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P0300 Random Misfire Diagnosis on 1997 Chevrolet Silverado 4X4 with O2 Sensor Code P0157

Model: 1997 Silverado 4X4 Fault Code: P0300 Posted: 2009-04-19 13:15

My 1997 Chevrolet Silverado 4X4 with a 350 engine was running smoothly when I left for the store. Upon returning, the vehicle started with a noticeable misfire and lost power while driving. After arriving home, I connected my OBD2 scanner and found two fault codes: P0300 (random misfire) and P0157 (oxygen sensor bank 2, sensor 2). I recently replaced the intake gasket, distributor (which included a camshaft position sensor), ignition coil, cap, rotor, spark plugs, and wires. Based on prior research, possible causes include a faulty coil or a burned valve. However, I believe that if there were a burnt valve, it would only affect one specific cylinder — not multiple cylinders as indicated by the P0300 code. This makes me lean toward a coil issue. I am aware that an oxygen sensor failure could contribute to misfires, but I suspect that my original intake gasket failure may have damaged the O2 sensor years ago. Given that this problem appeared suddenly with no prior warning signs, I'm concerned about underlying issues such as fuel delivery or ignition system faults. Any insights into whether a random misfire like P0300 could stem from a simple component failure (like an ignition cap) would be greatly appreciated.

Related fault codes

P0300

Comments (6)

Anonymous 2009-04-20 06:32

Check the engine timing — the distributor might be off by one tooth. Proper timing is essential for consistent combustion and can cause random misfires. Good luck, and please update if you find anything.

Anonymous 2009-04-20 06:55

Verify fuel pressure and flow. Low or inconsistent fuel delivery can lead to misfiring, especially in older engines like the 1997 Silverado 350.

Anonymous 2009-04-20 08:53

I've checked timing — it's correct. I haven't tested fuel pressure yet. Recently noticed a significant amount of oil smoke coming from under the vehicle, which is new and concerning. This makes me wonder if there could be an issue with the transmission converter or internal engine seals. The problem appeared suddenly without warning, so something unexpected may have occurred. I'll check fuel pressure soon and update on my findings. Also, I'm considering a possible electrical short — I measured 0.5 milliamps of drain, though fuses are fine. I've replaced the spark plugs with ACDelco parts, which were installed one year prior to this issue. I recently replaced the intake gasket following an AllData service bulletin recommending 'green' gaskets over older 'orange' ones — a known source of air leaks. After installation, the misfire returned but only at speeds between 55–60 mph. Fuel pressure is stable at 60 psi and within specifications. O2 sensor readings appear normal, though I can't graph them for full analysis. The engine starts immediately upon key turn — suggesting it's not a coil issue. My next steps will be to locate the electrical short, re-seat all spark plugs, and test for sparks in darkness (since each cylinder has its own ignition module). Thanks for the input, Sebastian.

Anonymous 2009-04-22 09:12

I replaced the fuel filter with no improvement. There's a slight flutter noise coming from the driver’s side exhaust. I pulled out all spark plugs and inspected them — they were in good condition except cylinder #3, which had a wet plug indicating possible fuel leakage or internal combustion issues. I tested compression on cylinders #1 and #3 and both read between 135–150 psi — within normal range. Due to weather conditions, I couldn't continue testing further. I'm frustrated but determined — this is my only vehicle for daily use. While I suspect a simple fix exists, it's not yet clear what it might be. Previously, after the intake gasket failure, I had an O2 sensor fault (P0171) on bank 2, sensor 2, which cleared after clearing the code. Recently, I received a new P0135 — oxygen sensor bank 1, sensor 2 — suggesting a possible O2 sensor issue. However, since the code changed, I'm less certain. My PCM was sent to Hypertech for diagnostics and reflash — they confirmed it's functioning properly. Once I complete compression testing on all cylinders, I should be able to narrow down the root cause. Thanks for the help.

Anonymous 2009-04-22 15:25

I feel silly now. After checking spark across all cylinders, I found that both cylinder #3 and #7 had very weak sparks. When I removed the distributor cap, there were no visible arcing signs. Then it hit me — the original cap had plug wires for #3 and #7 placed side by side directly next to each other. This likely caused interference and poor spark distribution. I retrieved my old cap from storage and installed it. The problem disappeared immediately. It was actually a faulty ignition cap causing misfires, not a coil or valve issue. This is frustrating because so much effort went into diagnosing complex issues — but the real fix was something simple and right in front of me. Thanks to everyone who contributed suggestions — your input made a big difference.

Anonymous 2009-04-23 11:46

Ok I feel stupid now. I went to check spark on all the cylinders. #3 and #7 were both really weak spark. So, I took the cap off and looked under it and there were no signs of arcing anywhere. Then it hit me, the cap had plug wires 3 and 7 side by side. So I went and found my old cap that for some reason I had kept around, put it on the truck and guess what, problem solved. It was the cap the whole time. Kinda aggrevating when you put all that effot and it turns out to be something simple right in front of your face. Thanx to everyone that put their thoughts in and tried to help me. It was all appreciated.