Diagnosing Fuel Economy Issues

[fullmetalGTI]

Hello all,

New here and hoping some of you may be able to help me.

I've been having issues with my 2009 Manual Transmission Base model GTI. For almost a year now my MPG has been a lot less than it used to. It decreased rather suddenly, from getting an average of about 30mpg between freeway and local driving, to an average of about 25mpg. I'm not sure what I used to get out of a tank, but now I usually get about 300 miles or less per tank, and from what I've read other people are getting about 350.

About 4 months before this started I had a check engine light for a bad Oxygen sensor, so I got a new one put in. I was in CA near San Francisco at the time. In the 4 months after that I drove across country and moved to Norfolk, VA. I mention this because I'm not sure if this change in location affected anything.

Since the decrease I have tried many things in an attempt to fix the problem, including replacing the spark plugs, running seafoam through the system, installing a new fuel filter, new air filter, and changing the oil. None of these things have worked.

Recently I had some engine trouble with my #3 Cylinder misfiring. When I took it in they had to clean the exhaust system because of excess carbon buildup restricting the exhaust flow and they had to change an ignition coil.

With all the information I have, I have theorized that the Oxygen sensor is the problem somehow. I think that it might be causing my engine to run too rich, leading to the high MPGs and excess carbon buildup which caused the other problems with my engine. The thing that gets me is that I have had no CEL for the oxygen sensor since the new one was installed, and I have taken my car into two shops to try and diagnose the problem, one local and one VW dealer. Neither could figure out the root cause of the problem.

Sorry for the really long explanation of what's going on. I just want to be able to get my car fixed so that she is back to normal. She hasn't been operating at full capacity for a while and she feels sluggish, even after I took her in for the misfiring cylinder.

Any help would be greatly appreciated!!

 

[eurocars]

How many miles? When you said they "cleaned the exhaust system", do you mean they cleaned the intake valves?

 

[Eric6014]

I'm in the same boat. Mine is worse than yours. I get about 15 or 16 driving around town 13 or 14 if I drive like an asshole (which I usually do). Most I ever got since I had the car was 26/27 on a long highway trip. I've been told I need to get my valves cleaned and that should help a lot. Just haven't gotten around to it yet cause it ain't cheap. I've replaced the fuel filter, coils and plugs. Didn't help at all.

 

[ROH ECHT]

Most of the time...a new MAF and/or the N75 does this. Collecting data will help determine if the N75 is suspect...the Maf and first O2 are a bit more difficult to detect if not completely shot yet.

 

[fullmetalGTI]

How many miles? When you said they "cleaned the exhaust system", do you mean they cleaned the intake valves?

Okay, so. Here's what they did. They installed a new fuel filter, used a fuel injector flush kit to clean the fuel system and injectors, inspected injectors and combustion, and they decarbonated the intake valves and chambers.

Most of the time...a new MAF and/or the N75 does this. Collecting data will help determine if the N75 is suspect...the Maf and first O2 are a bit more difficult to detect if not completely shot yet.

Appreciate the input ROH. What are the MAF and N75, and how can I go about troubleshooting? None of the shops that I have been to have been of any help so I guess it's up to me to figure out the problem so I can determine the solution.

 

[ROH ECHT]

The N75 is your boost controller...if you compare actual to specified boost or could view your boost on a boost gauge...you'd know if you need a new one. MAF could also have data viewed and tell you if it is reading at full potential or not. Both of these can cause poor fuel mileage. You can search these things and research the symptoms when they begin failing.
A bad 1st position O2 can do this and could also be checked along with MAF, boost, fueling, and all else with vcds/vag-com...which I assume you co not have. You could either begin replacing everything or purchase Ross-Tech's vag-com cable and download vcds for free and check all of your systems yourself.

 

[fullmetalGTI]

The N75 is your boost controller...if you compare actual to specified boost or could view your boost on a boost gauge...you'd know if you need a new one. MAF could also have data viewed and tell you if it is reading at full potential or not. Both of these can cause poor fuel mileage. You can search these things and research the symptoms when they begin failing.
A bad 1st position O2 can do this and could also be checked along with MAF, boost, fueling, and all else with vcds/vag-com...which I assume you co not have. You could either begin replacing everything or purchase Ross-Tech's vag-com cable and download vcds for free and check all of your systems yourself.

Thanks ROH. Appreciate your help. Looks like I'm going to have to get a vag-com so that I can figure this out properly. It will be good to have for future issues too so easy to justify the purchase.

I'll update you all once I successfully fix my baby.

 

[gdub9]

I had the same issue, was getting near 25-30mpg (city/highway), bought my intake and its gone down to 20-26. Not really too upset about it. I normally just reset my mpg pages on my MFD when I fill up and drive highway for a little bit to keep em high to make me feel better lol.

 

[flat tire]

That would likely be due to improper maf scaling, you will get better mileage in that case if you just unplug the maf. Or maybe you're just driving a lot more aggressively to hear your intake noises.