teke184

my dealer told me that my a/f ratios are probably all out of whack cause i don't have the air pump hooked up.

i'm pretty sure he's with me....since hundreds of guys run with no AIR.

but it made me think...

how does pumping air into the manifolds not throw off the o2 sensors?

probably a simple answer....just wondering...thanks

96SFLZ

I'd guess it's because the AIR pump only pumps air when the engine is cold (it's only function is to heat the cats quicker)...so it's in open loop and isn't paying any attention to the O2 sensors.

Cstick

When you go to check gases at the tail pipe you want the air pump disabled. This is one of the first steps my group had to do at school before we used our 4 gas analyzer. The air pump on will cause lean results with the machine.

96SFLZ is right- air pump is on during open loop to warm up the cats so they can start working, while in the mean time the O2sare not looked at by the computer until closed loop.

teke184

duh...

that makes sense....

yet another example of the dealership shoveling

then again the said the computer was "modified" because i have a wire spliced in for the shift light

Team ZR1

O2s do function in open loop, in fact rear O2s are measured on startup to determine in part what the injector pulse width will be for that drive cycle
If AIR is not functioning on cold start then the O2s are reading richer then AFR really is and PCM is adjusting pulse width so yes no AIR can effect the AFR esp if the PCM has not been recalibrated (tuned) for the mods.

Slim

The O2 sensors do not work for about the first 30 seconds after a cold start. It takes this long for them to heat up enought to operate.

In open loop the O2 sensors have nothing to do with injector pulse width calculation. The injector pulse width is calculated base on the flow rate of the injectors at the current difference between fuel rail pressure and intake manifold pressure and the fuel mass needed.

The fuel mass is calculated by dividing the mass of air that enters the cylinder by the desired air-fule ratio. The mass of air that enters the cylinder is estimated based on MAF and MAP and a lot of calculations.

The same calculations are used in closed loop to cacluate injector times, but the fuel mass is corrected based on O2 sensor feedback.

The purpose of the air pump is to heat the catalyst up to operating temperature as quickly as possible. This is done by running the engine rich and retarding the spark and then pumping fresh air (oxygen) into the exhaust manifolds. The combination of the excess fuel and the fresh air provides a burnable mixture in the exhaust manifold. The retarded spark timing creates enough heat to ignite the mixture. The basically creates a torch in the exhaust manifold that heats up the catalyst quickly.

Team ZR1

The O2 tests such as heater I/M tests are done with water temps less then 122 degrees and as per GM's own content states :

Catalyst Monitor Heated Oxygen Sensors

In order to control emissions of hydrocarbons (HC), carbon monoxide (CO), and oxides of nitrogen (NOx), the system uses a 3-way catalytic (TWC) converter. The catalyst promotes a chemical reaction which oxidizes the HC and CO present in the exhaust gas, converting the elements into harmless water vapor and carbon dioxide. The catalyst also reduces NOx, converting the NOx into nitrogen. Catalyst monitor HO2S (Post Catalyst HO2S) are always located downstream of the catalytic converter.

The PCM has the ability to monitor this process using the post catalyst heated oxygen sensors. The pre-sensors produce an output signal which indicates the amount of oxygen present in the exhaust gas entering the three-way catalytic converter. The post sensor produces an output signal which indicates the oxygen storage capacity of the catalyst. This in turn indicates the catalyst's ability to convert exhaust gases efficiently. If the catalyst is operating efficiently, the pre HO2S signal will be far more active than that produced by the post HO2S.

In addition to catalyst monitoring, the post heated oxygen sensor has a limited role in controlling fuel delivery.

If the post HO2S signal indicates a high or low oxygen content for an extended period of time, the PCM adjusts the fuel delivery slightly in order to compensate.

Catalyst Monitor Diagnostic Operation
The catalyst monitor diagnostic measures oxygen storage capacity of the catalyst converter. In order to do this, the heated sensors are installed before and after the 3-way catalyst (TWC). Voltage variations between the sensors allow the PCM to determine the catalyst emission performance.

As a catalyst becomes less effective in promoting chemical reactions, the catalyst's capacity to store and release oxygen generally degrades. The catalyst monitor diagnostic is based on a correlation between conversion efficiency and oxygen storage capacity.
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GM also states :

In order to comply with OBD II emission diagnostic requirements, an additional oxygen sensor is placed in the exhaust system behind the catalytic converter (post converter sensor). Post converter oxygen sensors are used on all General Motors vehicles that comply with OBD II. The job of the post converter oxygen sensor is to measure the amount of oxygen in the exhaust system after the catalytic converter.

This information helps determine catalytic converter efficiency and short term/long term fuel trim data values.

The only differences in the post converter sensor and the pre-converter sensor is the length of the wires, the wiring connector and protective wiring sheath. Depending on the design, there may be up to four heated oxygen sensors in the exhaust system, two before the converters and two after the converters
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Without AIR on cold start the heater tests can fail the 2 minute I/M test.