# Troubleshooting Poor Gas Mileage (VCDS Content)



## Matti von Kessing (Jan 17, 2011)

I've only been getting 23-26 mpg in my Rabbit. I'm lucky if I hit 300 miles on a tank of gas. I know the 2.5L is not known for the best fuel efficiency, but this seems low compared to what other people have posted here.

I've been catching up with deferred maintenance. I changed out spark plugs with OEM replacement a few weeks ago. Tires are aired up to spec. Neither of those things have really had a huge impact on fuel efficiency.

I started playing around with my VCDS today and checked fuel trim. I essentially followed this protocol:






My adaptation settles in at 2.6%, with the lambda control oscillating a little north of zero, which doesn't appear bad. Running rich, as my fuel efficiency would indicate. Then I cleared codes and looked again: with adaptation at 0%, my lambda control was oscillating around 20%. Very similar to the video. Its too dark to poke around in the engine bay and look for a vacuum leak, but I'm not throwing any codes, nor do I have a rough idle. Just bad gas mileage and a kinda-high lambda.

Is there a vacuum diagram floating around anywhere for the 2.5L? Are there any vacuum points that are more prone to leaking than others on the 2.5? I didn't see one in my Bentley, but it could be hidden somewhere I didn't look. 

Any ideas of other things to put on the list to check?


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## mk6matt (Jan 26, 2013)

Is the 2.6% partial or idle? You may want to look at block 31 to actually get a lamda value (Lamda > 1 is lean). I would also measure the vacuum at idle and see if it is abnormal (using VCDS). Do you find your idle particularly high? or is there any extra hang in the revs? I know when I had a vacuum leak recently I couldn't get it to idle much bellow 1100rpm and the rev hang was terrible. 

As far as vacuum lines go I don't think there are that many, these are the ones I know of (some may not exist on certain years): 

-SAI
-Crankcase and VC vent.
-Line from N80 to throttle body. 
-There is a line on some models that goes to the manifold just above the alternator. I've never gotten a close look at that one since neither it or the SAI hose are on my engine.

To find my leak I used a propane torch. I turned on the gas and waited to see if I could see a bump in the idle. It bumped ~200rpm. Turned out my vacuum line for the W/M split a little. Brake cleaner also is supposed to work well. 

Also if your MAF-less then (as far as I know) the only leak that would matter would be one post Throttle.

Hope that helps a little :beer:


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## Matti von Kessing (Jan 17, 2011)

Yup...you're right. I got my lambdas backwards. I'm running mostly positive, which means I'm lean, which means the car is compensating by adding fuel (which is what I was calling "rich"). 

Anyway, the 2.6% is at idle. I have zeros in my partial load correction. I'm not having any particular problems with my idle: it is steady at 700-800. Can I measure vacuum at idle on a MAF car, or is that MAP only? With no other symptoms of a vacuum leak, I'm inclined to start looking elsewhere, but you've given me a good starting point if I need to start poking around. 

A false lean condition could also be the MAF sensor reading false low, right? I'm going to data-log blocks 1, 2, and 3 on my way to work tomorrow to see if anything support that, but wouldn't a fouled MAF sensor also change my partial load adaptation value? Only idle correction and no partial load correction sounds more like a vacuum leak. 

I'll report back in about 12 hours.


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## mk6matt (Jan 26, 2013)

I agree if the MAF was bad it would likely cause issues at partial throttle too. You can log your vacuum by looking at the manifold pressure (MAP). This will be given in absolute pressure (mbar I think). Subtract 101.325 mbar to go to gauge and divide that by 14.696 to get psi (gauge). 

To confirm a lean condition I would let the car warm up and then clear the memory and log MAP, rpm, block 31 and block 32. 31 will give you your lambda value (a percentage of stoic, λ>1 is lean) and you'll be able to watch and see how its trimming the fuel with 32. You can also confirm the idle rpm and pressure with those readings (I don't know the blocks off hand).


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## Matti von Kessing (Jan 17, 2011)

I have a MAF, not MAP...so logging vacuum is out.


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## mk6matt (Jan 26, 2013)

You will have both a MAP and a MAF. VW went solely MAP in 09 but the previous years had both :beer:


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## [DubSTeR]01 (Nov 13, 2002)

Your MAP sensor will be in your intake manifold under the TB.
They are known to the coated in oil.


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## Matti von Kessing (Jan 17, 2011)

Thanks guys. Did not know that.


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## mkV Fanatic (Oct 23, 2014)

[DubSTeR]01 said:


> Your MAP sensor will be in your intake manifold under the TB.
> They are known to the coated in oil.


Are you able to clean a MAP sensor like how you can clean a MAF sensor? I've been experiencing what I think is a failing MAP sensor (2010 2.5) at 60, 000 miles... :facepalm:. But if you say they are known to be coated in oil is it possible to just clean the sensor or better off buying a new one. Perhaps the Bosch one since it's $36 instead of $133... Or is OE that much better?

http://www.ecstuning.com/Volkswagen-Jetta_V--2.5/Engine/Electrical/Sensors/MAP/


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## [DubSTeR]01 (Nov 13, 2002)

Yes, most people pull them out and try cleaning them first with intake cleaner, just like doing a MAF sensor.
And once you pull your TB off most people use rags and stuff them into the intake manifold to try and soak up any excess oil sitting on the bottom.


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## mkV Fanatic (Oct 23, 2014)

Ok I'll probly try cleaning this one an run it for a little but I'm gonna buy a new one anyway cuz I'm sure this will happen again. Is this unusual to happen at only 60k miles? I figured these would hold up better than MAF sensors would. And good point on soaking up excess oil in the intake. Maybe if I do that this won't happen again for awhile. Thanks


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