1996 GMC Sonoma P0175 & P0300 Misfire Fix: Fuel Pressure Regulator Replacement Resolves Issue
I recently resolved a persistent engine misfire issue on my 1996 GMC Sonoma with 168,000 miles. The truck has otherwise been reliable, with only routine maintenance needs like brakes and fuel pump replacements. The problem began when the engine exhibited slight idle sputtering—no check engine light (CEL) initially. I assumed it was carbon buildup, so I added Seafoam injector cleaner as recommended by NAPA. After driving several miles, the CEL started flashing intermittently, cycling on and off during starts and while driving. Over time, the codes evolved: P0300 (random misfire), P1153 (system too rich), P0175 (rich condition in bank 2), and P1441. After clearing the codes, the issue persisted with a longer stay of the CEL on, followed by a flash pattern before stabilizing. I had previously purchased a new O2 sensor (Bank 2, Sensor 1) but did not install it. Research suggests that Seafoam is unlikely to cause permanent damage, though some users report it clearing P0300—my case was different. After further diagnostics, the most consistent code returned was P0300 with a specific cylinder 6 misfire. I had already replaced the fuel pressure regulator (located under the intake manifold), and since then, the truck has run smoothly without any CEL activation or surging. Key observations: - The P0175 code likely resulted from a fuel delivery issue rather than being the root cause. - A noticeable symptom was delayed starting when warm—engine would fire easily when cold but surge until cruising. This is consistent with gas pooling in the intake due to poor fuel pressure regulation. I now believe this was a fuel system failure, specifically the fuel pressure regulator. The misfire (cylinder 6) and rich condition were secondary symptoms of an underlying fuel delivery problem. Recommendations: - For 1996 GMC Vortec engines, the fuel injector assembly (sometimes called 'injector spider') is prone to failure—consider replacing with a revised model if possible. - A tune-up has not been performed since purchase (~148k miles ago). I replaced plugs with NGKs based on recommendations; however, plug types (e.g., Platinum vs. P2/P4) are unclear. - Avoid aftermarket O2 sensors—opt for dealer parts when replacing. - If fuel pressure is low or fluctuating, use a fuel pressure gauge to confirm regulator function. This issue highlights the importance of diagnosing root causes before assuming injector or sensor failure. The fuel pressure regulator replacement has fully resolved the problem and eliminated all misfire symptoms. Thank you to Jeff for the clear guidance—this was a critical insight that led to the correct diagnosis and fix.
Hi Jan, Seafoam is unlikely to have caused your issues. The most probable cause is either a leaking fuel pressure regulator or a failing injector on cylinder #6—common in GM Vortec engines. A revised injector assembly (sometimes called the 'injector spider') is less prone to failure and should be considered if replacing. If misfires continue, it will damage the catalytic converter. For O2 sensors, always purchase from the dealer for reliability. You didn’t mention a tune-up—has one been performed? Also, check your distributor cap; poor quality aftermarket caps can cause cross-firing in these engines.