Contrary to what most think an over rich air/fuel mix will have less power, Lean mix has more power but will melt pistons and valves, lol, so there is a fine line.
I would pull a couple of spark plugs, from different cylinders, to see if they are dark brown or blackish, a sign of running rich.
Off-white, Light brown is normal
If spark plugs look normal you could have a small fuel leak.
O2 sensor's life expectancy is 100k, toward the end of life they will cause engine to run richer than needed.
The newer computer controlled systems are much better and easier to work on than the old carb and distributor systems.
The only reason people think differently about that is because you HAD TO work on the old systems ALL THE TIME so HAD TO learn about them.
The newer system is so much more reliable you just never have to learn about how they work, lol.
MAF(mass air flow) sensor measures the air coming into the engine.
Computer uses that data to set the 14:1 air:fuel mix
O2 sensor looks at the Oxygen content in the exhaust, too much oxygen means Lean burn, too little oxygen means Rich burn, computer adjusts fuel injector open time(pulse width) to fine tune the burn.
TPS(throttle position sensor) is the same as the accelerator pump on a carb, when your foot pushes down on the gas pedal there will be a lag time before MAF sensor picks up the extra air flow, to prevent this lag time the computer gets a "heads up" from the TPS to add more fuel immediately.
Accelerator pump added(squirted in) extra fuel when gas pedal was pushed down.
ECT(engine coolant temp) sensor, since there is no carb/jets there can be no Choke Plate to run cold engine richer with higher idle, the ECT sensor tells the computer the coolant temp so computer can run a cold engine richer with higher idle until it warms up.
The ECT is a two wire sensor, there is also a one wire SENDER used for the dash board temp gauge on all fuel injected engines.
IAT(intake air temp) sensor, similar to ECT in that it is a two wire temp sensor but for air temperature, colder air is denser so requires slightly more fuel in the mix than warmer air.
This is why in colder climates MPG will go down in winter months.
All sensors can be tested with a Volt or Ohm meter.
IAC(idle air control) Valve is a control not a sensor, computer uses this to set idle, because there are no jets there can be no "idle screw", the IAC Valve is basically a controlled vacuum leak, it is an air passage by-passing the throttle plate.
Computer opens this valve all the way when starting engine, so RPMs should go up to 1,500+ when engine starts.
Then computer will start closing it to set idle based on ECT sensor data, cold engine should idle at about 1,100rpm, then idle will start to drop as coolant temp rises.
Target warm engine idle for manual trans is 650, automatic trans 750
IAC valve can be removed and cleaned.
And can be used to test for vacuum leaks, warm up engine, unplug wires from IAC Valve, it will close, RPMs should drop to about 500 or engine may even stall, either is good it means no vacuum leaks.
If idle stays high then start looking for the leak.
The Computer runs things and also has a memory on how this one engine runs, so it is adaptive to a point, which makes it more reliable.
When you change or clean parts you need to let the computer know that there were changes and it should re-learn how things work.
You can reset the computer using a Reader/scanner, or you can unhook the battery for 5 minutes or longer, that will cause a reset when engine is started again.
Last edited by RonD; 05-04-2015 at 11:34 AM.