Low fuel mileage

My Average consumption dropped from 45.6 to 14.5 in 6k miles

http://s776.photobucket.com/user/Mathew_Bossinger/slideshow/

At 5300 Miles mileage dropped to ~ 32 so I changed the Oil to Pennzoil Ultra Euro 5W-30 L and it went up to 52mpg but I dont even have 4k on this Oil Change. I do mostly Interstate driving top speed is 80 Mph (Limits 75) Does every one else experience this fuel mileage loss at these miles? Tire Pressure is 40 PSI all around. I use Cruise Control on the Interstate so I don't get pulled over. I let it do its Regens, I took it to the dealer and they said it was Fine as long as no warning lights go off because it has a lot of sensors.

Holy crap. Temperature difference of 69*F to 11*F. Even that wouldn't drop you from 45.6 to 14.5 mpg. That 80 mph will kill your mpg. But not a 31.1 mpg loss. If still under warranty tell them to replace your stuck open thermostat. No insist they change it. Is there another VW dealership you can take it to. The one you were at is clueless. How many miles are you getting on a tank of diesel ? Maybe the brains aren't calculating your consumption properly but you're still getting like 600+ miles per tank.

It just started droping like a rock last week. Started at about 36mpg last monda Dec 16 It went down from 24.7 (12*)to 14.5 (11*) just today. I cant hand calculate it yet unless you want me to top off tomorrow? The next closest place I'm aware of is in the next state, its about a 60 mile drive one way.

Also I dont let it warm up, I start and drive just like the manual says.

How many miles on average have you been getting on a tank of diesel ? Starting and driving as soon as possible is the right thing to do. I'd first go back to the dealership that said nothing is wrong. I'm sure you've told them the problem. Insist that they change the T-stat. Are you getting any heat from the vents in the car ? With that drastic an mpg drop something is really wrong here. Hopefully someone else will chime in with some more suggestions on things that may be amiss. Do the new Passats have a temp gauge and if yes what is it showing after driving enough time that it should be at full operating temp on the coolant ?

Run this tank dry. Get fuel somewhere else and report back.

dzcad90 said:

Run this tank dry. Get fuel somewhere else and report back.

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Yes. That would be really crappy diesel.
But I wouldn't run the tank dry. Not good for the fuel pumps. I'm sure you didn't mean to say dry as in no fuel left.

I usually get about 575-700 depending on how much of a hurry im in and how big of a vehicle I can draft. I have always used nothing but mobil 1 diesel as I get the best mileage out of the local choices. Water temp is the normal 190 and heat works fine. I used over a quarter of a tank before my mileage tanked. I also use howes diesel treatment which boosted my milage.

09mlb86 said:

I usually get about 575-700 depending on how much of a hurry im in and how big of a vehicle I can draft. I have always used nothing but mobil 1 diesel as I get the best mileage out of the local choices. Water temp is the normal 190 and heat works fine. I used over a quarter of a tank before my mileage tanked. I also use howes diesel treatment which boosted my milage.

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If you're showing 190*F and good heat from the vents then your T-stat is working as it should. 575-700 miles per tank is very good. Sounds like you have a problem with your brain that calculates your mpg. Have the dealership fix it under warranty.

You are looking at average consumption 1 on your MFI. That is a calculation for the current trip, it will reset to 0 after the vehicle is parked for 3 (I think) hours.
Press the OK button on the wheel and see what average consumption 2 shows, that is your running consumption for the tank, or since new if you havent reset it.
You can reset each of those by holding the ok button in whlie the figure is displayed.
I reset both of the average consumption figures with each tank.

Bobmws said:

You are looking at average consumption 1 on your MFI. That is a calculation for the current trip, it will reset to 0 after the vehicle is parked for 3 (I think) hours.
Press the OK button on the wheel and see what average consumption 2 shows, that is your running consumption for the tank, or since new if you havent reset it.
You can reset each of those by holding the ok button in whlie the figure is displayed.
I reset both of the average consumption figures with each tank.

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^^^ This is my bet as to what's going on ^^^

I reset my #2 AVG's at each tank fill.. He's right the #1 trip AVG's reset at 2-3hrs, the #2 Trip AVG's have to be manually reset, they will automatically reset, but it's a really long interval, I think it's at 9,999 miles or 99 hours (or something like that. It's all well covered in the owners manual)

575-700 miles/tank is pretty darn good, just doing the numbers in my head, but that should calc out to mid to upper 40's mpg (if you have a 14.5 Gal tank like my Jetta)

Average 1 resets after the engine has been off for 2 hours. If you're going to show us what Average 1 says, you also need to show us what Route 1 shows for miles driven (current trip).

