18.8 AFR at WOT
 

Hi guys,

I just got an aam midpipe and axleback exhaust and I love it. The car feels more powerful but my gas mileage has gone to hell. Before the exhaust I could easily get 18 to 23+ mpg now I can barely get 13. I used the torqe app to datalog my afr and it says im at 18.8 afr (the commanded afr). I looked it up and this is supposed to be really lean and the car should feel like crap but it doesn't, just bad gas mileage, really bad. Am I really running lean or do I have bad o2 sensors? I dont know what else to look at to diagnose the problem so any help would be great. Thanks.

Also I noticed that my fuel gauge is going crazy. After my autox event I drove home on a little more than 1/4 tank and after about 30 miles the car went down to 1 dot and low fuel light came on. I filled the car up with 14 gallons and the gas started coming out of the filler cap. Kinda weird for that to happen when the car takes 19 gal and I only put in 14gal.

The fuel issue is most likely related to the fuel starve issues the Z has due to the fuel tank/fuel pickup design. Because of the "saddle" design of the tank, fuel sloshes from one side to the other causing odd fuel readings. Just be happy the car didn't starve and strand you.

Now on to the AFR. Is the check engine light on? Also did you use an impact gun when doing the exhaust work (that's a big no-no)? I would suspect that if the O2 sensor is bad that you'd get a CEL. Also is the 18.8 AFR a sustained reading or just a momentary spike?

Whats your AFRs show at idle? Is your car tuned? Can you see each 02 individually with that program?

More torque/power = shittier gas mileage, almost without exception...

As to the ultra lean AFR... perhaps a tune...?

Thanks I actually got a fuel starve at my last event and the car died on me. I learned to not run on low fuel anymore.


I did use an impact gun to get the exhaust off where it connects to the cats, later I learned thats a no no. I think its OK since the car is not throwing any CEL's though. last night I did a pull to redline and measured the commanded AFR I guess I need to be using the the measured one. I have attached a log from today with both the measured and commanded AFR's. Can anyone take a look at let me know if things look OK and maybe if there is a reason why my gas mileage has dropped so much, maybe in need of a tune? I see that when I get on the gas the measured AFR is between 12.0:1 and 10.99:1 with the richer afr's at redline.

Mitco my AFR's at idle looks to be 14.7:1 as they should be. And no I cant see the individual O2's just the overall. Also I am not tuned just stock ECU.

Just looks like I may have been reading things backwards. My measured AFR is rich and the commanded afr is what is at 18. Is the ecu doing this to try to lean out the measured AFR's?

It should be commanding ~13 or so IIRC under load. As soon as it starts to get loaded it switches to open loop and the car is just going to meet its target AFR without even looking at the O2s. If your car idles fine right at 14.7:1 with no trim codes or other funky O2 codes then your good. I dont know how much I would trust that program your using. At 18:1 under load you would have 0 power.

Have you tried resting the ECS by disconnecting the battery and letting it sit for a few minutes?

Its possible the program isn't 100% accurate but its all I have :-(. The ecu is commanding 18 but the actual afr is around 11 and at redline its at 10.

Yup I disconnected the negative terminal and let it sit for about 15 minutes. I did this about 700 -1000 miles ago.

[QUOTE=redline10000;2886499]Its possible the program isn't 100% accurate but its all I have :-(. The ecu is commanding 18 but the actual afr is around 11 and at redline its at 10.

Its pretty rich for NA, id suggest getting a tune for it to wake it up a bunch. Also I believe that ECU commanding is wrong. If it was commanding 18 it should be doing around 18. Under open loop conditions (under load) the ECU will blindly meet a AFR target based only on the metered air going into the engine. So the fact that it is not listening to that value under open loop tells me that value is incorrect.

[QUOTE=Mitco39;2886515]Thanks for clearing that up for me, much appreciated. I will look into getting a tune but hopefully I'm good running the car the way it is now. At autox I redline the car a lot so I don't wanna mess up the engine with the car being so rich.

Rich might over time hurt your cats, but thats about it. Once your cats are removed running rich wont hurt anything, it just wont give you optimal power.