Quote:
Originally Posted by ike84
So today I got to test my AFR guage. Its actually a solenoid controller from innovate motorsports that integrates a Bosch wide band afr sensor. What I saw was rather interesting (and a bit concerning).
On startup at idle, AFR was running 15.1-15.3! Too lean! Under light loads this would fluctuate back down 14.5-14.9. At higher loads this would go lean again, 14.5-15.3. At WOT it dropped way rich, down to 10.5-11.3.
Interestingly, with foot off gas, it would peg lean (over 22).
Numbers in the drive home were a little less lean but Instill saw 15+ a fair bit of time.
I can explain the WOT richness - switching into open loop, the computer is fueling a 3.4L engine. Even with the biggest air intake in the world that ****************s just not gonna match up on a 2.7
The closed loop operation afr bothers me though. I would assume that with as sophisticated as our computers are (combined with what I perceive to be a general cover-your-ass approach from Porsche in regards to performance aspects of this car) that the computer would shoot for 14.7 or less at all times. I was really quite shocked to see 15+ so much.
With that being said, if the computer is gearing the fueling to hit a target above 14.7, I think that may be just one more piece of the puzzle as to why we see issues like bore scoring, overheating, etc.
I did have my battery disconnected this weekend so maybe it just needs a few days to relearn short term trims again.
Anyway, I'll keep things updated over the next few days to see if things change much.
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Ike
That all seems like pretty normal numbers to me.
A couple of things to keep in mind:
14.7 fuel air ratio is not what the engine needs under all conditions to perform best.
An example is WOT.. it should go rich. Heavy engine load requires a richer condition.
Foot off the gas your getting 22: exactly you don't need fuel in the air to coast. your getting 22 because you don't need fuel under that condition but the engine still pulls in air.
Hell you don't need spark. Some modern day engines cut off fuel and spark all together under off throttle conditions. An AFR sensor will see all air (or nearly so) under off throttle conditions.
Another thing: 14.7 fuel to air is the ideal ratio for Catalytic Converter performance.
Not engine performance under all conditions.
On a modern day computer controlled engine the primary function of the DME/ECU system is "Catalytic Converter performance, engine performance is secondary.
The DME/ECU under closed loop conditions constantly fluctuates the fuel air mixture above and below the 14.7 fuel air ratio. That is why O2 sensor signals constantly fluctuate in a sine wave. The DME/ECU sort of looks for an average under closed loop conditions.