Initial diagnostics showed misfiring. Replaced plugs, wires, rotor, distributor cap and coil. Had to replace injector for cyllinder #8 which fixed the misfiring, but still running really rough. Mechanics are baffled!! Any ideas? It runs roughest between 1500 & 2000 RPMs
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misfires are from mechanical faults ( loose valve seat insert, problem hydraulic tappets 0 injectors -- wiring,injector tips , low fuel pressure ECM to electronic problems , plug, HT lead distributor cap (if no multi coils) rotor button, coils and ignition control module
run the fault codes to get closer to the fault
It could be a fuel problem, the pickup coil in the bottom of the distributor, or the ignition module among other things. Someone would need to figure out if the ignition system is causing the problem or if one of the injectors is failing.
It is hardly ever the spark plug or only the spark plug - generally for some reason there is a weak or lost spark and the reason must be found and rectified by carefully checking and then replacing any components found to be edfective - plug wires, coil, distributor cap, rotor (if fitted), wiring, connections and could conceivably even be the ECU. Perhaps the misfire is due to injector trouble...
Sometimes a fault with persist even after it has been "cured" until the fault code is erased with diagnostic equipment.
I WOULD REPLACE DISTRIBUTOR CAP AND ROTOR.MARK LOCATION OF ROTOR OR MAKE NOTE SO YOU CAN INSTALL NEW ROTOR IN CORRECT FIRING POSITION. IF THIS CAR HAS A LOT OF MILES I WOULD REPLACE DISTRIBUTOR CAP, ROTOR.SPARK PLUGS AND WIRES.REPLACE IGNITION COIL.CLEAN FUEL INJECTORS, FILL GAS TANK UP TO 3/4 TO FULL TANK OF GAS POUR CAN SEAFOAM IN FUEL TANK IT WILL FLUSH OUT THE FUEL INJECTORS WHILE ENGINE RUNNING OR DRIVING CAR. REPLACE FUEL FILTER. REASON I SAY DO A TUNE UP PROBLEM COULD BE MINOR FIX.COULD NEED TUNE UP.IF TUNE UP DONT HELP THEN YOUR PROBLEM IS DISTRIBUTOR OR THE CAMSHAFT SENSOR INSIDE OF DISTRIBUTOR.THAT WOULD SET OFF CHECK ENGINE LIGHT.BUT I WOULD SAY TUNE UP COULD FIX PROBLEM.
P-0304 indicates number 4 cylinder misfire. Are you using Toyota parts dept spark plugs? More often than not, non-toyota plugs make the v-6s run poor, right out of the box....
Try this:With the engine running, pull the connector wire off of the #4 injector. Note to see if there is any change with the idle quality. No change? You may have a bad injector.