This could have other causes also, especially if the wires have been replaced. It could be a failed (stuck open) EGR valve. I don't recall ever having an ignition coil failure on one of these vehicles, that would be kind of rare; if it is coil related though it would most likely only cause a misfire during conditions where peak firing voltage was required such as accleeration or under load. If it misfiring at idle, it is more likely to be somethign else such as fuel injector, EGR valve, or erratic crank position sensor signal.
The most likely cause on this engine however, is a fuel injector assembly failure. THese are known problems on these engines. THe TSB I have provided you a copy of below states that the repair is free to California registered vehicles, most likely as a result of some specific action unique to that state; however the problem can and does appear in vehicles in other states as well (the repair is not free however). I would suggest taking this particular problem to your favorite service facility for diagnosis; there are lots of things you can repair in the driveway at home, but isolating this particular symptom to a root component will likely require some test equipment for an accurate diagnosis.
File In Section: 06-Engine Emissions
Bulletin No.: 99066F
Date: March, 2003
99066F - SPECIAL POLICY ADJUSTMENT - SEQUENTIAL CENTRAL PORT
FUEL INJECTION (SCPI) FAILURES IN CALIFORNIA ONLY (YF5
CERTAIN 1996, 1997, 1998, 1999, 2000, 2001, 2002 S/T, M/L, C/K, G,
P, W4/NPR TRUCKS AND 2003 NPR TRUCKS EQUIPPED WITH 4.3L
(RPO L35 - VIN CODE W OR RPO LF6 - VIN CODE X), 5.0L (RPO L30
- VIN CODE M) OR 5.7L (RPO L31 - VIN CODE R) ENGINE AND
CALIFORNIA EMISSION EQUIPPED (RPO YF5)
This bulletin is being revised to add the 2002 and 2003 model years to the SCPI Special Policy on certain S/T, M/L, C/K, G, P and W4/NPR truck models. Please discard Special Policy Bulletin Number 99066E, dated February, 2003.
Some customers of 1996, 1997, 1998, 1999, 2000, 2001, 2002 model year S/T, M/L, C/K, G, P, W4/NPR trucks and 2003 NPR trucks, that are registered in California, equipped with 4.3L (RPO L35 and VIN Code W, or RPO LF6 and VIN Code X), 5.0L (RPO L30 and VIN Code M) or 5.7L (RPO L31 and VIN Code R) engine, and California emissions (RPO YF5), may experience a "Service Engine Soon" light, misfire, rough idle or hard start due to a deposit build-up on the Sequential Central Port Fuel Injector (SCPI) poppet valve(s). The deposit build-up may cause injector poppets to stick closed. Certain fuels have been found to interact with the SCPI system to cause the deposits.
SPECIAL POLICY ADJUSTMENT
This special policy covers the SCPI failure condition described above for a period of ten (10) years or 200,000 miles, whichever occurs first, from the date the vehicle was originally placed in service, regardless of ownership.
The repairs will be made at no charge to the owner. This special policy applies ONLY to repairs requiring SCPI system servicing, injector cleaning and/or MFI assembly replacement of the SCPI system. The customer should not be charged for performing a system check when it is determined that the SCPI system is not the cause of a customer complaint (labor operation T5532 is provided to submit claims for such system checks). Any additional necessary diagnosis and repairs that are not related to the SCPI condition are not covered by this special policy. The customer should be informed that any further service that is not covered by new vehicle warranty will not be covered by this policy.
Involved are certain 1996,1997,1998,1999, 2000, 2001, 2002 S/T, M/L, C/K, G, P, W4/NPR and 2003 NPR model vehicles, registered in California, equipped with 4.3L (RPO L35 - VIN Code W, or RPO LF6 - VIN Code X), 5.0L (RPO L30 - VIN Code M) or 5.7L (RPO L31 - VIN Code R) engine; and California emissions (RPO YF5). This Special Policy covers all vehicles within these model years, with these engine and emissions RPO's.
System Check: Use strategy-based diagnoses listed in the front of the Driveability and Emissionssection of the service manual. If the SCPI system is operating properly, inform the customer that the vehicle does not have the condition listed in the owner letter. If poor driveability conditions persist, inform the customer that any further diagnosis and repairs will be at their expense if the vehicle is outside the parameters of the new vehicle warranty.
SCPI Injector Cleaning Procedure: If diagnosis leads to sticking poppet nozzles, use the service procedure from Service Bulletin 00-06-04-003B to clean the SCPI poppet nozzles. Please note that the service bulletin term for SCPI is Central Sequential Fuel Injection (CSFI), but is referring to the same fuel system assembly. If the poppet nozzles have previously been cleaned and the sticking condition has reoccurred, refer to the correction paragraph below.
SCPI Fuel Tank Fill Pipe Assembly for 1997-99 M/L Van and 1999 - some 2000 C/K Truck: with 4.3L, 5.0L, 5.7L engines and built prior to listed VIN/Production dates on Service Bulletin 00-06-04-018: If diagnosis leads to sticking poppet nozzles on these models, use the service procedure from Service Bulletin 00-06-04-018 to replace the fuel tank fill pipe assembly, if this procedure has not been performed previously. Previous service procedure can be verified by checking GMVIS for Labor Operation L1065 on "M/L" trucks with replacement part number 15050573; or Labor Operation L1065 on "C/K" trucks with replacement part numbers 15747585 or 15747588.
Correction: If, after cleaning the SCPI poppet nozzles, the normal service manual diagnosis still indicates that the SCPI is the cause of the customer complaint, or if the injectors have previously been cleaned and the vehicle has again experienced sticking poppet nozzles, refer to Service Bulletin 00-06-04-003B and replace the SCPI fuel assembly with the MFI fuel assembly. Please note that the service bulletin term for SCPI is Central Sequential Fuel Injection (CSFI), but is referring to the same fuel system assembly.