I am getting P0302 Cylinder 2 misfire detected and P0303 Cylinder 3 misfire detected. I was also getting P0704 System Too Lean (Bank 2) but that cleared up and it runs smoother after replacing the upstream O2 sensor on that side. Not detecting any vacuum leaks. Any ideas what might cause two adjacent cylinders to misfire?

 

Welcome.

 

Wear and tear from time and mileage. Investigate the ignition system. Coil pack, spark plug wires, and spark plugs.

 

Ask here before replacing good parts. It saves you money.

You replaced a good oxygen sensor for nothing.

What are you talking about here? 4.2 v6 engine in what?

What you likely are heading for on a 1997-2004 4.2 is intake manifold gasket and isolator bolt replacement. That engine would be 20 years old at a minimum and overdue. My first go around started with a P0302 and P0174. You also might need a tune up.

 

Thanks to all who replied! A smoke test revealed leakage at the isolator bolts / middle intake plenum gasket area. I've pulled the upper and middle intake plenums and will replace the bolts/gaskets. And also the injectors, EGR, and PCV valves since I'm there already. Not seeing any sign of leakage at the lower intake manifold gaskets from the smoke test. Would it also be prudent to replace those gaskets anyway (are they typical failure points)? The IMRC linkage appears to be in good condition.