Ok I have a 1.8 b6 Quattro 6spd. 111k miles.
deleted cat otherwise stock.

I Recently replaced timing belt, water pump, fuel pump/filter and battery. Shortly after one of the coil packs took a crap so I replaced all of them with a set of premium napa replacements.
All was well, so I left on a summer adventure. 5000 miles into it I fueled up with full tank of 93 octane gas. Shortly after fueling my fuel Econ went from 33mpg@ 80mph down to 25mpg@80. Along with a drop in performance/ power.
Annoyed I put in a bottle of dry gas thinking the gas was full of water, after that tank I refueled with a full tank of 91 from a seemingly main stream gas station. (trying to avoid crap gas)

That didn't help after two more horrable mpg tanks of gas I swoung into a napa and swapped out plugs with single platinum core plugs and a fresh air filter just because it was about that time.

After that it ran a little better but MPG still sucked.
2 days later (now) it's hesitating/shuttering randomly but mostly during accelerating and mpg is all over the bored.

Just pissed after getting stupid mpg I filled up with 85 oct fuel to my Supprize mpg with back up to normal but random shutter/stumble still persists and if its at WOT it pretty much falls on its face.


All hoses are in great shape, no vacume leaks clean connections everywhere.
Clean smooth idle only code being thrown is p0420 for my down stream since I removed cat.

I'm stumped everything is new that could fail.

 

sounds like a possible fuel system problem, you may have gotten crap gas causing a restriction in the fuel pump strainer, fuel filter or possibly the fuel injectors. Before you go replacing any of those, try running a good fuel system cleaner through i.e. Seafoam, techron, regane, etc

 

Ok so I used 3/4 bottle seafoam in my gas and 1/4 in the oil cuz its due for an oil change. Performance increased slightly along with fuel Econ but when I re fueled both went back down.

I'm starting to wonder if the plugs I put back in are running too cold

 

if u run seafoam thru the engine u should always change out your plugs. Have u tried to unplug ur maf while the car is idle. If the problem goes away when u unplug then ur maf if bad

 

Check the plug gaps. I'm not 100% but I believe you should be a .028. most plugs need to be gapped out of the box.

the shaking sounds a like like misfires, from your descriptions, but that should throw codes. even if it was from bad gas, I suspect you should be getting a code/blinking CEL

the final possibility that comes to mind is that the timing is off after the belt change. 1/2 to 1 tooth would give you crappy performance, as you describe, but not really screw anything up. exhaust gases would be wonky too