I recently turned 80k in my GLI and did a tune up, which included plugs, fuel filter, air filter, oil, oil filter. I also cleaned my MAF with the CRC MAF Cleaner. The first tank of gas after the tune up my MPGs dropped about 5mpg, the car use to get 28-29mpg mixed and now it is at 22-23mpg and has stayed like that for a month now. It is driving me crazy because now I fill up an extra time a month now 4 times instead of 3. The car is driving the same route everyday and gas is from the same gas station every fill up 93octane. The car runs fine, no loss of power, no funny idle, it feels jsut as strong as before the tuneup. Could the MAF be failing, is there a way to see if its not working to full capacity, I have vag com.
I doubt your MAF is causing a 20% drop in MPGs, I've used MAF cleaner on mine before with no negative (or positive) effects. I'm pretty sure if your MAF were failing it would cause a CEL and top-end power loss.
Your theory is super easy to test:
Get your engine warmed up, then pull into a driveway.
Reset your trip MPG thingy.
Drive around the block, try to maintain a constant speed, no hard acceleration. If you've been running your AC, keep it on.
Pull back into your driveway. Write down your trip MPG.
Turn off the car, pop the hood and unplug your MAF. Start the car (you should get a CEL).
Reset your trip MPG thingy again.
Drive around the block again, try to mimic your last trip exactly.
Pull back into your driveway. Compare MPG.
Turn off car, pop hood, plug MAF back in, start and drive to clear CEL.
Fouling your MAF can *easily* drop you 20% on mpg. It will simply run rich enough to keep the motor safe.
Try disconnecting the MAF (obviously leave it in place on the intake, just unclip the wiring). Tuck the harness someplace safe and do your normal commute to work. See if the car runs any different.
To be honest though - you make have cooked it, since they are very sensitive. Also double check to make sure you have it screwed in in the correct direction - they have arrows imprinted on them for airflow direction. Off the top of my head I believe you could easily bolt it down in the opposite direction - I dont think it's shaped where it only allows the proper placement.
How many tanks of gas have you gone through since you've seen this drop of MPG?