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Using Air fuel Learning 1 ABCD for diag?

8K views 4 replies 3 participants last post by  Devin Buxton 
#1 ·
I have been having issues with rough idle/hesitation with my car lately (08 wrx) and its only been getting progressively worse. After some reading i was told to use the A/F Learning 1 ABCD gauge to diag a possible intake leak/vacuum leak/boost leak. I believe D has been disabled by my tuner since it reads 0 no matter what i do to the car. B was pegged at 9.5% the entire ride home and after asbout 5 min of idling. C ran around -1.4% to -.40 %. When I got home i let the car idle for a bit while i looked under the hood to listen for any hisses (lol yeah right). between typical scoobie noise, belts, and the rough idle i couldnt hear a damn thing. when i got back into the car to shut it down I saw A was at 7% and still climbing (ran at 1% the entire ride home until parked). So to sum it up, someone told me these numbers can tell me whether i have an intake leak/vacuum leak/boost leak/bad MAF sensor/probably more that im forgetting. cobb specs say it shouldnt be outside of +-5%. can anyone tell me which one tells what story?
 
#2 ·
Here are a couple of quick pages to give you some basic info on A/F learning:

https://cobbtuning.zendesk.com/hc/en-us/articles/200025044-AF-Learning-1-A-B-C-D-Analysis gives some diagnostics based on whether you are seeing positive or negative values. Negative generally means boost leak, while positive would indicate a vacuum leak or bad MAF.

https://cobbtuning.zendesk.com/hc/en-us/articles/200025104-A-F-Learning-Ranges gives you a basic idea of how Subaru does their fuel trimming. The A, B, C, and D indicate different load ranges the engine is experiencing and the respective trim the ECU is going to use to compensate. The A/F Learning 1 parameter I think you are referring to will jump between them because it shows which range (A, B, C, or D) the ECU is using at that moment.

I would suggest either taking your car to your tuner to have them check it out, or if you are confident in your ability to find a leak, you could attempt to find and fix it on your own. Probably the most common place for a leak to occur is the hose between your throttle body and intercooler, so I would start there.
 
#4 ·
I've been trying to diag a poor idle issue with the air fuel learning. I'd like to have this all tested before i go back to my tuner and bug him for some tweaks on the tune. AFL-A (idle) will continue to increase for as long as you let the car idle. The highest I've seen it is 9.5%. AFL-B (light cruise?) when i reset my ecu it read 13.9%. AFL-C (cruise/load) hardly ever moves regardless of what i do but when it does it shoots up. sat at -.50 for the longest time then as i went up a hill it shot up to +5.9% then went stagnant. I tried unplugging my MAF sensor to see if it was bad, but still had a rough idle. Can anyone dumb these trims down for me so i know what im looking for? I'm going to say since its dumping fuel i have an intake leak, but not a boost leak? and possibly a vaccum leak which i plan on testing this week with a gauge. I'm hoping i can get an intake leak test tool to put some pressure though the intake without it running. any info and of course any tips would be helpful, both with the AFL and leak test.
thanks guys
Here's one method: How To Boostleak Test a Subaru

Here is one possible way of obtaining a tester from @boo-key: http://www.wrxtuners.com/forums/f194/boost-leak-testers-61737/

Doing a quick google search for "WRX DIY leak tester" gets you a lot of good results for testers, methods, and general information. I know when I was going through finding a leak, I made my own tester using this method. Basically some hose, misc plumbing parts, and a compressor with a low(ish) PSI regulator is all you need. Just be EXTRA SURE you disconnect/remove/pinch all the vacuum lines they recommend in the write-ups and remove your oil cap or you will be worse off than you are now. As I stated, there are other methods that are commonly used such as smoke testing, using a propane torch or spraying connections/hoses with soapy water and looking for bubbles that are all covered in depth within the Google monster.

I also mentioned the TB coupling earlier, but another good one to check is the hoses behind the intercooler. That is another common area that will crack/come loose with time. You know what. Just check all the hoses. They all come loose at some point ;)

Without being able to see the car/hear what is happening, it is hard to say for certain what you are dealing with, but my gut is telling me vacuum leak. So I would start there.

And when I mentioned talking to your tuner, I was talking about having them go over the car, not give you a new tune. A lot of shops will do basic diagnostics, but I may be wrong in your case. At any rate, they probably would have some good insight on what your potential problem is because I bet they have seen it before.
 
#5 ·
Thanks for the insight. In terms of the idle issue its not a fluctuation of rpms but more a small "buck/hiccup". at first start up the idle will fluctuate terrible until it starts to get warm (above 1/4 on temp gauge). As for the hesitation, when i come to a full stop and try to pull out with some authority behind me (possibly let the clutch out a little too quick) the car will fall on it's face for a hot second, then backfire and be fine after that. The backfire isnt loud and it sounds more like "wuapop" rather than just a big loud bang. my best guess is its dumping fuel at the intersection and when i get on it all that extra fuel is causing it, which is what my A trim tells me its doing. Doesn't burn oil never needs coolant... hoping my intake leak test shows something leaking otherwise im going to have to... (gulp)... compression test :crying:
 
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