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1997 Chevrolet K1500 P0304 Code: Diagnosis and Possible Causes After Multiple Repairs

Model: 1997 k1500 Fault Code: P0304 Posted: 2021-12-27 20:10

Hello, I own a 1997 Chevrolet K1500 with a persistent P0304 diagnostic trouble code. Despite replacing the fuel injectors, cleaning valves, spark plugs, distributor cap, rotor, and ignition coil, the code continues to appear. Originally, it was a steady code that returned within seconds after clearing—now it only shows as a pending code. When I removed the spark plugs, all appeared fouled except cylinder 4, whose plug had no electrode remaining—a sign of severe backfiring or misfire. My main questions are: Could a backfired combustion event travel far enough to affect the engine control unit (ECU)? And is it possible that the engine runs smoothly despite this, suggesting a non-critical issue? I’ve performed a compression test on warm cylinders: - Cylinder 2: 175 psi - Cylinder 4: 100 psi - Cylinder 6: 150 psi - Cylinder 8: 185 psi Cylinder 4 shows significantly low compression. I’ve also observed a white, crusty buildup on one valve in cylinder 4, which appears to be partially open (about 1/8 inch). The spark plug in that cylinder has a rainbow color but is dry. I recently replaced the distributor, hoping it would resolve misfires, but now the code is fluctuating between P0300, P0301, and P0304 depending on whether I clear the ECU. Additionally, the P0122 code shows a live data value of 0, with the fuel system remaining closed—this may indicate an issue with sensor readings or fuel management. I’m now considering a cylinder leak-down test to confirm if the low compression is due to worn rings, leaking valves, or other mechanical issues. I also wonder whether a head gasket failure could be contributing to this problem. Any insights on what might be causing this persistent P0304 code would be greatly appreciated.

Related fault codes

P0304

Comments (9)

Anonymous 2021-12-28 11:19

Performed compression test on cylinders: 2 at 175 psi, 4 at 100 psi, 6 at 150 psi, and 8 at 185 psi. Cylinders 2 and 8 are within normal range, but cylinder 4 shows significantly low compression.

Anonymous 2021-12-28 17:28

Low compression in cylinder 4 is concerning—should be around 180 psi. Adding a few squirts of oil into each cylinder can help identify if the issue is due to worn rings or leaking valves. A leak-down test would also confirm whether there's internal leakage, which could point to valve issues, worn cam lobes, or weak/failed valve springs.

Anonymous 2021-12-30 15:35

I added oil to cylinder 4 and retested—compression rose from 100 psi to 130 psi. This suggests potential ring wear or internal leakage. The valve in cylinder 4 appears white and crusty, with a visible gap of about 1/8 inch, indicating it may not be seating properly. I’ve also replaced the distributor, but now the ECU is throwing P0300, P0301, and P0304 intermittently. The spark plug shows rainbow coloration (indicative of partial combustion) but is dry—this could point to poor fuel delivery or misfiring. I plan to perform a full cylinder leak-down test soon and will check the left-side cylinders tomorrow.

Anonymous 2021-12-30 15:37

I only saw P0304 once today after clearing the ECU multiple times while attempting to set the distributor. However, after using sea-foaming intake cleaner, I noticed a temporary appearance of P0301. This led me to suspect ignition-related issues—though I’ve already replaced all ignition components except the ICM (Ignition Control Module), which is located near the coil and has not yet been replaced.

Anonymous 2021-12-30 19:14

You reported 100 psi in cylinder 4—not cylinder 1. That’s a critical point: low compression in cylinder 4 suggests mechanical failure, especially given that it's also showing valve issues. You should perform a full leak-down test to confirm whether the problem is due to worn valves or internal engine components. Also, check for low compression on cylinders 1 and 6—your current data shows only one side tested so far.

Anonymous 2021-12-30 21:30

Apologies for the confusion—I meant cylinder 4 has 100 psi, not cylinder 1. I will complete a compression test on the left-side cylinders tomorrow and schedule a leak-down test in the coming days. The valve issue was also observed in cylinder 4: it appears white and crusty with an open gap of about 1/8 inch. This could indicate poor seating or internal contamination—worth investigating further.

Anonymous 2021-12-31 08:57

The valve not seating properly is likely the root cause. How many miles has your truck accumulated? If it's over 80,000 miles and the engine was previously rebuilt, it’s worth considering a full head inspection or valve job. However, at that mileage, worn valves are common—especially if the engine wasn’t maintained properly post-rebuild.

Anonymous 2021-12-31 10:46

The truck was originally purchased from someone who claimed the engine had been completely rebuilt at around 80,000 miles. I’m not sure whether it was a full rebuild or just a partial overhaul—this could affect long-term reliability.

Anonymous 2021-12-31 17:28

Over 80,000 miles is significant. If the engine was fully rebuilt at that point, valves should still be serviceable—but wear over time can degrade performance. A leak-down test will provide definitive insight into whether the issue stems from valve failure or other internal components. Until then, it’s best to avoid replacing parts without confirming mechanical root causes.