How are these sensors related to the ECU deciding how much fuel to inject? I can understand the IAT being used in open loop mode, but in closed loop mode is it ignored completely? If the O2 sensor keeps the fuel mixture in check, then how do IAT sensor mods make it richer in closed loop?

2002 Cavalier 2200 5spd

If the air is colder, it is more dense. So there is "more air" going in when it is colder, so the engine can put in more fuel in which gives you more power. If the PCM thinks the air is colder, your car will run rich.

People new to tuning assume they can put a resistor on the IAT wires to send a "cooler" signal to the PCM, unfortunately, the computer will figure this out VERY quickly and you will be right back where you started.


"The Blue Bullet"

Yes, I know the ECU figures this out by reading the O2 sensor. My question was under which conditions does the ECU use the IAT sensor to determine the A/F ratio, and when does it look to the O2 sensor?

2002 Cavalier 2200 5spd

Solid Snake wrote:Yes, I know the ECU figures this out by reading the O2 sensor. My question was under which conditions does the ECU use the IAT sensor to determine the A/F ratio, and when does it look to the O2 sensor?

The PCM uses almost every sensor on the car to determine how long to pulse the injectors.

The IAT is used as said above sort of. The computer wants to run as lean as possible for fuel economy, and emissions. so colder air is more dense, so it will pulse the injectors a littel less.

The O2 sensor is ignored in open loop. Unilt the car has hit closed loop, which the car must have been on for at least 60 seconds, must have hit a set speed, usually under 10mph, the coolant must be a certain temp, different for every car, some other conditions must have been met to, varies though.

If the O2 sensor is giving out a signal above 450mV, than it is rich, so it will go lean, if it is below 450mV, it is lean so it will go rich,

The PCM tries its best to keep at 450MV, and at 14.7 A/F ratio, stochometery (sp).






- 2004 Cavalier - 124k, owned since new

Yes, I already know this, but what happens if the two sensors give contradictory data? For example, the IAT may say "It's really cold out, inject more fuel" but the O2 sensor is saying "There's no oxygen in the exhaust, we're running rich". Which one does the ECU listen to?

2002 Cavalier 2200 5spd

It doesn't wokr like that ^.

The Ecm as preset data. If the IAT say it's 35 outside, the ecm will run a bit richer but the 02 sensor is to monitor the IAT.

If it's really 35 outside, the added fuel for the denser air will end up to the same mixture as hotter with less fuel.

So the resistor trick doesn't work because it because richer.

Colder air will take more gas but it won't make the engine go from 14:1 to 12:1.

example:
IAT = 70
02 = 14:1

IAT = 35
02 = 14:1

Yes the 02 adjust the mixture when the other sensor don't work right. But most of it is the IAT that tell the temperature, the Ecm go with the table he got for the temperature of the IAT, then the Ecm adjust the injector and the 02 tell the Ecm if the ratio is right. If the IAT data is wrong(the ratio is way too rich), the Ecm will adjust from what the 02 say.



Gilles
2.3 Ho