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Misfire mystery!
So my 99 ranger 3.0 started running real rough. I had the codes pulled and it showed misfire in cylinder 3 and running lean on both banks. I replaced the coil pack , put new plugs and wires and even put a new injector in number three. It runs better but the shanking is still there slightly and it’s still showing misfire on #3. What am I missing here? |
After engine is warmed up and idling
Unplug the IAC Valve's 2 wire connector Idle should drop down to 500 or so, or engine may stall, either is good it means no vacuum leaks. If idle stays high then there is a leak and it is causing Lean codes, and misfire most likely. What did the old #3 spark plug tip look like? Being at the back of the engine the oil in valve cover will pool up there and bad valve guide seal can allow oil to be sucked in to that cylinder, spark plug tip would show that, black residue. Engine never actually "runs Lean", the 2 codes means computer is adding more fuel, to both banks, to get O2 sensors into correct voltage range. Computers main job is to calculate the amount of gasoline to add to the intake based on the air flow into the intake. Computer knows its controlling a 3 Liter engine, it knows the RPM and it knows the throttle position, so it "knows" how much air is coming in, the MAF sensor confirms that and off-sets it based on air Weight, which changes with elevation above sea level and temperature. Computer is "told", at the factory, that fuel pressure at the fuel injectors is 50-60psi, it has no sensor to confirm that. Computer then calculates how long to open each fuel injector to get the 14.7:1 air:fuel ratio needed for gasoline. If O2 sensor on that bank shows too much oxygen, Lean, then computer opens injectors a little longer, +1 fuel trim, if O2 still shows Lean then +2 and +3 and +4, ect........until O2 shows correct level of oxygen in exhaust. This all happens in seconds, so engine is never actually running Lean If fuel trim gets about +20 for any length of time computer will set Lean code for that bank and turn on CEL to notify driver that its calculations are off. Lean condition on BOTH banks can be a upper intake vacuum leaks, MAF sensor issue, or low fuel pressure Because these effect BOTH Banks. If just one bank is effected then could be O2 sensor or lower intake vacuum leak on that side I would see if you can test your fuel pressure, 1998 and up used 50-60psi, 1997 and earlier us 30-40psi #3 is at the engine of the fuel rail so could be most effected by lower fuel pressure |
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