About a week until I get to be back with the coupe, and a couple of things have happened.
The coupe seems to be choking itself out on startup.
I'll need to see more when I'm in person, but I'm thinking that the P0101, plus the car sitting, hasn't been nice to it.
And for Black Friday, I got Whiteline Performance sway bars (which appear to be rebranded Eibach sways), 24mm front and 22mm rear, along with some sway bar end links.
My big worry is now that the car is at 140k miles, the fuel pump and alternator may give out.
And the clutch is being wonky now.
My dad took it out for a drive to a friends house, and it suddenly wouldn't go into any gear but reverse.
Next day it worked just fine, so I think its a clutch cylinder issue or the clutch needs to be replaced.
I got my work cut out for me, diagnosing the CEL and this startup, figuring out the transmission wonkiness, and installing the sway bars and end links.
And then getting her legal again.
Hopefully after all this, I can take the coupe on some nice drives and get her feeling good again.

 

Oh man, you got your work cut out for you. I though you were around 100K miles. Make sure your battery is charged.

I pulled the sway bars and AMG brakes off of "Carly" (262k mile C230)
to put on "Inga" (the ~40K mile coupe). But now I need to focus on Christmas, getting tree, decorarting etc, keep wifey happy.
So, sway bars maybe after Christmas and AMG brakes when the brakes that are on there wear out.

Yes, if you are at 140K you will need alternator and starter soon. I wouldn't wait till they break.
Just do them both at once. Starter is tricky but I wrote a whole thread on it and posted a video.

1st time around like you now around 140K, I cleaned the starter (it was packed full of black dust from the brushes)
and replaced the brushes. Find an auto electric shop in your area.
It lasted another 100K + miles.
Alternator I had original one rebuilt. You will have either Bosch or Valeo. If you go new try to get new try to find Valeo if you can. Everything else is Chinese. Valeo in made in Poland.

Fuel pump held up until 200K miles. Just make sure you are replacing the fuel filter at least every 60K miles. Do one hose at a time so you don't mix them up. Take the fuel pump fuse out and run the car till it uses up the remaining pressure in the line.
Regarding the pumo,
Put oil reisitant tie wraps on the connections. They get stretched when you remove the pump. Use a tie wrap tool to pull them tight. I found out the hard way. I was warned this could happen by my mechanic AFTER I'd replaced the pump, and sure enough one popped off and the car died. Lucky me, we'd just returned from a jaunt to Half Moon bay, and I was less than a mile from home. Coulda been sooo much worse. We were on Skyline and all over remotte places.
Coasted into a parking spot and walked home.

 

Never heard that a fuel pump is getting weak, it usually dies suddenly (without a jab). To de-pressure simply push on the shrader valve at the fuel rail and open the gas cap.
Clutch replace is no fun the dual mass flywheel is very expensive, I'm in the state of denial, just replaced the head gasket.

 

Only thing about pushing scharader valve is gas spewiing out of it, hence i prefer to
remove the fuse, run the car till it stalls and then push it once all pressure is removed just ot be certain.

 

Finally home and unboxed the sway bars and my new sway bar end links.
According to my dad, the coupe starts up differently each time.
Choking out, running perfectly fine, or a rough idle.
I need to figure out what is wrong with this motor.

 

What a great reunion with my coupe!
I can't wait to see what great adventures are in store for us this winter!

Hot damn.
I might need an exorcist.

 

Hot damn is right, as a uni student with one of these cars, and very little money, i feel you, but it aint the end of the world.
Ill try to organize my little 2 cents here in the order or least labour to trouble shoot this thang.

First off your plugs, are they gapped correctly to 0.8mm ? a too large or too small gap WILL cause misfires especially at startup.
I had that issues for a bit with ngk bkr6e but i gapped to 0.8 and it was fine.
If its the OEM sparkplug laser iridium part no. it should be pre-gapped properly.

Rule out MAF wiring real quick, you mentioned the coupe starts up different every time, if you unplug the MAF and start the car and the startup was improved, its likely wiring or power to sensors could be part of the issue, check the fuse next to the ESP fuse in front fusebox. i cant say its maf cause you got a new one, but again vacuum leaks can set off MAF code.

