Horrible MPG in 03 Manual TDI Wagon

I just picked up a 2003 Manual 5-speed Wagon and got an Ultraguage in the mail today. I plugged it in and was depressed to find that my average mpgs is around 20 with about 50% city and 50% freeway. On the freeway I am lucky if the mpgs gets up to 35mpg.

I have two bad glow plugs, and code P0106. I cleaned out the air intake and MAF to see if that would help. The code went away, and hasn't come back yet.

My new glow plugs are coming tomorrow. I am on my first tank so I haven't confirmed the mpgs, but I am about at a half tank and have travelled about 213 miles (29mpg). I was hoping to see higher numbers on my new vehicle...

Does the Ultra gauge need to be calibrated?

Also, have you done any hand calculations on fuel used Vs. miles traveled?

I wouldn't believe that number until you run a full tank through the car and check it.

is that a full, vented fuel tank, or just till the pump clicked off.??

The difference can be several gallons of fuel.

whitedog said:

Does the Ultra gauge need to be calibrated?

Also, have you done any hand calculations on fuel used Vs. miles traveled?

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The only hand calculations I have are above. I haven't calibrated the ultraguage, but I was under the impression that would only be a nominal difference. I will do that tonight.

speed185187 said:

is that a full, vented fuel tank, or just till the pump clicked off.??

The difference can be several gallons of fuel.

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Just till the pump clicked off. These have a 14.5 gallon tank, right? I will have to look into the vented fuel tank option. I just got this car last week, so I'm not sure how to do that.

Vent the tank, reset the ODO and see how far you go..

sammyman said:

The only hand calculations I have are above. I haven't calibrated the ultraguage, but I was under the impression that would only be a nominal difference. I will do that tonight.

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There is a more than nominal difference. Mine was way off until I calibrated it. About 25%, at least, as I recall.

sammyman said:

Just till the pump clicked off. These have a 14.5 gallon tank, right? I will have to look into the vented fuel tank option. I just got this car last week, so I'm not sure how to do that.

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A set of pictures is available here that details the procedure.

Basically there is one screw holding the rubber shield around the filler neck in place. After opening the fuel cap (as you would to fill the tank), you unscrew that screw and gently pull the shield out (I just let mine droop down, you may want to wrap it with a rag so it doesn't touch the paint directly, if you're picky).

There is a tube sticking out of the filler neck towards the rear of the vehicle. Inside it is a plastic assembly with a pressure relief valve in it. You gently push that out (not in, unless you WANT it to fall into your tank).

You then remove the spring and other guts and put the now empty white plastic tube back in the side of the filler neck where you got it from, gently put the rubber guard back over the filler neck, replace the screw, and put the fuel cap back on. All done.

I'm gonna vent it tonight. Very cool. If it is already vented then I am probably getting worse mileage than I am estimating.

I also live at the top of a very large hill which may contribute to bad mileage.

Also, good to hear the ultraguage could be way off. I'm off to the garage to vent and calibrate. I'm keeping my fingers crossed that I don't have a major issue here.

sammyman said:

I'm gonna vent it tonight. Very cool. If it is already vented then I am probably getting worse mileage than I am estimating.

I also live at the top of a very large hill which may contribute to bad mileage.

Also, good to hear the ultraguage could be way off. I'm off to the garage to vent and calibrate. I'm keeping my fingers crossed that I don't have a major issue here.

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I tend to open up my TDI a lot in the mountains and I still get above 30 mpg lol.The only time I have ever dropped below that is when I am full out racing around hair pin turns or running on dirt.

It's possible, too, that you might have at least one brake dragging or a wheel bearing that might be going bad, too. Those will both contribute to poor fuel ecconomy.

My correction factor is -53%, i.e. out by more than a factor of two.

I an not sure why the car currently being vented or not could possibly have affected the mpg calculations, you look at miles-driven versus fuel-dispensed and that's it. The only time it could affect the figures is when one tank is vented and the next is not or vice versa, and in that case the two tanks would still be correct in aggregate.

Ultimately you just need to be patient before finding out what the typical mpg is like, it takes a while before you can get accurate figures simply because you really need at least a couple of tanks. The first tankful is unreliable because you don't know how much was in the tank to start with.

As well as the city versus highway balance, you also needed to consider average journey length to know what mpg to expect... Diesel engines take a long time to warm up.

Run thru about 5 vented full tanks of fuel and then decide what kind of mileage you're getting. 1 or two tanks prove very little.

"I also live at the top of a very large hill which may contribute to bad mileage."

Remember that hills work both ways.

Sammyman, I just read this thread twice and my understanding is that the tank has never been re-filled? Then you claimed to use hand calculations but how do you know how much fuel you have used? Could you post your raw data numbers so we can check your calculations?

I agree with the guys above- wait until you run at least a few tanks of fuel through it to determine your MPG's. Make sure you either remove the vent guts, or vent it by hand each fill up. That is the only way you can be absolutely sure you fill it up the same way and with the same amount each time, otherwise it's just an approximation. Do hand calculations for your MPG's everytime, and use your Ultragage for refference until you have it dialed in. Also, if your Ultragage is anything like my ScanGageII than it reports accurate MPG results only when I drive it in town since thats where it is calibrated most accurately-If I drive on the hwy which is out of the norm for me, the MPG reports as being very low(35MPG) and I know for a fact I am actually at 55 MPG under those conditions. You may want to go through the A4 Faq section on the forum and study up about these engine platforms if you aren't familiar with them. The Faq contains some info on common problems with these vehicles to watch out for that can affect fuel economy:

http://www.tdiclub.com/TDIFAQ/


Also, like your doing, stock the forums and dig in and you will discover many new things about the car that can be maintained so that you will be rewarded with ultimate fuel mileage.

..

Yeah, and whatever you do don't do something like put larger nozzles and a better intercooler in the car... while both of those can produce better mileage, they only do so if your right foot remains under control, and mine is definitely not.

I can easily go from ~42-44 around town to upper 30s simply by changing how much I want to smile..... Likewise, if I am willing to drive 55 (I'm not) I can get low 50s on the highway.