ok so i have been experiencing bad highway gas mileage. 19 to 20 in town, only 25ish on the highway. i have had it up to 31 before. i tried changing spark plugs already, to no avail. i left them at their stock gap settings, so that could possibly be an issue. i had one dtc in with the cel lit up a couple weeks ago. it was p0171, system too lean bank one. that is why i chnaged plugs. the maf was replaced last september with a used unit with 56,000 miles on it. i have put on 12,000 since then so that could be athe issues.
anyways, i got p0171 come up today again. it is in limp mode, only boost up to 4psi. i dont know if the gas mileage and code are the same cause because a lean code means not enough gas, not too much. i think ill try cleaning the maf tomorrow, other than that i wanna hear what you got. incase your a noob and cant tell by my car by my avatar, its a 01 1.8. not that it matters but its a manual. no engine mods.

 

Well besides getting worse gas mileage then your G ma's wheel chair.....
Cross these off if you have thrown these in. Fuel filter, air filter, check tires, run some seafoam, checked the cats and 02 sensors should be fine if no codes but what the hell. Use the cruise . I know you may know most of this stuff but maybe not .
Pay attention on your driving habits. Go a little easier on it and don't let it idle if your doing it already.

 

haha very funny you are! fuel filter and seaform are of no concern. ran well last fall, started acting up this spring. the cruise was used the whole way up to the casino and back. i only passed a couple cars and kept it under 90 the whole way. i wasnt racing, mostly cause i was in limp mode. the gas mileage is a concern, yes, but i am more concerned about the engine code. i had this same code last september and, with the gracious help of AUDIFORUMS, was able to figure it was the maf. thats why it was replaced. I was previously informed if it was an o2, i would have gotten an o2 specific code. mind you it took a couple weeks for the code to generate again. it is kind of a long term thing, thats why i think it might be my plug gap, but not sure

 

i would change your plugs first, then look at your primary O2 sensor. since you get a code for it every once in a while it could be on its way out and reading lean even if you are actually running rich. if your plugs are fouled/blackened more then they should be that would be another sign.

 

thats the thing. i know a lot about sleds, and i let the same knowledge hold true for my car. for sleds, the best color for plugs is cardboard brown and thats what i had in my old plugs, but i still changed them as per the recommendation of this website. i still got the lean code on the old plugs though. i really dont wanna start throwing random parts at it to see if it works, especially something like a o2 sensor that costs a lot. ill pull the plugs tommorrow and if anything is awry, ill post pics, but im guessing theyll be fine. and like i said ill buy maf cleaner but i doubt it will do anything. its not like this problem is a huge deal, but i want the car to be running perfectly without mil's. p0171 is the only code i pulled by the way. zero misfires.

 

well what you could do to narrow it down even more is get a copy of VAGCOM and log your O2/fuel and MAF readings. if the MAF readings are low/erratic, that would be the problem. if they arent, i would bet its the primary O2. when was it last replaced if ever? 100k miles is a good replacement interval for O2 sensors. couldnt hurt to pull and regap the plugs first i guess.

 

i would get vag com but it costs $$$.... i got a couple exra bucks because of the summer job but i was plannin on savin it. i will pull the plugs tomorrow and gap them correctly (plus delete the code) and post if i see any odd coloring. my current milage is over 127 so i know they (the o2 senors) should have been repaced by now. the thing is i asked the only other owner the other day if he replaced the clutch before cause its about time to go out and he kind of hesitated on the answer meaning he doesnt even know about cars so he is of no help. what throws me off is the milage/performance in town is normal, but on the highway it gets goofy.

 

Checking for boost leaks is easy enough and cheap to do (tennis ball method). When I had a boost leak, I was getting a system lean code and poor gas mileage. And the highway is where you will be boosting the most.

 

[QUOTE]ran well last fall, started acting up this spring./QUOTE]

IAT sensor?

 

well no need to worry anymore i guess. i was going to try a few things yesterday. i stopped at the store and when i turned the car back on the mil went away. so im not really going touch it unless you guys think its a better idea to.