Since you've replaced your MAF at the dealer, please accept my condolences, the dealer quoted me 1500usd for a new MAF sensor for my 24 year old car and i laughed and walked away, got a siemens sensor off ebay and didnt look back (solved my p0101)

I digress.
Whilst the engine is running you can also use something like water, carb cleaner, wd40, contact cleaner etc and spray every single conceivable mating surface or gasket area that could possibly leak boost or vacuum, you ideally want to check after the MAF and IF you spray a leaking vacuum spot, the idle with surge / raise or lower (surged for me).
A boost leak before the maf could possibly cause p0101, however it would need to be a huge leak like a cracked intercooler or popped off charge pipe (also check the 2" rubber joiner that connects the bypass valve to the SC body itself, not the black plastic piece, its underneath the bypass with 2 hose clamps, i found it popped up once.

M111 evo vacuum lines:
Towards the firewall, the intake manifold has 2 vacuum lines, one with a check valve, going to the Secondary Air Injection Valve, and another going to a hard vacuum line which runs along the wiring harness into the cabin, i believe for headlight adjustment, rear head rests and brake booster sense.
On the Secondary air injection valve, there is the aforementioned vac line to manifold at the top of the valve, a soft line at the bottom of the valve which connects underneath Throttle Body Elbow, and the remaining vac line on top of the SAI valve goes into the EGR switchover valve (Circle shaped valve connecting to throttle body elbow with 0.75" reinforced hose). Look up DoitVehicles video on c200k SAI he details how to check the EGR valve, cause technically if this is stuck open it will act like TB is open slightly and also cause lean condition and throw off MAF (cause ecu isnt expecting it to be open).
Im sure you also know about the 2 nipples underneath the intake manifold going to the PCV as well, these can leak vacuum.

It is advised to check all of these for leaks, as they will cause a lean condition, which if bad enough would cause all your symptoms.

Again MAF, Misfire and Lean codes could all be explainable due to a vacuum leak, but the TPS code is usually set off by dodgy/waterlogged connections, slow to respond TB or, *check the connectors at your ECU (inside front fusebox) for oil contamination caused by leaky vvt magnet, cam pos sens, crank pos sens*. This will manifest as oil in ecu connectors (and ecu if left alone for too long) and O2 sensor connector.

The throttle body code is weird, honestly everything else gives away a huge vacuum leak or sensor input issue, but the TB code isnt one ive commonly seen before unless the pedal is bad and well the actual TB itself i havent really seen fail.

So another thing to check is how fast your throttle butterfly closes, open it manually (obv with key not in ign) and pull away your finger to let it snap close. If the butterfly returns slowly, or with slight steps, look up how to clean a M111/W203 bypass valve/throttle body, simple just removing clips, motor, greasing end of the motor shaft a little and then checking back and making sure the TB snaps closed like its supposed to.
If this is fine, your TB wiring is fine. It must be a vacuum or boost leak. Do a throttle and bypass valve(this needs 2 ppl) reset procedure once you have ever disconnected the TB wiring.
ALSO another wiring thing could be mixing up the barometric pressure sensor connector (on top intake manifold 3 pin) with the actual MAP sensor which is located down by the brake booster hard line.



Finally, is your fuel pump original, it could very very well be on her way out, mine died due to sitting during my headgasket job, was original, your car sat during the HG job too. If fuel filter is changed within 60k and the original pump was changed, its not fueling issues, if pump is original check rail pressure before startup and check for bleed down of fuel pressure too, indicative of a bad FPR/filter (FPR in filter).

If you get through or dont get through with this very haphazardly laid out troubleshooting advice, ill try to get reminders from this thread and reply, could have written this better but exam fatigue hitting hard.
for any lovely w203 owners, please dont throw parts at your 203 without confirming fully that the part in question is defective, esp when it comes to OBDII errors. This is honestly how most 203 owners end up selling the car due to mental and financial migraines.

