My alternator dying?
 

The other day while driving, my car's acceleration was stuttering randomly. No engine codes on the way to work. On the way home I got code P0172. I checked all the items under the hood for anything that was loose and everything seems fine. I read somewhere about possible electrical or voltage issues so I switched my scan gauge over to show voltage. While driving home I was watching the voltage. It goes down sometimes while giving it gas down to 12.3v then when I left off gas back to 14ish. When not on the gas it seems to flux around 13.5 and 14.2, but there was one case where it dropped lower and my car almost stalled. From what I'm reading it sounds like it may be a failing alternator. In the past I've only ever had batteries die on me but my car starts fine so I don't think it's the battery. If I clear the P0172 code it will stay away for a day then eventually come back.

If it is the alternator, should I stick with a stock one or should I get a 'better' one and if so where should I get one from?

I'll make a deal with you. If you tell me what the code means and you still want an alternator, I'll help you change it.

P0172 SUBARU - System Too Rich Bank 1
Possible causes - Faulty front heated oxygen sensor - Ignition misfiring - Faulty fuel injectors - Exhaust gas leaks - Incorrect fuel pressure - Faulty Mass Air Flow (MAF) sensor
What does this mean? When is the code detected? - Fuel injection system does not operate properly. - The amount of mixture ratio compensation is too large. (The mixture ratio is too rich.)
Possible symptoms - Engine Light ON (or Service Engine Soon Warning Light) - Excessive Fuel Consumption P0172 Subaru Description With the Air/Fuel Mixture Ratio Self-Learning Control, the actual mixture ratio can be brought closely to the theoretical mixture ratio based on the mixture ratio feedback signal from the heated oxygen sensors 1. The Engine Control Module (ECM) calculates the necessary compensation to correct the offset between the actual and the theoretical ratios. In case the amount of the compensation value is extremely large (The actual mixture ratio is too lean.), the ECM judges the condition as the fuel injection system malfunction and light up the MIL (2 trip detection logic).

Read more: http://www.autocodes.com/p0172_subaru.html

I just cleaned the MAF sensor with maf cleaner so we'll see if that helped but it looked clean to me. No changes to my car since our last debacle with the injector last year.

Well, damn. I was sure it would come out all grungy. I don't think it's your alternator. How many miles now?

How about the actual connector to your maf? Any of the leads look questionable? Some delicate wires.

I've driven my car twice now after spraying the MAF cleaner on it. So far the car is running fine. Voltage stayed a steady 13.9-14.2 while driving, usually 14.1 or 14.2. If it continues to run well I think we can probably say it was the MAF. Although maybe just unplugging and re-plugging it in was what did it because I did that too when I cleaned it.

My car has 58k miles. I'm considering paying a shop to change my spark plugs since I don't know if I feel like spending 4+ hours doing it myself with jacking up the engine (trying not to break stuff).

def your alternator

P0172 has nothing to do with the alternator. You have a prob causing the engine to run rich which is causing the car to almost stall out which is causing the engine RPMs to dip and the alternator is running too slow so the voltage is getting low. You have it backwards- the alternator voltage is not causing the car to stutter, the car stutter is causing the alternator voltage to go down.

What is your AFR at idle and what's your total fuel trim (LTFT + STFT) at idle and 2500 RPM?

Everyone always wants to clean the MAF like it'll cure everything, but it is almost never the solution. What have you done to the car lately? Have you done any mods or anything?

DO NOT change out the alternator without having it properly tested. When you start throwing parts at a problem you have not properly diagnosed you're just going to cause more problems. You'll run out of money before you run out of guesses.

Don't waste your money on MAF cleaners unless you leave in an exotically screwed up environment. They all heat up super hot when you start your car to burn off any contaminants, you're likely doing more damage cleaning it than just leaving it alone.

Of the top of my head without looking it up have you been using Torque and a Bluetooth ELM module or a legit scan tool?

Also as a side note, I thought my local Costco sold awesome fresh top-tier 92 octane gas. I started having similar rough idle and hesitation issues with only 26k miles/headers/tune.

After 2 tanks of Shell my car had new life breathed into it. It's likely something internal could have been gunked up, possibly the O2 sensor. Check your fluids, try a GOOD and proven gasoline like chevron or shell 92+ and have your battery checked too.
At that age you may have just deep cycled it one too many times.

Stocky, don't be discouraged if the code pops again. We're just getting started.

The MAF cleaning didn't cure it. Last night the voltage was going down to 13.0 and went up to 14.3 which I haven't seen it go over 14.2 until last night.

Codes did not reappear yesterday nor this morning on the way to work.

-Not sure what my AFR at idle, nor trims is but I'll check with my scan gauge.
-I have not changed anything in my car for over a year with the exception of in the spring I started using 5w-30 oil for the summer. I'm going to put 0w-20 back in end of the month when it's time to change the oil.

-I'm using a Scangauge2.
-I live one block from Costco and use their premium gas. Perhaps they had a bad batch as it started after I filled it up, or perhaps it's coincidence. I did check the receipt just to make sure they put premium in it as I thought perhaps they put in regular instead.
-My battery is maybe a couple years old at most as I got it just over a year ago. It's a Shorai 3lb Battery LFX36L3-BS-12. I went on vacation for 2 weeks and plugged the trickle charger into it, but when I got home the trickle must have stopped at it had no charge. I used it to charge the battery and everything was fine after that. That was the week prior to filling up the gas and starting to see the issue.
-I have a voltmeter but don't really know how to use it. I'm assuming I could use that to check the battery and alternator?

I don't understand. Are you going only by the voltage reading, or is it still stuttering? Bring it by Saturday and we'll dig deeper. I'm not against swapping in my alternator to troubleshoot but we can do much more before we go there.