There is a tech bulletin about misfires on 4.2 engines, here's part of it, if you want the whole thige post your email.
4.2L LL8 Engine - Flashing and/or MIL/SES Light, Rough Idle, Misfire(s) DTC(s)
P0300, P0301, P0302, P0303, P0304, P0305, P0306 (Install AIP Seal)
TECHNICAL SERVICE BULLETIN
Reference Number(s): 06-06-04-048B, Date of Issue: January 12, 2007
2004-2007 Buick Rainier; 2002-2007 Chevrolet TrailBlazer; 2002-2007 GMC
Envoy; 2002-2004 Oldsmobile Bravada; 2005-2007 Saab 9-7X; with Vortec(tm)
4.2L Inline 6 Cylinder Engine (VIN S - RPO LL8)
This bulletin is being revised to add the new part numbers for the Spark Plugs and
Ignition Coils. Please discard Corporate Bulletin Number 06-06-04-048A (Section
06 - Engine /Propulsion System).
Related Ref Number(s): 06-06-04-048, 06-06-04-048A, 06-06-04-048B
Some customers may comment that, after severe weather that includes large amounts of rain in a
short period of time, the engine has a rough idle and/or flashing MIL/SES light. Upon further
investigation, there may be the following DTC(s): P0300, P0301, P0302, P0303, P0304, P0305 or
This condition can be aggravated if the vehicle is parked nose down on an incline during this type
of weather. The customer may also comment on repeat occurrences of this condition because water
may be passing over the Air Intake Plenum (AIP).
The dripping rain water onto the engine cam cover will collect at the coil(s) and may seep past the
coil(s) into the spark plug(s) well of the cylinder head and may affect the operation of the spark
plug(s) and coil(s), causing the misfire(s).
IMPORTANT: Before replacing the AIP seal, refer to Misfire diagnostics in SI to
determine if water intrusion was the cause of the misfire.
DIAGNOSTIC AIDS FOR MISFIRE
Refer to SI for Base Engine Misfire without Internal Engine Noises.
If no trouble found (NTF) using SI document on Base Engine Misfire without Internal
Engine Noises, then refer to SI for Misfire DTC(s).
If Misfire diagnostic leads to the removal of the coil(s) and spark plug(s), refer to the
following SI Documents:
? Air Cleaner Outlet Resonator Replacement
? Ignition Coil Replacement Removal Procedure
? Spark Plug Replacement
4.2L LL8 Engine - Flashing and/or MIL/SES Light, Rough Idle, Misfire(s) DTC(s) P0300... Page 1 of 6
Monday, February 21st, 2011 AT 8:15 PM