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Discussion Starter · #1 ·
I'm having some odd issues with a recently purchased 2007 subaru outback 2.5l with auto transmission where (with alternator plugged in and battery reading 14-14.5v) the engine idles a little rough at 700rpm, the brake and battery light are on, and it will nearly stall out when I hit the gas but no CEL. When I unplug the 3pin connector on the alternator the idle smooths out at 800rpm, brake and battery light turn off, and the engine is responsive and crisp when I hit the throttle. The engine has a new remanufactured alternator (previous owner replace this) so I don't believe its the alternator but it definitely seems to be charging system related given the symptoms I'm seeing.

Anyone have any idea what this might be? Any help is greatly appreciated.

Full story is I bought the car with a blown engine and recently installed a used replacement. Original failed the #1 cylinder big end bearing after running low on oil from a head gasket leak. I only used the short block from the used engine, all the intake, wiring, alternator, throttle, etc is from the original engine. With the alternator unplugged the engine runs smoothly with no weird noises, drives nicely and pulls pretty hard for a 2.5l. With the alternator plugged in it idles lower and runs rougher, it will drive but it bogs with throttle and wants to stall out with additional engine loads like turning the wheel at while stationary.
 

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Discussion Starter · #3 ·
Turns out I'm an idiot. The issue was the 7.5amp fuse on the positive signal wire was blown. I had visually checked it before and I thought it was good but it was dirty so I didn't see the blackened area. I with the key on I checked the voltage drop between the positive signal wire at the alternator pin and the battery and was reading 12.6v but it should have been reading zero. This prompted me to check the resistance on the fuse and when it read open circuit everything made sense. Replaced the fuse and car runs perfect now.

I thought about not posting this but I spent a lot of time googling symptoms and found nothing. Hopefully this post saves someone else some time and headache later.
 
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