← Back to list

1998 Chevy Silverado 350 V6 Misfire Code P0305 and P1351 - Diagnosis and Fix

Model: 1995 Chevy Silverado Fault Code: P0305 Posted: 2016-05-17 09:43

I recently encountered two persistent engine codes on my 1998 Chevrolet Silverado 1500 4x4 with a 350 V6 engine: P0305 (cylinder 5 misfire) and P1351 (ignition system fault). I've already replaced the ignition module, spark plugs, coil packs, distributor cap, rotor, IAC valve, and fuel injectors — yet the P0305 code continues to reappear. The issue seems to be specifically tied to cylinder 5, as I observed weak orange sparks at that plug (especially during idle), with a dark black, oily appearance indicating possible internal combustion issues. Additionally, I noticed white smoke coming from the exhaust during deceleration and idle — which appears correlated to cylinder 5. After inspecting the exhaust manifold, I found one bolt on the left side of the engine (second from driver's side) was stripped due to previous ownership. As a temporary fix, I reinstalled it with silicone sealant and allowed it to set for 24 hours. The truck also idles very low (around 400–500 RPM in gear), which makes driving uncomfortable but doesn't result in stalling. I've been advised to perform a compression test and check for vacuum leaks, especially around cylinder 5. A MAF sensor code (P102) briefly appeared but did not persist. I’ve borrowed a scan tool to monitor ECT, MAP, and other parameters, though its full capabilities are still being evaluated. I plan to conduct a compression test tomorrow and will update with results. After replacing the battery and alternator — both of which were failing due to a faulty voltage regulator in the alternator — the P0305 code disappeared entirely. The spark at cylinder 5 improved significantly, and no smoke or misfires remain. This suggests that the root cause was not mechanical but electrical: insufficient voltage leading to weak ignition in cylinder 5. This experience highlights how a failing alternator can mimic severe engine misfire symptoms, even with all components replaced. For other owners experiencing P0305 on older GM trucks, I recommend checking battery health and alternator output before assuming mechanical failure.

Related fault codes

P0305P1351

Comments (14)

Anonymous 2016-05-17 09:48

The user has replaced ignition module, spark plugs (AC Delco), coil packs, distributor cap, rotor, IAC valve, and PCM-related components. All parts were replaced with no resolution of P0305 or P1351.

Anonymous 2016-05-17 10:19

This is a 1998 Chevrolet Silverado 1500 4x4 with a 350 V6 engine using the 'Spider' fuel injection system. P0305 indicates a misfire in cylinder #5. Diagnosis steps: 1) Spark check at cylinder #5 — is there spark? How does the plug appear (wet, dry, black, tan)? 2) Injector — the spider system is located inside the intake manifold. Try adding fuel additive like Seafoam or Techron. Perform a fuel pressure test under KOEO conditions; observe if pressure drops more than X psi in 5 minutes. 3) Mechanical check — conduct a compression test and inspect for vacuum leaks, especially at cylinder #5. Note: Was P1345 present before replacing the distributor cap? Avoid adjusting ignition timing by moving the distributor. Search 'GM P1345' for tutorials. Ensure AC Delco plugs are used.

Anonymous 2016-05-17 12:38

Last night I installed a new ignition module and haven’t tested the spark yet. The spark at cylinder #5 was weak and orange, with only cylinder #5 showing misfire symptoms. Plug condition: dark black and oily — signs of oil contamination. I have a brand-new spider injection system in place; no compression test has been done yet. I did not see P1351 before — this is the first time it appeared after replacing the ignition module and coil.

Anonymous 2016-05-17 12:39

During deceleration, white smoke exits the tailpipe. This occurs intermittently and appears linked to cylinder #5. I also observe backfiring and a wet plug sensation at idle — consistent with misfire behavior.

Anonymous 2016-05-17 12:41

The exhaust manifold bolt on the left side of the engine (second from driver's side) was stripped by a previous owner. As a temporary fix, I reinstalled it with silicone sealant and let it sit for 24 hours. This may be contributing to white smoke during deceleration — though it only occurs intermittently and is tied to cylinder #5.

Anonymous 2016-05-17 12:42

Confirmed: this truck is a 1998 Chevrolet Silverado 1500 4x4, 350 V6 with Spider injection system.

Anonymous 2016-05-17 13:10

The truck idles very low (400–500 RPM) when in gear and tends to want to die — but never actually stalls. This may be related to ignition or fuel delivery issues, especially under load.

Anonymous 2016-05-17 14:15

A compression test is strongly recommended. Please confirm: Is the vehicle stock? Are there any other codes present? Does it lose coolant? Any modifications? Do you have a scan tool? Can it read ECT, MAP, and other sensor values?

Anonymous 2016-05-17 15:16

P0305 appears twice out of three total codes. The other code is P1351 — no modifications to the engine. The truck has a 6-inch lift with extra cab. No antifreeze loss. I’ll perform a compression test soon. A brief P102 (MAF sensor) appeared but didn’t persist. I was considering replacing the MAF sensor at over $100, but I’m holding off due to ongoing misfires. P0305 is particularly frustrating — it can stem from many causes. Thanks for your help so far. After clearing these codes, I need to pass a smog check and get my tags.

Anonymous 2016-05-17 15:21

I borrowed a scan tool from a friend. I’m still evaluating its capabilities — will update on whether it can read ECT, MAP, or other sensor data.

Anonymous 2016-05-17 15:38

P0102 is a Mass Air Flow (MAF) sensor issue with low signal frequency. Do NOT use an oiled air filter (e.g., K&N) with MAF-equipped engines. Check for vacuum leaks after the MAF sensor. Note: A diagnostic code does not always mean a part is faulty — it only indicates a system anomaly.

Anonymous 2016-05-17 16:30

I will perform a compression test tomorrow and update with results. Thank you for your guidance.

Anonymous 2016-05-18 10:13

Today I picked up the P0305 code again — it appeared twice. I’ll share my compression test results soon.

Anonymous 2016-05-21 14:17

The truck is now fully resolved. The root cause was a failing battery and alternator with a defective voltage regulator in the alternator. This led to insufficient voltage, resulting in weak spark at cylinder #5 — barely enough to ignite, but sufficient for all other cylinders. I observed only an orange spark (not hot) at cylinder 5 during testing. After replacing both the battery and alternator, all codes disappeared. No smoke or misfires remain. The truck runs smoothly now. This case serves as a reminder that electrical issues — especially in older GM trucks — can mimic severe mechanical faults. I hope this helps others facing similar P0305 problems.