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1998 Chevy S-10 P0300 Code: Rough Idle, Rich/Lean Running Issues After Rebuild at 133k Miles

Model: 98 Chevy S-10 Fault Code: P0300 Posted: 2007-05-14 20:45

I own a 1998 Chevrolet S-10 with a 4.3L V6 engine. The vehicle has been professionally rebuilt at 118,000 miles with a 30-over bore and has run smoothly until now—at 133,000 miles—when it began exhibiting a rough idle and consistently triggers the P0300 fault code on my OBD2 scanner. I've already replaced spark plug wires, spark plugs, distributor cap, fuel injectors (originally from a Tahoe), and the catalytic converter. I performed a visual leak check on the upper intake manifold using B-12 spray while the engine was running—no visible leaks were detected. The issue persists. Diagnostic readings show the engine is running rich on one bank and lean on the other, which aligns with a misfire pattern. Here are the key parameters from my scanner: - Absolute TPS %: 4.7 - Engine RPM: 1259 - Calculated Load %: 7.8 - MAF (Mass Air Flow): 12.41 g/sec - MAP Pressure: 36 kPa - Coolant Temperature: 118°F - LT FTRM1 (%): 0.0 - ST FTRM1 (%): 46.0 - LT FTRM2 (%): 0.0 - ST FTRM2 (%): 4.6 - Vehicle Speed: 0 mph - Fuel System 1 & 2: Open I'm seeking advice on potential causes for this persistent P0300 code, especially given the engine is already rebuilt and has been running well previously. Any insights or troubleshooting steps would be greatly appreciated.

Related fault codes

P0300

Comments (5)

Anonymous 2007-05-15 06:44

Try disconnecting the oxygen sensors on the bank that's running rich to see if the P0300 code clears. Also, check for fuel pressure drop when the engine is off—this could indicate a leaking injector or faulty fuel rail. Ensure all fuel injectors are connected to their correct engine lines.

Anonymous 2007-05-15 07:28

I noticed your fuel system is in open-loop mode. This is typically normal during cold starts, but at 118°F coolant temperature, the engine should be in closed-loop operation. Closed loop is essential for accurate fuel trim readings—especially STFT (Short-Term Fuel Trims). Without it, the ECU cannot properly adjust fuel delivery. There are several reasons why a vehicle might remain in open loop, but the most likely cause is faulty or malfunctioning oxygen sensors. You didn't include O2 sensor data—I recommend checking their voltage behavior. If the Bank 1 front O2 sensor is intermittently stuck low (0.1–0.4 volts), it may be causing excessive fuel enrichment by driving up pulse width. Additionally, a STFT reading of 46% appears highly unlikely—most modern systems cap STFT at around 28%. This suggests a possible data entry error. If the value is truly 46%, it indicates an engine running very lean (not rich), and the ECU would be adding fuel repeatedly until reaching its maximum limit, which could trigger P0300.

Anonymous 2007-05-15 14:17

One key question: Which bank is Bank 1 and which is Bank 2? This helps determine whether the misfire is on the side with cylinder #1 or the opposite side. I’ve double-checked the data—46% is listed correctly, so I’ll verify the O2 sensor readings to confirm accuracy.

Anonymous 2007-05-15 15:44

Bank 1 refers to the side of the engine where cylinder #1 is located. Follow the spark plug wire from the distributor or ignition coil to identify which bank that corresponds to.

Anonymous 2007-05-16 07:19

Oxygen sensor readings and fuel trim values are closely linked. Monitor your O2 sensors in real time—especially the front O2 sensor on Bank 1. If it's intermittently stuck at low voltage (0.1–0.4 V), it may be faulty and causing the ECU to overcompensate with fuel, leading to a lean condition and misfire. Confirm that the sensor is switching between high and low voltages; if it fails to switch or suddenly sticks, it’s likely defective.