Also, 575 miles per tank would be considered on the poor side of things, 700 would be average. Remember that the Passat has an 18.5 gallon tank. If you refuel when the low fuel light comes on, you're probably looking at 16.5 gallons.

575/16.5=34.85 MPG
700/16.5=42.42 MPG
800/16.5=48.48 MPG

Cold and adverse conditions knock the socks off FUEL mileage. FUEL MILEAGE!

That said I suspect somewhere just south of 40 mpg average unless for very short commutes with no chance of a warmup.

Cogen Man said:

Holy crap. Temperature difference of 69*F to 11*F. Even that wouldn't drop you from 45.6 to 14.5 mpg. That 80 mph will kill your mpg. But not a 31.1 mpg loss. If still under warranty tell them to replace your stuck open thermostat. No insist they change it. Is there another VW dealership you can take it to. The one you were at is clueless. How many miles are you getting on a tank of diesel ? Maybe the brains aren't calculating your consumption properly but you're still getting like 600+ miles per tank.

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Cogen Man said:

How many miles on average have you been getting on a tank of diesel ? Starting and driving as soon as possible is the right thing to do. I'd first go back to the dealership that said nothing is wrong. I'm sure you've told them the problem. Insist that they change the T-stat. Are you getting any heat from the vents in the car ? With that drastic an mpg drop something is really wrong here. Hopefully someone else will chime in with some more suggestions on things that may be amiss. Do the new Passats have a temp gauge and if yes what is it showing after driving enough time that it should be at full operating temp on the coolant ?

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Let's not jump to conclusions here. First things first, is the vehicle actually delivering poor fuel economy or is the OP being mislead by his interpretation of what the MFI is telling him?

So far we can't tell because he hasn't provided us with some key information...

• Route mileage for MFI 1
• MPG and Route mileage for MFI 2
• Pen & Paper calculations for an entire tank
• Picture of trip odometer that includes a picture of the fuel gauge

VeeDubTDI said:

Let's not jump to conclusions here. First things first, is the vehicle actually delivering poor fuel economy or is the OP being mislead by his interpretation of what the MFI is telling him?





So far we can't tell because he hasn't provided us with some key information...

• Route mileage for MFI 1
• MPG and Route mileage for MFI 2
• Pen & Paper calculations for an entire tank
• Picture of trip odometer that includes a picture of the fuel gauge

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From the three pictures that the OP linked to, the OP is completely relying on the MFI to get his/her TDI's fuel economy numbers (two of the pictures had the exact fuel economy that he/she mentioned in their first post).

bubbagumpshrimp said:

From the three pictures that the OP linked to, the OP is completely relying on the MFI to get his/her TDI's fuel economy numbers (two of the pictures had the exact fuel economy that he/she mentioned in their first post).

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Yes, but the pictures only tell part of the story because MFI 1 resets after the engine has been off for 2 hours. So we really have no idea how many miles those pictures are covering.

If you want to rely on the MFI for fuel economy numbers, then MFI 2 is the better option (as long as you reset it with every tank).

If his fuel economy had seriously dropped to 14.5 MPG, he would be out of fuel after only 250 miles.

VeeDubTDI said:

Yes, but the pictures only tell part of the story because MFI 1 resets after the engine has been off for 2 hours. So we really have no idea how many miles those pictures are covering.

If you want to rely on the MFI for fuel economy numbers, then MFI 2 is the better option (as long as you reset it with every tank).

If his fuel economy had seriously dropped to 14.5 MPG, he would be out of fuel after only 250 miles.

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Yup. That's what I was getting at. For the average person (myself included) that hasn't read their owners manual enough to know how the MFI works...it's completely useless as a fuel economy reference.

bubbagumpshrimp said:

Yup. That's what I was getting at. For the average person (myself included) that hasn't read their owners manual enough to know how the MFI works...it's completely useless as a fuel economy reference.

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It all depends. It can be very useful if you know how to use it. I have found its calculations to be very consistent.

I just love people who can't hand calculate math problems and rely on computers to do the work for them...

VeeDubTDI said:

Let's not jump to conclusions here. First things first, is the vehicle actually delivering poor fuel economy or is the OP being mislead by his interpretation of what the MFI is telling him?

So far we can't tell because he hasn't provided us with some key information...

