2002 Subaru Impreza WRX 2.0 Turbo voltage and rpms dropping close to stalling

Hello All,

This is my first post so please be gentle as I am a novice and wanting to learn all I can to not be a parts changer.

I have a 2002 Subaru Impreza WRX 2.0 Turbo manual transmission and for about a year now its been having an odd issue. Last winter it started only in the mornings to stall out on me, and only every now and then. It has 225,000 miles on it and it would always start right back up and it was so sporadic I didn't really pursue it, thinking it was just due to it being cold outside etc.. Fast forward to a spring 2018 and my alternator bearing starts to go , you could hear it pretty good when near the car , so instead of being smart and getting an OEM Subaru alternator to take out the old and put in the new ( since the OEM one had lasted so so long ) i tried to take apart the current one and replace the bearing. Things did not go well and I messed up the alternator shaft during disassembly so I had to get a new alternator. My local subaru dealership did not have one in stock so I had to get a duralast from Autozone. Installed the duralast and thought all was well. About 30 days in to the duralast one morning my battery and break dash lights came on while I was driving so I went to Autozone , they bench test the alternator and it tested bad and they swapped me out for a new one that they tested before they gave it to me and it tested good. The car would only act like it was going to stall every now and then during all this time. Then 2 months go by on 2nd duralast and while driving during this time the battery voltage would drop at the battery and the inside air fan for blowing air speed would lower on its own. I turned a/c on or off and no difference voltage drops at battery and fan speed for inside air slows, so I do not think the problem is load dependent. If I am coming to a stop or slowing down where RPM's rapidly decrease it would act like it wanted to stall out or struggle to a stable idle. I put new belts on and tensioned properly to make sure since new alternator that old belts would not be contributing or slipping. That seemed to help but the problem came back even stalling out one time at 60 MPH on the highway ( it only did that one time ). I took the car to a mechanic I trust and he hooked it up to more scopes and back probing and you name it he was hunting for what was wrong. But of course while it was in with him and I was there the whole time in the bay with him and the car it did not act up at all. The voltage dropped a little a few times, but nothing major. He said he could not really tell what was happening and since the alternator was just changed recently its possible the voltage regulator is intermittently going out. So I bought a Subaru reman alternator OEM from my Subaru dealer and warrantied out the duralast to get all my money back on that thinking going back OEM would solve it. It sadly did not. ( side note the 2nd duralast bench tested bad when I got all my money back as well . Even now with OEM alternator its still doing this and now more consistently. I have noticed if I am coming to a stop and push clutch in and let engine RPM begin to drop to come to stop I can count on the voltage drop and fan slowing to happen every time. It will still happen when I'm running at street or highway speeds as well but never to point of complete stall out like the one time at 60 MPH. I have a Cobb access port that I am using to monitor battery voltage when this occurs and it goes from 14 V down to sometimes 11.9 V when it drops and then comes back up. My battery voltage is spot on what it should be with car off. Im not sure if the new alternator is also bad or if the IAC valve isn't functioning properly or if I have a grounding issue, throttle body needing to be cleaned etc.. Please help. This is my daily driver and I have 4 kids so money is tight to just take it in for diagnostics at a shop. I can't figure out what would cause the voltage to drop and also cause the RPM's to dip & come back up like it can't hold idle or struggling to find idle for a moment like its doing .

Looking forward to my first adventure on this with you all. Thanks in advance for all your help and wisdom !!

Side note : no DTC's / or codes are present or were present during all of this.

so system voltage is dipping below 12 volts when this occurs?

It does dip a little below 12 v on occasion but typically it drops from 14 v to very close to 12 v so 2 v drop / dip and then it regains and stabilizes. During the dip fan speed lowers and if I’m coming to stop with rpms dropping the car seems to have a hard time establishing proper rpm for idle when voltage has dropped to the low point

The engine will run quite happily down to only 10v to me this looks like an idle speed cntrol issue I think I would start by giving the throttle body and IAC a good clean. As the engine returns to idle to prevent stalling engine computer should be able to adjust the IAC to prevent stalling to meet extra loads from the electrical system, AC and power steering.
When you are cleaning the throttle body visually check all the intake pipe work for leaks. If there is no improvement then the next step is looking at live data ie fuel trims, MAF output, calculated load .................

Dirty Throttl body video

Thank you Andy.MacFayden - this I think did the trick. Sorry for the delay lots of work and trying to make time to look at the throttle body. It was indeed dirty, not to bad but the plate was covered near the rim of the plate on both sides of the plate and the area of the throttle body where the plate rest was dirty as well. I ended up removing the throttle body and giving it a good cleaning, and cleaned my IAC valve as well since its connected to my throttle body - took it off and cleaned. Did this a few hours ago and so far so good. Its not acted up a bit. Fingers crossed.

Thanks to everyone that posted and replied - this will take a load off my mind with this being solved out now.