If you have an O2 sensor, ohm meter, and a propane tourch, you can hook the ohm meter up to the sensor and apply heat and watch the resistance change. It's a good way to test an O2 sensor.

This all make sense to me now that I think about it. More oxygen means less fuel, which equates to a lean A/F condition since the PCM is calculating the fuel. I guess I didn't get a misfire code since the coil packs were operating as spec'd.

 

Here'a a pretty cool link: http://print.google.com/print/doc?articleid=lNSfppkI1RH.

Here's how the PCM detects misfire:
The catalytic converter is one of the most expensive emissions-reducing components on your car, and misfires are its worst enemy. A misfire is a result of incomplete combustion, and one cause may be a defective spark plug. Misfires send unburned hydrocarbon gases into the exhaust and into the atmosphere. But the unburned fuel also travels through the catalytic converter and may permanently damage it. For this reason, automotive engineers spend a lot of time designing ways to detect misfires. The most common approach takes advantage of the fact that each cylinder contributes to the rotational velocity of the crankshaft. If a cylinder misfires, the crankshaft velocity decreases momentarily until the next cylinder in the firing order takes over. The PCM uses a sensor on the crankshaft to compute the rotational speed and another sensor on the camshaft to determine which cylinder misfired.

Hope this helps

 

Good info!

 

Bud, I believe your wrong, sorry not gonna trust you on that one. If you have a misfire, meaning the plug didn't fire the air/fuel then the O2 sensor would pick up a lower a/f ratio therefore sensing it rich. If you have a vacuum leak, meaning unmetered air, then you will have more air than fuel and it would be a lean condition.
If the ecm sensed the misfire then it should accomodate by increasing the pulse width on the injector trying to lean it out. It is possible that it would make it lean but it would sense the richness first and rule out anything after that. The readings on the O2 sensor wouldn't matter to the ecm because the O2's would be in failsafe mode, giving a substitute value of .450V making the O2 sensor useless after the c/e light came on for misfire faults.

 

Bud is correct. Incomplete combustion in the cylinder leaves some oxygen unreacted with the fuel. If all of the oxygen and fuel reacted in a stoich mixture, there will only be a tiny amount of oxygen left in the gases that exit thru the exhaust valve. Since the O2 sensors detect oxygen only, incomplete (or none) combustion will show up as more oxygen than expected in the exhaust stream. The PCM thinks this is lean because it expects a complete combustion process and fully reacted gases reaching the O2 sensor.

mudz5.3 - what makes you think that increasing injector pulse width will result in a leaner condition???

 

I was thinking about the injector pulse and it came out wrong. As far as the stoich mixture goes, explain a rich mixture that an O2 sees. I know the fact of a incomplete burn or no burn will increase the oxygen in the exhaust but the 02's will see the extra fuel and cause it to show rich. But then again, since the a/f ratio is 14.7:1 typically that it will see more air now that I think about it, sorry thinking out loud. Anyway think about a rich mixture that an o2 will see. It sees lack of oxygen because there is more fuel due to a misfire.

 

Please already...I am positive about this...12 years as an ASE Certified Master Technician, Advanced Performance(L1) Certification, and Undercar Specialist. I have been diagnosing misfires and 02 Sensor problems since GM started using 02 sensors with feedback carburetors. A dead miss will produce more oxygen in the exhaust stream because it is not being burned with the fuel. A 4 or 5 gas analyzer will also see high hydrocarbons due to the unburned fuel, but this has absolutely no affect on 02 readings....an 02 sensor is actually a mini battery with zirconia inside. It references oxygen in the exhaust stream to ambient oxygen (that's what the little holes on the outside of the sensor are for, a reference to ambient oxygen). Lean=Low Voltage=High Oxygen - Rich=High Voltage=low oxygen.

Rich readings are generally signs of excessive fuel problems ie. leaking injectors, bad FP Regulator, partial misfire, etc.

You can actually get a voltage reading from an 02 sensor with it unplugged...it produces it's own voltage - try it, just be prepared to clear the 02 code. Run the vehicle, let the 02 sensor heat up, and unplug it. Get a multimeter and put the probes to the 2 sensor pins. (not the heater pins...there should be 4 wires for a heated 02 sensor) You will see voltage, (might have to set the meter to millivolts), I promise. Now remove a spark plug wire and see what happens.

Now newer style Air Fuel Ratio sensors work completely opposite and on a higher voltage scale. (Low voltage=Rich, High=Lean) I'm not really sure if GM uses these kind of sensors though...have many years with Toyota, both as a Master Diagnostic Technician and as a Service Training Specialist in the New York Region. But that's a whole other story...

 

Air fuel ratio=14.7:1 correct? Ok, 14.7 parts air, 1 part fuel. So what have we learned today? Misfire=lean condition. Don't worry, I'm Ford Certified....ok that doesn't mean a damn thing but it sounds cool huh?

 

In reality you can have a rich or lean misfire, but an 02 sensor will read a dead miss (100% misfire) as lean. 100% misfire will yield high hydrocarbons and high oxygen content out of the tail pipe. A rich condition will yield high CO (Carbon Monoxide) and low 02 out of the tail pipe. N0x may go down, and C02 will also be low. Lean will yield the exact opposite as rich on the sniffer...02 up, C0 down, NOx up, C02 down. (High C02 numbers are desirable, C02 (Carbon Dioxide) is a by-product of complete combustion.

A 100% misfire is not a rich or lean condition, it is a dead miss. The 02 just shows it as lean.

Man, I need a beer.

 

Ok, well like I stated in my last statement about the 14 parts air to 1 part fuel that I can see it being a lean condition. It just didn't make sense till that clicked with the fuel ratio. You would figure that if it doesn't fire then you would have excessive fuel but then again since its more air than fuel you will have more oxygen which is what the o2 reads. My mistake, I am open enough to admit it. I am a bmw tech and I was thinking about our o2's and how we are always seeing lean condition, not because of misfires but because of vacuum leaks. We see rich condition with bad ign coils, probably the partial burn or leaking injectors and the like. But anytime there is a misfire it will put the 02's into failsafe and run a substitue value of .450. Sorry for the confusion.