Mileage drops from 44 to 34 mpg

A few weeks ago I managed to get 44 mpg with a cetane additive on a highway road trip on a cold, windy day. Speeds ranged from 35 mph in the snow drifts to 70 mph on the open freeway (2003 VW GLS wagon, automatic).

Since then a loose tie rod was replaced and the front end straightened so the drift to the right was corrected. I also had the two tires with the slow leaks seated and sealed properly.

I was expecting a slight improvement with these changes, but on this last road trip, the mileage dropped to 34 mpg. It was another windy day with some snow drifts and icy patches, but nothing as bad as the prior trip. The return trip over the same route on a wide open road yielded a little better 36 mpg.

Fuel, terrain, driving behavior, load, etc. were essentially the same coming and going.

The first few road trips I was getting 39-41 without the additive.

There isn't a fuel leak, so what could cause such a dramatic change?

You say it was windy ... if you had a tailwind on the first trip, and a headwind on the second, then there would be a big difference in MPG between them.

I would check wheel alignment/tracking/toe given the recent history.

I suppose in Wisconsin the winter additive is standard everywhere, but did you fill up anywhere South? Did you fill up at the same location? Just wondering if the winter additive formula was the same each time.

My wife's 02 Wagon auto has had the same mileage drop this winter.
I aired the tires up to 40 psi and the next tank was 38 mpg. She will be filling again soon so I will have more data.
I think a combination of cold, blustery weather, short local drives and winter fuel are the biggest culprits along with lower tire pressure

i hate these types of threads.

detroitmike said:

i hate these types of threads.

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Yeah, it's definately a FAQ. Somebody should write a sticky covering this stuff ...

The fuel is exactly the same as is the additive.

Wind was predominantly from one side or the other - not a head or tail wind.

I may be a newbie on this forum, but I'm certainly not a newbie to driving. I've combed through the FAQs and there is nothing specific to a significant drop in MPG when the conditions are essentially the same.

If the answer is so simple, why not point to it rather than a holier than thou non-answer?

chzweeth said:

...
If the answer is so simple, why not point to it rather than a holier than thou non-answer?

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Nobody said it was simple (that's what makes it difficult to answer). But the simple fact is that we don't have a sticky/informational post on here which at least says the basics.

So what exactly is the problem with us pointing that out? Why get defensive?

chzweeth said:

The fuel is exactly the same as is the additive.

Wind was predominantly from one side or the other - not a head or tail wind.

I may be a newbie on this forum, but I'm certainly not a newbie to driving. I've combed through the FAQs and there is nothing specific to a significant drop in MPG when the conditions are essentially the same.

If the answer is so simple, why not point to it rather than a holier than thou non-answer?

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It's not always so simple. All the above is good advise.
Crosswinds are almost as bad as headwinds, it seems.
Try it w/o additive. Really, I do better w/o additive and I pay a lot of attention to the mileage and mine has been very good this winter. It's still been below 0F most morings and this will be another 800+ mile tank. 52ish mpg again. Commuting 40 miles one way.
Make sure the autofail 01M tranny is actually going into 4th gear and the torque converter is locking up properly. (any CEL/codes?)
Short drives in cold weather kill mileage, how long were the trips?
Verify that the coolant temp is actually getting up to normal. The dash gauge is no real indicator, it will read 190 when the coolant is anything above 165, which is far from ideal.

Thanks for the response.

The 44 mpg trip was 505 miles with 1/3 off cruise on two lane, twisty-turny roads along the river and ridges @ 55 mph or less, 1/3 on divided 4 lane on cruise @ 70 mph, and 1/3 off cruise with snow drifts @ 35-45 mph.

The 34 and 36 mpg runs were 275 and 319 miles respectively on the same 4 lane freeway. The outbound was mostly on cruise @ 70 mph with some lowering due to the drifts and ice; the return was about 40 miles in town and then on cruise @ 70 mph on clear roads.

It was actually windier on the 44 mpg run and more into the wind, with far more changes in speed due to the roads and terrain.

If I had seen a modest change in mpg, I might chalk it up to the conditions; but when the conditions are worse and the mpg is better, that has me puzzled.

I started using the additive because the prior owner used his own homemade bio-diesel and the car also sat for a year. Yes the fuel filter was changed and it wasn't too bad.

The timing belt, water pump, turbo and fuel injectors were also recently replaced by the previous owner.

I also checked the cetane of the fuel available in my area and found the winter blends were in the 40 to 42 range. In all the years I've driven diesels, I have only used the dirt cheap truck stop diesel when there was no other alternative.

The car also had some hard starts when it was parked overnight in the cold we've been recently experiencing. I changed the oil from the 10w/30 to the 5w/40 and that has helped.

Prior to the 44 mpg run, I was getting 39-41 mpg on the highway without the additive and the repair to the tie rod and tires.

My thoughts are I have an injector that isn't working properly or a minor fuel leak (the pavement is dry and I had them look for leaks when it was last in for service).

