1997 Buick Riviera 3.8L Supercharged - P0301 Code, Misfire at Stop, 99,000 Miles
My 1997 Buick Riviera with a 3.8L supercharged engine is exhibiting a persistent P0301 misfire code when idling in drive, both cold and warm. I've already replaced the #1 spark plug, ignition coil pack, all vacuum hoses, intake manifold gaskets, and the supercharger seals. The EGR valve and associated tubing were cleaned and rebuilt; the fuel injector was disassembled, cleaned, and tested. I also replaced the camshaft position sensor and verified ECM functionality using a scan tool—O2 sensor readings appear within normal voltage ranges. A compression test showed 155 psi for cylinder #1 versus 170 psi in the other cylinders. A bleed-down test revealed approximately 5% pressure loss across all cylinders. I previously noticed a slight lifter noise from cylinder #1, which disappeared after flushing with Ames oil flush and changing both oil and filter. After two flushes and another oil change over the past 3000 miles, the noise ceased. The rocker arm appeared tight during inspection but no visible damage was found. For about a month, the P03-1 code cleared itself, only to return recently. I am now considering replacing the hydraulic lifters as a next step—though I'm open to other diagnostics or solutions before proceeding.
Update: I removed the valve cover and inspected the rocker arms, push rods, and internal components. Everything appears intact with no visible damage, bent valves, or cylinder wall scoring using a bore scope. I also performed a pressure test by connecting an air line to the #1 spark plug hole at 100 psi—there was approximately a 5 psi drop in pressure. Despite these findings, the P0301 code persists and I still experience rough idle when stopped in drive. After evaluating all components, I have decided against replacing the hydraulic lifters at this time.