Long-Term Fuel Trims too high...AEM CAI to blame?

[mysleeper6]

I bought a used AP from another member (thanks, LOLWOT!), and it arrived yesterday. I am itching to try it out on my MS6, but I felt I should make sure the car is performing correctly on the stock map first. I have an AEM CAI as my only current breathing mod, and I have never thrown a CEL.

I have a DashHawk and I had noticed that my LTFTs were a little high, so I thought maybe I should clean the MAF sensor and filter. Good intentions being what they are, I managed to get one of them accomplished (cleaning the MAF; it WAS dirty). I should also note that it did not appear as though there is any damage to the MAF or the housing. I disconnected the battery for a bit then drove the car conservatively for about 30 miles, and already my LTFT's were settling in at 14.1 under even light throttle. When coasting the STFT's would go to zero, and the LTFT's would go to about 10, but as soon as any throttle input was given it jumped back to 14.1. At that point I got impatient and drove it like I normally do, with basically the same results.

Now, by my understanding, my STFT's should become my LTFT's eventually; that is to say that if the ECU is adding fuel in the short term at between 0 and 5, eventually my LTFT's should reflect that and settle down to between 0 and 5, all other things being equal. However, I never see anything as high as a 14 on my STFT's so why am I getting that as my LTFT?

I don't have any logs to share at this time, but if I was to log this what PID's should I display? I was thinking RPM, STFT, LTFT, and Actual AFR? And does anyone think (or better yet, know) if this is an expected issue with the CAI?

 

[MShreve]

I can't address your questions directly. But AFAIK, you should expose the ECU to ALL types of driving while it's in a learning state. Meaning equal input of WOT, idle, normal driving, etc.

There's a good read on M S F about BPV spring tensions affecting LTFT's as well. I'd link to it but this forum won't let me.

 

[mysleeper6]

Thanks. Anybody else?

 

[FORZDA 1]

I bought a used AP from another member (thanks, LOLWOT!), and it arrived yesterday. I am itching to try it out on my MS6, but I felt I should make sure the car is performing correctly on the stock map first. I have an AEM CAI as my only current breathing mod, and I have never thrown a CEL.

I have a DashHawk and I had noticed that my LTFTs were a little high, so I thought maybe I should clean the MAF sensor and filter. Good intentions being what they are, I managed to get one of them accomplished (cleaning the MAF; it WAS dirty). I should also note that it did not appear as though there is any damage to the MAF or the housing. I disconnected the battery for a bit then drove the car conservatively for about 30 miles, and already my LTFT's were settling in at 14.1 under even light throttle. When coasting the STFT's would go to zero, and the LTFT's would go to about 10, but as soon as any throttle input was given it jumped back to 14.1. At that point I got impatient and drove it like I normally do, with basically the same results.

Now, by my understanding, my STFT's should become my LTFT's eventually; that is to say that if the ECU is adding fuel in the short term at between 0 and 5, eventually my LTFT's should reflect that and settle down to between 0 and 5, all other things being equal. However, I never see anything as high as a 14 on my STFT's so why am I getting that as my LTFT?

I don't have any logs to share at this time, but if I was to log this what PID's should I display? I was thinking RPM, STFT, LTFT, and Actual AFR? And does anyone think (or better yet, know) if this is an expected issue with the CAI?

My experience says the +14 LTFT is about right for the CAI and won't cause you any significant problems. The STFT will jump all over the place with changing throttle, but I recommend you set the cruise control and cruise at a set speed (like on a highway trip) and you will see the LTFT come way down. Now, when you get back into "town" with stop and go throttle variances the LTFT will go back up.

Basically the CAI design makes the MAF sensor see less air and the ECU gives less fuel, BUT when the O2 sees lean, the ECU must add fuel (fuel trim) to get the AFR back up where it should be. There are a few annoyances (for me anyway) with this though. You will run rich during very quick WOT shifting and get stumbles and backfires which hurt ultimate power output, but no damage to the motor.

What you REALLY should want is an AP map (street)tuned for your car in your area to get the most out of the AP and your car. A canned OTS tune (from ANY product, even OEM) isn't going to net you a great tune.

I've spent a bit of time tuning my setup and I see essentially zero LTFT values and have great power output on the OEM turbo with lots of boost.

 

[mysleeper6]

Basically the CAI design makes the MAF sensor see less air and the ECU gives less fuel, BUT when the O2 sees lean, the ECU must add fuel (fuel trim) to get the AFR back up where it should be.

Thanks, Forzda. I've learned a lot since I posted this, and I guess I was already in the right ballpark with my suspicions. From following along in other threads I know you are very knowledgable, so this is really confusing me. It seems counter-intuitive. The CAI lets the MAF see less air, not more?

And thanks, I am going with an OTS map that I will play with in ATR (to gain more understanding of the whole process, i.e. what affects what) and then get a real tune when I finish modding. That seems to be the smart way to go.

 

[FORZDA 1]

Thanks, Forzda. I've learned a lot since I posted this, and I guess I was already in the right ballpark with my suspicions. From following along in other threads I know you are very knowledgable, so this is really confusing me. It seems counter-intuitive. The CAI lets the MAF see less air, not more?

And thanks, I am going with an OTS map that I will play with in ATR (to gain more understanding of the whole process, i.e. what affects what) and then get a real tune when I finish modding. That seems to be the smart way to go.

Yeah, the MAF location in the pipe affects how the air hits the sensor.

The CAI is actually capable of flowing more air, but the CAIs generally make their power increase "mechanically" by the MAF location causing a leaning out of the mixture, not necessarily adding fuel. If the MAF sees less air, the ECU pulls the fuel back to try and hit the targeted AFR. Less air needs less fuel.

When in closed loop you'll get the high fuel trims, but when the ECU goes open loop the fuel trims are dropped and the CAI allows more air to lean the mixture. As the ECU doesn't apply trims, the actual AFR is leaner resulting in more power.

Practically ALL cars run very rich at higher rpms, so leaning it a bit will net a bit more power.