Engine Performance problem
1990 Chevy Silverado V8 Two Wheel Drive Automatic 118000 miles
1st Test 522 ppm HC (limit 220) 5.41 CO% (limit 1.2), CO2% 11.5, O2% 0.1; High idle OK at 43ppm HC, CO% 0.01, CO2% 13.6, O2% 1.4; Always ran good but sometimes hesitant starting. ALDL jumper test does not yield any problem codes. Chgd oil, air filter, sparkplugs, MAP sensor, TPS, Idle air control valve, distrib cap & rotor, and PCV grommets. Burned 15 gal. Fuel w/CRC guaranteed to pass additive. Retest failed at 243/1.99/13.8/0.1, High idle OK at 32/0.02/13.6/2.1. Changed sparkplug wires and ohm checked circuitry from PCM end. Coolant temp sensor seems to be right on, 213 ohms at 195F. Burned 6 of 11 gal treated w/CRC G2P, but didn't follow instructions to get near empty. Added 12 gal fresh fuel and retest failed at 296/2.22/13.6/0.2, Hi idle OK 51/0.06/14.4/1.4; Fixed air leak from old o-ring on oil fill cap. Fixed deteriorated ground lugs & studs and got cruise control working again. Changed ignition module. O2 sensor soon cools below 600F in idle as shown by change in check engine light flash rate using A-B jumper on ALDL connector. So we're dealing with problem in open loop conditions. Runs fine but idle exhaust is still kinda stinky unless I'm getting a whif of minor oil leakage as it cooks behind the engine, after it seeps out of the oil pressure switch which is inconveniently located behind the distributor. Have doubts that it will pass retest but must try before going further. Gas mileage seems to have gone from 17 to 20 MPG. Alternator causes a distinct whine in radio with pitch that follows RPM. Wonder if such electronic noise is adverse to the computer, what with all the wierd things that harmonics can do in other realms. Unless I get lucky and squeeze under the limits, I foresee more of this baffling mystery, drudgery, and expense continuing towards injectors, fuel pressure regulator, internal oil leak, maybe bad PCM. Any opinion?