In closed-loop mode the ECU reads off of O2 sensor and adjusts the air to fuel ratio by adjusting the amount of fuel injected to *try* and keep the ratio at 14.7:1

In open loop mode (anything BUT cruise and idle) the ECU reads the throttle position as either high or low, even though S5's have a full range TPS. The ECU has two "fuel maps", so to speak, one for the low position, one for the high. The ECU will then adjust the amount of fuel injected based on RPM signal.

So... where exactly does the AFM come into all this?

 

The ECU still uses the AFM at ALL times after startup. during closed loop the ECU uses the O2 sensor for aditional help to determin the correct amount of fuel to be delivered.

In open loop the ECU still reads the AFM which is telling the ECU how much air volume is entering the engine and also uses the intake air temp sensor in the AFM to help determin how much air mass has entered the engine.

The ECU does not use the TPS to determin the correct mixture. It does use it to determin throttle opening for emmisions, idle, accel enrichment and maybe a few other things.

If you want to think of it as fuel maps, you could say there are 3. Startup which does not use the AFM at all and runs off a preprogrammed map in the ECU, closed loop map and open loop map.

The S5 full range TPS is NOT used for fuel at all and is only used for OMP operation. The ECU uses the narrow range TPS just like the S4 narrow range TPS.


All times after startup.

You can test this your self. Unplug the AFM while the engine is idling. It will instantly stall out. Keep the AFM unplugged and start the car, the engine will rev up and shut down after a few seconds.

 

Hmm.. thats interesting, I didnt think it was so integral to operation. Since it is so depended upon, does it matter that on a 0-5V device, my readings are 0-3.8V max due to 17 year old corroded connections?

 

hmmm so the engine will stall if the afm is not connected...My friend was messing with my AFM when were looking for the tps test plug....the car has been doing that since, the engine just dies a few seconds after it starts up should I replace my AFM or is they're a way to fix this?, I already double checked everthing in the intake box .

sorry for the thread jacking yo :S

 

No worries.

Chances are its not your AFM, but rather a disconnected vacuum hose or the like. But if you really feel like it you can check your AFM by tapping into the 4th wire from the left if you have the connector facing you. You can also measure it at the ECU. It is the bottom right pin on the middle connector. Measure the voltage on this line. With the key to the ON position (but not started) the voltage shoiuld be between 4 and 5V. Once the car has started, it will drop to about 1 or 2. If you're not reading any voltage, check your wiring first before you replace your AFM

 

oh ok, I'm replacing all my vac lines today any ways, hope that helps me resolve my problems! thx for the info flipstar

 

Does it matter? Possibly not to much if your engine is running fine. The volts drop the more the flapper is opened. I Forget what I would get at idle, I think 3v or something around there so I think you are fine.