1 Lucky Texan

It does seem like you could have 2-3 different problems. clunking could be bushings, definitely check the rear bushings on the front lower control arms. steering could be air in the power steering fluid, or maybe slipping belt or even separated craank pulley slipping? I don't think we have that power light but I'm not sure. You are checking trans fluid while idling right? some folks forget as it's the only fluid check like that.

maybe exmaine the diff's drain plug magnet for chunks or excessive 'sludge' ?

found a couple more instances of the bird symbol discovery; https://www.bangshift.com/forum/forum/bangshift/project-cars-update/the-5-000-bangshift-project-car-challenge/40543-1987-subaru-dual-range/page50

maybe an old Nissan trans or JATCO logo ? Subaru Robin brand? some japanese foundry/casting plant mark? maybe look through here;

try to drain some trans fluid; use a clean drain pan/container, put a quart of water in it, mark or make note of the level, dump and dry it. pull the plug on the trans, put plug back in after you drain a quart (yeah, it will be messy - you could use a transfer pump and suck fluid up the dipstick tube) put in a bottle of Trans-X, check the trans level, if still a little low, you have some fluid in your new clean container you can use to top-off.

well, kinda bad news

knock sensor might be cracked /bad - not uncommon on older soobs and may not set a code. Affects power and can cause the car to tick/knock under load......cheap ebay sensors work for most folks. If the tick is better/gone with high octane fuel, almost certainly the knock sensor is bad. You could also have excess carbon build-up. thud? worn bushings or motor or trans mount maybe....check the bushings at the rear of the front lower control arm especially, lots of force there. check trans fluid level and condition - you must check the level while idling, after moving the shifter thru each gear. Top up 1/2 pint at a time, the marks are NOT 1qt. apart. On older soobs, some folks can cure delayed engagement by using Trans-X, might need some every year or 2. brakes are fairly simple and easy to work on, but there are a LOT of areas where things can need help. Run decent tires, do a good bleed, don't use 'economy' pads, name brand ceramic are probably the best. make sure calipers and other parts are free to move and brackets/bolts are secure, etc.

when I had my WRX at the track the first time, I had an instructor off and on. They were trying to get people to not treat the accelerator like a switch - to roll-on and roll-off appropriately. Once, my instructor praised me "good throttle roll-on!" I didn't tell him I was flooring it just before the apex and he was feeling the turbo spool on the exit!

any chance a milkshake or sugary drink was spilled there?

coupla odd ideas - try fueling with hoses pulled from the charcoal canister - one at a time, might lead to where a blocage is. (Flooded charcoal can or insect/mud nests, sometimes there's a cable going to a solenoid on the can - cable gets lose or is bad, etc.) did you change a front axle? On automatics, you can sometimes feel vibrations when stopped in Drive, if the vibration stops when you slip into N, that can be due to a rebuilt axle.

wiring harness, ECU, maybe fuel pump, maybe the muffler,,,,,,,,....I dunno, a swap is gonna be MAJOR surgery.....

Long story short, in a conversation with AWD Tuning, the local shop that repaired my WRX's transmission, they report good success with 'HD' axles made by FEQ , they get them from WorldPac. I have no idea how many models they offer or any other details.

if you go back, take some starter fluid and squirt in the intake to see if it will fire.

inspect rear bushings on front lower control arms.

just a guess, maybe something could be fabricated from tubing with the ends smashed flat?

I think a P0440 is an evap leak? first things to check on an older car; condition of gas cap seal, and inspect filler tube for rusted-thru holes.

Scan the ECU for pending codes. Maybe you could arrange to test drive another new one. Borrow a car or have someone take you to a dealer. If you still feel there's a difference, you may need to demonstrate the difference to a service writer or tech. Otherwise - you may just need to wait until it pops a solid CEL.

it may take a lot of pumping but, aren't there hand-operated vacuum pumps? maybe loaned-out by parts stores. usually used for brake bleeding. www.youtube.com/watch?v=h0Z4WPqhLqc

system is supposed to give you a coupla stops after an engine stall while driving. I doubt the system is intended to retain a full vacuum overnight (Perhaps a new car CAN, and an older car can't - but it really doesn't NEED to.) as, before driving in the morning vacuum will be re-established quickly after starting the engine. BUT, bleeding brakes with full brake pedal travel has occasionally been reported to compromise master cylinder seals.... dunno if that could match the symptom you have or if you bled the brakes that way......? do you still have the old booster? maybe a test for it could be rigged-up?

there are black(?) and green test or programming connectors, not sure on the white. The green should be disconnected. It will cycle evap, fans, and other solenoids/relays as a sort of diagnostic. hoping you get better info from one of the gurus.

not sure what's normal, have read often that it is "1 or 2 stops" so, maybe you're on target? If you still have a concern, perhaps you could compare vacuum gauge readings; examine hooked up normally, then compare to a reading with the brake booster hose pulled and its port plugged?

wow, some of you guys really have skills and confidence when it comes to challenging car repair. on the road > in the junkyard !

if the cat got that hot, it may now be bad. But the cause was likely the misfiring you experienced. too much unburned fuel going into it. can you clear-up the sequence of events for us? I'm guessing you did some work some time ago, then the car had low power/missing, then the cat got red ? or, did the issue with low power happen immediately after some work? what was done JUST before the problem started? I'm confused

needs an OEM thermostat. some reports of overheating due to clogged heater cores. (maybe test by bypassing the HC with appropriate piece of pipe and maybe xtra hose?)

seems to cross to a Russian part? REMKAFLEX RARC01196