaporative emission (EVAP) canister vent solenoid may cause this condition. Because of this, an engine misfire may develop.
A new EVAP canister vent hose assembly, P/N 15759042, that includes the valve and the solenoid has been developed to correct this condition.
The new solenoid has a revised mounting bracket and a sealant has been applied to the top lid seal surface of the solenoid to address water intrusion. Use the procedure and the part numbers listed below.
If standard diagnostics do not lead to the resolution of the misfire, do the following steps:
1. Remove the fuel rail at the # XXXXX and # XXXXX fuel injector from the engine and drain the fuel into a suitable container. Inspect the drained fuel for the presence of water. Refer to the Fuel Rail Assembly Replacement procedure in the applicable Service Manual.
2. If the presence of water is detected in the fuel rail, remove the fuel tank and inspect the fuel for the presence of water. Refer to the Fuel Tank Replacement procedure in the applicable Service Manual.
3. If water is present in the fuel tank (other than from a water enriched fuel fill), replace the EVAP canister vent hose assembly. Refer to the EVAP Vent Valve Replacement procedure in the applicable Service Manual.
4. Disconnect the EVAP canister line connections and remove the canister. Inspect for the presence of water at the canister line connections and inside the canister. If water is present, replace the EVAP canister. Refer to the EVAP Canister Replacement procedure in the applicable Service Manual.
5. Flush and clean the fuel system. Refer to the Fuel System Cleaning procedure in the applicable Service Manual.
Parts are currently available from GMSPO.