Fuel Consumption Mystery: Troubleshooting Tips

I've often had the impression that my Passat TDI 90 hp, manufactured in 10/95, consumes significantly less fuel when the tank is full compared to when it's half-empty. Therefore, I recently conducted a test because I've been driving a lot over the past two weeks:

Route A: Dresden - Munich, 458 km. Dresden: full tank. Munich: full tank, 22 liters used. Moderate driving, not exceeding 140 km/h, no air conditioning. Calculated fuel consumption: 4.8 liters per 100 km.

Route B: Munich - Dresden - Hamburg, 1006 km. Munich: full tank. Hamburg: full tank, 68 liters. Driven more moderately, not exceeding 130 km/h, without air conditioning. Calculated fuel consumption: 6.8 liters per 100 km.

Consumes 2 liters per 100 km more fuel, despite even more moderate driving?
It's a very strange phenomenon, isn't it?
Do I now have to refuel every 500 km or so to ensure my car uses little fuel?
Perhaps one of you might have an explanation for this...

Thank you for your suggestions.

SunMan.

Two ideas:

Are you sure you always want to fill up completely? The fuel pump shut-off mechanism doesn't necessarily have to behave in a predictable way; this can explain differences in the amount of fuel dispensed, sometimes by as much as a few liters.

What about wind? If you've experienced headwinds once and tailwinds once, then it fits very well.

Best regards,

Jan.

Hi Jan6k!

Regarding headwinds/tailwinds: On the first leg, I mostly experienced headwinds due to the fuel-efficient driving.

Regarding fuel consumption: 20 liters for every 1000 km is an increase in consumption (so effectively, 20 liters for the last 500 km), which shouldn't completely empty the fuel tank, I think... it's probably a maximum of 5 liters or so.

I've noticed this phenomenon for quite some time now, but this time I had the opportunity to check it out in detail...

One (though perhaps somewhat weak) idea is that the constant 'fuel-efficient driving' might cause the engine control unit to make some 'unintended adjustments' (BMW and Mercedes, for example, have engine control units that adapt to the driving style, although this may only apply to automatic transmissions), or simply cause the exhaust system to become clogged, which in turn increases fuel consumption. There's a reason why people are advised to occasionally 'burn off' a car.
That would mean I would have to drive at full throttle for 30 to 40 kilometers approximately every 500 kilometers.

enigmatic, enigmatic...

Anyway, thanks a lot!

SunMan.

Hi,

Were the traffic conditions during your tests comparable?

I'm wondering how the fuel tank filling process could possibly be related to the amount of fuel injected – or rather, it shouldn't be, right?
Unless the tank is empty...

The traffic conditions were quite similar both times... both were consistent highway driving with moderate traffic... no traffic jams, no congestion.
The route from Munich to Dresden is actually a bit more hilly than the route from Dresden to Hamburg.
*?*

I have one more idea to add.

What would happen if there was some dirt in the tank? I didn't notice anything unusual in the 'circulation,' but could it potentially cause the engine to consume more fuel towards the end to achieve the same performance?

Hello,

"This only happens if you calculate fuel consumption using the MFA (Multi-Function Display), or if dirt prevents enough fuel from being drawn in, so that the maximum speed cannot be reached because only air is being sucked in, which may affect the injection timing and trigger an error message."

Best regards, Rainer.

Unfortunately, I don't have MFA.
The error memory shows no errors (it was last checked for errors 200,000 km ago).
I haven't tested the top speed on the Hamburg route, but this morning on the highway, I felt that if I drove above 130 km/h for a while and then floored it, it took much longer to reach 180/190 km/h compared to if I had already been driving at full speed for a while, then drove at 130 km/h for a few minutes, and then floored it again. <--- Is this an indicator of something?

'Thank you!'

No, I think you either made a calculation error or experienced headwinds, had the handbrake engaged, etc.

I'll repeat the test next time. Ideally, I'll run one test with DD-M and then refuel, and another with DD-M-DD and then refuel... let's see what results I get.
due to statistical significance

'Thank you anyway...'

SunMan.

Hi!

How accurate are the fuel pumps, actually?
I recently filled up a diesel tank in an A3 with 66 liters.

Best regards,
Uwe

Hi there,
The difference in fuel consumption might be due to the fact that Munich is located at a lower altitude than Dresden, meaning you were generally driving 'downhill' for the entire route (or 'uphill' from Munich to Dresden).

Definitely, your test will be more reliable if you always drive it in the same direction.

Hi Hans,

p.s.: please don't correct me on geography, especially if I'm wrong about the elevation difference between Dresden and Munich , as that's just a guess.

Hi,

So, to lift a car weighing a maximum of 1.2 tons by 500 meters, it shouldn't require that much fuel.
Otherwise, the roofer on the building across the street would have to have a tanker truck parked next to the brick truck .

Best regards, Rainer.

@rainer:
That's right.
This forum simply lacks the 'think again' warning message next to the submit button.

Hi Hans,

Hello.

I've never driven from Munich to Dresden. However, I've driven a similar highway stretch of about 50 km with an elevation difference of approximately 200 meters many times (around 100 times) here in Austria. The fuel consumption is about 7 liters per 100 km uphill and around 5 liters per 100 km downhill.
With a heavy and less powerful vehicle, you almost drive uphill at full throttle, and downhill it's more like letting it coast, even though you're maintaining a constant speed of 130 km/h.

Best regards,
Ernst