• Route mileage for MFI 1
• MPG and Route mileage for MFI 2
• Pen & Paper calculations for an entire tank
• Picture of trip odometer that includes a picture of the fuel gauge

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Not jumping to any conclusions. Just asking questions to try to drag some of those key info points out of the OP that you mention. By telling the Op to have the T=stat changed maybe the dealership would show him how the MFI stuff works.
Sometimes it's like pulling hen's teeth to get the needed info to give help on a forum. We should all just say. Take it to a freaking mechanic.

Just got the Car back from the dealership today. They said they tested my Diesel fuel and said it was just a bad tank. Back up to 39.5 already. Thanks for all the replies. As a side note, I was using Mobil 1 Diesel that I would drive 8 miles out of the way to get to but I wont be using that brand any more.

09mlb86 said:

Just got the Car back from the dealership today. They said they tested my Diesel fuel and said it was just a bad tank. Back up to 39.5 already. Thanks for all the replies. As a side note, I was using Mobil 1 Diesel that I would drive 8 miles out of the way to get to but I wont be using that brand any more.

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Did they change the fuel filter? What exactly did they say was wrong with the fuel? How did they test it? Did you see the test results? Post a copy if you can.

The reason I ask, even though it's a Passat, I'd be a little concerned about the impact this may have on the HPFP especially if they say it was gasoline contamination.

I just cannot imagine how they could have "tested" it properly that quickly, I'm certain they don't have the necessary lab equipment to do it properly and can't imagine how they could have sent it off to a lab and gotten results that quickly over Christmas... Brings back memory's of the styrofoam cup test for gasoline..

Or they kept the car for a day. Let the mfi reset. Charged VW a couple hours of warranty work without touching the car. Handed the owner the keys and blamed bad fuel. Owner is happy, dealer is haopy , life goes on.

kydsid said:

Or they kept the car for a day. Let the mfi reset. Charged VW a couple hours of warranty work without touching the car. Handed the owner the keys and blamed bad fuel. Owner is happy, dealer is haopy , life goes on.

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LOL.. Knowing dealers, that makes the most sense so far.

09mlb86 said:

Just got the Car back from the dealership today. They said they tested my Diesel fuel and said it was just a bad tank. Back up to 39.5 already. Thanks for all the replies. As a side note, I was using Mobil 1 Diesel that I would drive 8 miles out of the way to get to but I wont be using that brand any more.

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How did they determine that it was bad fuel? Do they have a testing facility in their shop?

What a load. Glad your dash is displaying numbers that you're happy with again.

kydsid said:

Or they kept the car for a day. Let the mfi reset. Charged VW a couple hours of warranty work without touching the car. Handed the owner the keys and blamed bad fuel. Owner is happy, dealer is haopy , life goes on.

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That brings up an interesting point. The whole "could not duplicate problem" thing. I wonder how many technicians who have written that up even tried to duplicate the problem?

09mlb86 said:

Just got the Car back from the dealership today. They said they tested my Diesel fuel and said it was just a bad tank. Back up to 39.5 already. Thanks for all the replies. As a side note, I was using Mobil 1 Diesel that I would drive 8 miles out of the way to get to but I wont be using that brand any more.

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In the Omaha area, Performance VW supposedly has one of the top Diesel mechanics in the country. Thought I had a letter from VW sometime back about mileage loss issues. For the life of me, I haven't found it.

My mileage hasn't been horrible, but it has dropped some. I don't push it that hard and most of my driving is in town lately (60/40). My avg in town by the MFI is upper 30's but I know this is about 2-3 miles to high based on calculator-in-hand at the pump. So realistically, my in town is more like 35-37 mpg. Highway was see high 30's to low 40's but rare lately.

Granted it's been in the 20's 'F (-4 'C) .. and we did have a couple of days sitting around -11 'F (-23 'C).

I did get about 53 mpg towards the end of a 300 mile drive from Chicago to Omaha. Illinois has a 60 mpg speed limit and Iowa jumps to 70 (or 73 ;-)).

By the time head winds and the faster speeds in Iowa started.. my best ever mileage was 49.2 mpg (MFI showed 53 but calculator at the pump was 49.2).

If I find the fuel letter.. will let you know.. meanwhile my winter driving (w/snows blizzaks LM-32) and temps in to 20's, 12 mile ride to work .. sits in the mid-to-uppers 30's (at the pump). Still seems low.

Local diesel stations: Shell, Hyvee, Sinclair, other .. most move diesel at a reasonable pace and in town not in the sticks.

http://www.fuelly.com/driver/midwesttdi/golf