Homebrew bio could well have damaged the injectors/nozzles and would explain some of the low fuel mileage you are getting. Poor spray patterns can make for big issues over time.
Look into the tranny shifting properly and TCC working as it should.
How are you filling the tank?? Search "ventecomy" and how it will make your fuel fill ups more accurate. A couple qts more/less per fill, especially on a partial tank, will make for widely varying mpg results.
Injection timing. Depending on where its set (TB was done by owner using homebrew bio, so I question it) can result in poorer cold starts and less than optimal performance/mileage.
So there are lots more possibilities and not always so easy to answer a question point blank.

jettawreck said:

Homebrew bio could well have damaged the injectors/nozzles and would explain some of the low fuel mileage you are getting. Poor spray patterns can make for big issues over time.
Look into the tranny shifting properly and TCC working as it should.
How are you filling the tank?? Search "ventecomy" and how it will make your fuel fill ups more accurate. A couple qts more/less per fill, especially on a partial tank, will make for widely varying mpg results.
Injection timing. Depending on where its set (TB was done by owner using homebrew bio, so I question it) can result in poorer cold starts and less than optimal performance/mileage.
So there are lots more possibilities and not always so easy to answer a question point blank.

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JW: These are key and often overlooked issues as far as mpg's go.
My trans TC did not lockup in 4th for the 1st 76k miles I owned the car and I was clueless about it. Never saw better than 35-37 mpgs.
One day it started locking up and mpgs rose to 40-42.
Since then it has varied from 34-46.
My wife never waits for the foam to settle so her mileage runs are always more than they actual would be if she brimmed it.
It does not take much to radically change your numbers.

Yes, I'm very much aware of the issues with foaming when fueling up. This may be my first VW TDI, but it isn't my first diesel.

The car now has 160k, so we're well past breaking anything in.

The turbocharger, timing belt, water pump, etc. as well as the nozzles were replaced about 30k miles ago.

As I drive I keep an eye on the rpms. I've been pleasantly surprised at how low 2,600 they are at freeway speeds, even up steep grades.

MAF, fuel temp sensor, etc.?

chzweeth said:

Yes, I'm very much aware of the issues with foaming when fueling up. This may be my first VW TDI, but it isn't my first diesel.

The car now has 160k, so we're well past breaking anything in.

The turbocharger, timing belt, water pump, etc. as well as the nozzles were replaced about 30k miles ago.

As I drive I keep an eye on the rpms. I've been pleasantly surprised at how low 2,600 they are at freeway speeds, even up steep grades.

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Just because you are aware of the diesel foaming issue, does that mean you fill to the same level each time or just let it settle down and let the pump shut off? There is a lot of variance in the VW fuel tank due to the expansion tank that is in the system. They use the same tank system for the gasser models.
I don't know what rpm the auto runs at "freeway speeds" (whatever that is maybe anywhere in the 65-85mph range), so that is useless info. Maybe post an rpm at a specific speed, such as my 5 speed runs about 2100 at 60 mph.
Haven't answered the injection timing question(s). Some of your issue point directly to retarded timing, in my opinion. TB and other parts were replaced be previous owner. That may/may not be a good thing. Just because he perhaps seemed to know VW TDIs enough to work on them doesn't mean he really did things correctly. Many, many of these engines have been brought to an early death by TB changes that were done incorrectly.
I have no fondness for the 01M automatics, but you should be getting better numbers than you appear to be.
You've been given some good things to check out by Mike and others, as well as plenty of things to search out in the FAQs/stickys, but instead seem to be looking for a easy cure.
Best of luck getting it figured out. JasonTdi is in Madison, WI and is a very respected TDI guru. If you don't have VCDS and can post some logs (coolant temp/timing graph/etc/etc), I would suggest taking the car to one of the forum gurus.

Yes my 5 speed turns about 2100 at 60 too.
The 01M is at least 200 rpms higher in 4th.
When the TC was not locking it would be even higher and would "hunt" around 100-200 rpms. Definitely bad for FE

Wife just filled up; 40.5 mpg

I appreciate all of the efforts to help. For those that find these threads boring or whatever, my sincerest of condolences.

I think I discovered and corrected the issue. It is one of the variables mentioned in my original post and one that I assumed was a constant. If I am correct, it is also a simple correction, as I suspected.

We'll see on a short run of 250+ miles planned for later this week.

And yes, as ALWAYS, I will top off the tank and let the foam settle at the start and again at the end for as accurate an MPG calculation as possible.

Care to throw us a bone, or going to keep us in suspense until the outcome?
Inquiring minds want to know.

This last run was 300 miles exactly to and from the same diesel pump. Before the trip, I fueled it up to the tippy top w/ cetane additive as always. I did the same on my return.

Conditions were cool, dry with little to no wind. About 50% of the trip was on the interstate @ 70 mph, about 45% on 2 and 4 lane highways @ 55 mph and the other 5% in town with a fair number of stops and some traffic.

The result: 44.7 mpg

The ONLY other significant change was the tire pressure.

The technician that fixed the poor seals and the one bad valve on the tires that were the source of the slow air leaks only filled them up to 33 psi. I had assumed the tires were fully inflated to the proper pressure of 44 psi.

These are Firestone FR380 All Season 195/65R15 tires and they came with the car when I bought it. They have some tread left so I'll drive them for a bit, but I plan to replace them before next winter.

So it was something rather simple after all

Back to normal, winter diesel is done and snow tires are 1/2 off[I left the fronts on for mud season].
Latest fill 42.5mpg

I wish winter was done. 10F this morning and 3-6" snow/slop forecast for tonite just to make the drive to work interesting.
Last tank was 52.2 mpg. 828 miles.