Fairtax4me

Get a leak-down test done. That will tell you if the compression is low and where it's going.

Dashboard has to come out to get the heater box out. Back flush is an easy option. Peak makes a flush solvent that works pretty well. They recommend that you run the engine for 3-6 hours of normal driving to get the best results, then flush the system thoroughly with water. The citric acid approach sounds like a good idea as long as it doesn't corrode the aluminum.

Ehhh kinda looks like you may have a compression issue on number 4. Burned valve, or broken ring land possibly. How long has it been running rough?

There's no specified minimum that I can find. Typically the difference in cranking amps is about 50-75 amps. Ex. 475 for manual, 550 for auto. This can vary between battery manufacturers. The FSM does specify base capacity of 48Ah for manual, 52Ah for automatic cars. Battery size numbers 55D23L, and 75D23L respectively. But I haven't found any cross-reference for those numbers yet.

They don't have to be exactly exact, as long as they're within that range. Some people think its better to get them as close as possible, but in most engines the plugs all wear at different rates anyway.

Auto trans cars use a higher amp battery because there is more electrical load on the battery and alternator due to the electronics associated with the automagic trans. (Trans control unit, valve body solenoids, etc) You don't want to put the lower amp rated battery for manual trans in an auto trans car.

Bulbs are not exactly interchangable so its hard to install and plug in the wrong type of bulb. But it is easy to misalign some bulbs in the bulb housing. Some bulbs have a notch or a small hole that has to be lined up with a similar notch or pin in the lamp housing. With limited space to access the bulb on newer vehicles it is not always easy to get the bulb lined up perfectly. Misalignment will throw off the beam pattern and cause low visibility.

IIRC, in the other connector there should also be a pin with gold on it. That's the sensor signal pin. Connect gold to gold. The other two (silver) are for the heater element and orientation doesnt really matter for those. If you get a code for O2 sensor heater circuit reverse those two and see if it goes away.

V powers are great plugs for that engine. Should have been gapped at .039-.043" though. The 5-6 are different heat ranges. Nothing to be too concerned with since those two are commonly recommended. The 5 just means its a slightly higher heat range than the 6 plug. If you have a torn plug wire boot the wires need to be replaced. The old 2.2 isn't as picky about wires as people make them out to be, but you are much better off with a quality set of wires. That said, I've used CarQuest house brand plug wires on several cars, and those will work fine for that engine. It can be kind of tricky to get the wires clicked on all the way. Put di-electric grease on the ends, and around the top where the boot seals against the plug well. Hopefully a set of wires will have it running right again.

Subaru recommends adding a "conditioner" to the coolant, which is basically a stop leak type product. Those engines had trouble with the head gaskets weeping externally, so the solution (rather than better gaskets) was to add crud to the coolant. Looks like someone went overboard on that one. Scrape out the big stuff. When you get it put back together you can try running a flush solvent in it. The stuff in the block won't hurt anything, but if there's a ton of it in the radiator that will cause problems. But the radiator is easy to replace. My worry with that much stop leak crud in the system would be that it has partially clogged the heater core. The heater core bypass route of the cooling system is vital for proper cooling system function on these engines.

Best way is to do a leakdown test. You could also check valve lash clearance. A bent valve will hang open slightly and clearance will be wider, but it may not be excessive so clearance isn't a guaranteed way to check. The at home quick way is to slap a timing belt on it and fire it up and see if it runs smooth. Did it jump time? Belt break?

More than likely they unplugged something during the oil change or in the course of getting to the headlamp bulbs to change them. They turned the key On to check bulb operation with the sensor still unplugged and it set a code, and turned the Check engine lamp On. On the newer cars when the CEL is On the cruise is disabled, which is noted by the cruise light flashing, and the traction control system may also be disabled. Its a feature designed so that if you're driving and there is a fault that causes the CEL to turn on, but there are no other signs of a problem, you'll still take the car to get it checked out. People red to ignore the CEL if it comes on but there is no trouble. When the CEL turns on it's because there is a fault that will increase emissions output (which is the whole reason the EPA mandated that CEL in the first place) and can also cause damage to the catalytic converters (which are expensive) and ignoring the light can lead to expensive repairs, which results in an unhappy customer, and lots of unhappy tree-lovers because of that extra smog coming from the tail pipe. Despite that, most people don't care enough to get the codes read and find out why that one tiny yellow light is on. (Yellow light = Go Faster, right?) But people notice and care ALOT if the cruise doesn't work! "Gotta have my cruise control!" they says! Something wrong with muh cruise I gotta get that fixed A-S-A-P! They take it in and fix it and the EPA is happy. But enough of that, the only thing I would be concerned with is that they didn't have the decency to check the code and turn the light off before they gave the car back to you, and then basically blamed it on you. Kinda half-astery there IMO. Its common for newer cars to have a somewhat longer starting time after clearing codes to turn the CEL off. I wouldn't worry about it too much unless it becomes a frequent problem, or the CEL comes back on. Id recommend finding a different shop for your next repairs.

No no, with the drivers cam in timing position the valves are OPEN on that side and turning the crank can cause them to collide. Put the sprocket on, and turn the crank counter clockwise (slowly) by hand or with a strap wrench, until the timing notch on the sprocket lines up. Line up the crank FIRST, then you can turn the cams to their proper positions.

Awesome!

I'm usually able to bleed a whole system with one quart, but It kind of depends on what method you use and how long you allow the lines to gravity bleed. I would recommend having at least two quarts on hand, then you know you have enough.

The main sprockets are typically re-used on modern chain setups. Smaller spockets like idlers usually get replaced. If you find there is excessive wear its a good idea to replace. There are nylon tension guides that you should definitely replace. Those can become brittle with age and break Water pump is also driven by the chain.

According to rockauto the mounts are different left/right. Did you get both mounts with the same part number?

Need to put the whole 2.5 manifold and wiring harness on it. The computer needs the MAP sensor for proper fuel mixture calculation.

It common for the coolant temp sensor to fail and cause starting issues, and also cause poor fuel economy and rough idle. They're fairly inexpensive and can be considered PM if it doesn't fix it. If you can use an ohmmeter you can test the sensor for proper resistance at a few different temperatures.

Sticks out of the top left side of the front cat. Kind of points toward the Inner cv joint on that side.

Leakdown test will confirm a bad head gasket. But if the coolant is disappearing slow and no trace of it on the ground or on the engine anywhere, probably a head gasket. Certain leaks will not push coolant out of the overflow, but will cause rough cold starting (due to coolant in the cylinder) and low coolant level in the bottle as coolant burns off a little at a time.

When it does this and you push the AC button on/off do you hear the compressor clutch "click" to engage? Do the cooling fans turn on? Have you tried setting the mode to defog to see if the compressor engages?

If you erase the code does it come back right away? I always try to check in the connector for dirt/corrosion to rule out a poor connection. Also make sure none of the sensor wiring is chafed/cut. If that checks out, sensor's probably kaput.

No issue. The cams rotate back because the valve springs push back on the cam lobe, when the lobe rotates out of the way it allows the valves to close. Just use a wrench to turn them back the direction needed to line up the timing marks when you're ready to put the belt on. The DOHCs can be tricky to keep the sprockets from turning. I get the belt on the lower sprocket then pull up on the slack part of the belt to keep tension on the belt against the sprocket while rotating the upper to the timing mark.

Pull the block and inspect the cylinder walls for scoring first. Might need a bore and hone as wtDash said. Subaru sells 0.25 and 0.50mm oversize. Works out to 0.020", that's the max Subaru recommends. If it needs overbore you order all 4 pistons the same size to match the new bore.