 

P0101 is a dead MAF at least it was for me on my Ford.
Easy to check just check with scanner and it doesn't move it's dead.
All the other errors will follow due to MAF.

 

Correct it could defo be related to MAF
Not necessarily do you get such bad misfires, a tps code and lean condition from MAF alone, i know from experience.
Physically damaged MAF would result in limp as soon as you hit boost or open throttle, or js after a couple mins of driving.
Worn or oil tarnished MAF could give rotaryfox's symptoms, however usually not accompanied by lean codes. This sim4le ecu works with part load lambda correction and complex manifold pressure differential calculation stuff to apply injector pulsewidth. If there is a vacuum leak especially one sizeable this is all thrown out of the window. Injector flow is thrown off due to low vacuum in manifold, stfts max out, the ecu has to apply excessive correction or excessive trims to compensate for the unmetered air causing leaner than target afrs and thus thinks the MAF is lying, even with a perfectly working MAF. IIRC this ecu has logic to detect electronic throttle being jammed open or closed at any position. It probably threw the tps code after a while of driving with a bad vacuum leak concluding the butterfly wasnt shutting all the way. Or it wasnt calibrated since removal.

 

Did you huy a MAF off Ebay.?

Only time I got P0101 was my dad's old Ford Escort and it was DEAD DEAD DEAD!
Lot of other codes reated to fuel etc. but never P0101 on rthe coupey,
I forget but I think if you use scanner (Torque on phone is good for this because you can make a graph.)
If it stays at 5v and doesn't moce it's MAF...or Zero I forget.
Also bought a MAF on ebay once wow, it was so bad, put the old one back in and ran ok again
just with lean P0171. and no boost. You need NOS Siemens or one from Merc.
$$ but worth it.

 

Sorry for the late reply, y'all, and thank you for all your help so far!
Been busy with university and will go back home in a week to actually work on the coupe.

I got pregapped plugs from Rockauto, NGK 5344 Laser Iridium; Pre-Set Gap: 0.040". I think that's about 1mm, but that is the OE-recommended spec, iirc.

I'll try unplugging the MAF and checking the fuses; somehow, that went over my head.
I got my MAF from parts and services and installed it myself, so not too murderous on the wallet.
Just wanted to make sure I got something that would actually work, but here we are.

I wouldn't be surprised if it was a popped hose from the intercooler or something, getting my hands in there to remove and replace that little clip was a PAIN.
I'm considering actually redoing all the vacuum lines atp, with the rubber being old it might be worth the time and effort.
I will check the EGR valve as well, since I did drive the car around with no boost because of the SAI being cracked. Maybe this did some lasting damage?
For the oil contamination, I have the pigtails, but will get some contact cleaner and check.
Last I checked, the TB was closing fine compared to the bypass valve, no steps or nothing, but maybe the car sitting for long periods of time is messing with it?
I have become a pro at doing the bypass valve reset, but for TB is it the same procedure?
I'll check those two 3-pin connectors, I honestly might've messed them up.

My fuel pump and filter are original, I do have a replacement filter that I should probably throw in.
Rail pressure was good last time I checked, but I'll check again.

Good luck with exam season, I gotta lock in for my finals but here I am lol

 

I'm leaning toward it being related to the oil-tarnished MAF, accompanied by some vacuum leaks.
And the misfires are only on cold start when the car has been sitting for a long time, not when driving under heavy load. There is no limp mode, but I do feel a slight power decrease compared to before.
Also, the TPS code isn't a permanent issue; it reoccurs after the car has been sitting.
I presume that the battery constantly dying from sitting isn't healthy, but wouldn't it technically reset adaptations?

Something else that was brought to my attention that I was told could be worth checking is the grounds.
After doing the headgasket, it's possible the connections aren't fully secured and could be causing some issues.

 

I got the MAF from the dealer in person, not online.
I still have the older one that I could use for testing.

Also, another important detail that could help, when trying to get all the sensors prepped for smog, when trying to get the catalyst to be ready, with 15-20 mins of 55-60 mph driving (tried both 5th and 6th gear), thats when the CEL usually pops up after a reset and clean.