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1999 Chevrolet Silverado 4.8L P0304 Misfire Diagnosis and Fix: Step-by-Step Guide

Model: 99 Silverado Fault Code: P0304 Posted: 2011-03-02 21:00

I own a 1999 Chevrolet Silverado 4x4 with a 4.8L V6 engine, currently at 181,000 miles. The truck has recently triggered a P0304 diagnostic code, indicating a misfire in cylinder 4. Since I purchased the vehicle in August 2010, I've noticed a recurring issue: when it rains heavily, the engine runs roughly. Initially, I suspected faulty spark plug wires—common on older trucks—and replaced them with new ones along with the spark plugs. After the replacement, the P0300 (multiple cylinder misfire) code cleared, but a persistent P0304 code appeared. The truck performs poorly when starting cold; it idles roughly and runs unevenly at low speeds. However, after revving the engine aggressively or driving through 2nd and 3rd gear for several minutes, the symptoms improve significantly. I've tested cylinder 4's spark by disconnecting its plug wire—spark is present, ruling out a faulty ignition system. Given that the misfire does not resolve under normal conditions but improves with aggressive engine operation, I suspect a failing fuel injector. A lean condition (indicated by a P0157 O2 sensor low voltage code later) supports this theory. I am now planning to test and replace cylinder 4's fuel injector. Has anyone else experienced a similar issue on the 1999 Silverado with the 4.8L engine? What steps have you taken to diagnose or fix P0304? Thanks, Dave

Related fault codes

P0304

Comments (8)

Anonymous 2011-03-03 06:26

Try swapping the coil pack from cylinder 4 to another cylinder (e.g., cylinder 2) and see if the misfire follows. If it does, the issue is likely with the coil pack. Even if a coil produces spark in open air, it may fail under compression—this is common on older GM engines.

Anonymous 2011-03-03 07:01

The 1999 Silverado 4.8L does not use a distributor; it relies on a coil-on-plug system. This helps clarify that ignition issues are likely related to the coil packs or injectors, not distributor-related components.

Anonymous 2011-03-03 18:13

A failing fuel injector is a strong possibility—especially since the misfire improves after aggressive engine operation and a lean condition (P0157) has now appeared. A defective injector can cause inconsistent fuel delivery, leading to cylinder-specific misfires and O2 sensor feedback errors.

Anonymous 2011-03-03 23:15

If the issue were due to a cracked spark plug or improper gap, it would likely manifest consistently across starts and not improve after aggressive driving. Since the symptoms resolve under load, this points more strongly toward an injector failure than ignition-related issues.

Anonymous 2011-03-04 05:34

Update: The misfire no longer improves with engine revving, and a new P0157 code (O2 sensor circuit low voltage—Bank 2 Sensor 2) has appeared. This lean signal strongly suggests an injector failure in cylinder 4 or another cylinder. I plan to test the injectors tonight while out of town for work. Thanks for all the input!

Anonymous 2011-03-04 06:23

A single-cylinder misfire is unlikely to directly cause a P0157 code—this typically indicates a system-wide lean condition. However, a failing injector could generate both a cylinder-specific misfire and a lean signal. I recommend starting with a backpressure test and replacing the O2 sensor as a first step, since faulty sensors can complicate misfire diagnosis. A new O2 sensor may resolve the issue or at least help isolate the root cause.

Anonymous 2011-03-05 08:16

FIXED! After swapping injectors between cylinders 2 and 4, the P0304 code followed cylinder 4—confirming a failing injector. I purchased a new injector for cylinder 4 and installed it. The truck now runs smoothly with no misfires or O2 sensor errors. Thanks to everyone for the helpful suggestions!

Anonymous 2011-03-05 16:15

Great job on resolving the issue! This is a common but often overlooked problem on older GM trucks—especially with high-mileage 4.8L engines. A proper injector replacement can make all the difference in performance and diagnostic clarity.