1997 GMC K1500 5.0L Random Misfire (P0300) - Lean Condition, High IAC, All Components Replaced

The vehicle has 88,000 miles on the odometer. This is a key detail for long-term wear concerns.

Check the IAC valve and its wiring — including connections at the intake manifold. A faulty or stuck IAC can cause high readings and misfires, especially under idle conditions.

Double-check EGR valve function, fuel pressure, oxygen sensors, and catalytic converter health. These components are common contributors to lean misfires in older V8 engines.

The EGR system works fine — it can be cycled with a scanner. Fuel pressure is stable after new pump/regulator installation, and all four O2 sensors read similarly before and after the cat. How could a catalytic converter affect idle performance? It seems unlikely unless severely plugged or malfunctioning.

If the converter is clogged, it may create backpressure that affects engine breathing. Are the post-cat O2 sensor readings matching pre-cat readings? A mismatch could indicate a blockage.

All O2 sensors are reading correctly and show consistent values — no significant difference between before and after the catalytic converter.

A new catalytic converter doesn't guarantee performance. If the engine is burning oil or coolant, it can rapidly clog a converter in weeks. This could be an underlying issue causing lean conditions and misfires.

The catalytic converter has only been used for about 15 minutes total — all components were replaced less than one hour ago, with minimal runtime. The O2 readings before and after the cat are identical using an E-test device. Why would IAC values be so high while exhaust remains lean? This is a major diagnostic puzzle.

The cold air intake uses a dry cotton filter — no oil-soaked K&N element. Could the airflow change from this modification be affecting idle stability or IAC calibration?

We cleared the misfire code and found a loose ground connection on the engine block — tightened it, and installed new spark plugs as a precaution. Initially, the truck ran smoothly, but after just one mile or two, it re-activates P0302 and begins running lean again.

Could the ignition coil or its wiring be faulty? Especially since #2 cylinder is misfiring despite strong spark readings?

We tried a different ignition module — it had a coil mounted on the same bracket, so we also tested that. The #2 injector was leaking and has been replaced. All cylinders fire well, and all injectors pass balance tests. Fuel pressure is stable with no bleed-off.

Confirmed: fuel pressure isn’t bleeding off, intake gasket shows no leaks, spark plugs and wires are new, and the #2 injector is not leaking — all injectors pass balance tests. What else could be causing this persistent lean condition?

All injectors perform equally on balance test — no discrepancies in flow or timing.

What is the next diagnostic step you're considering? Any plans to check for valve carbon buildup or perform a leakdown test?

All cylinders show good compression (~170 psi). Next steps: cylinder leakdown test, then re-install intake with new gasket. If needed, remove heads to inspect for carbon deposits or valve issues.

I work at a local shop and we’ve spent significant time troubleshooting this — the dealer also couldn’t find a solution after two weeks of diagnostics. The truck runs fine when codes are cleared, but reactivates P0302 within minutes and goes lean again. One theory: engine control parameters may be set too close together, triggering SES light falsely — though this is speculative.

Have you pulled the intake manifold or head? Carbon buildup on exhaust valves (especially #2) could cause misfires. A seafoam treatment may help clean deposits and restore performance.