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I replaced the fuel filter on Petra last weekend (its a pretty easy job and my thanks to Scuba and PassatVariantPunk for the write-up and photos, just remember to buy the clamps before you pull the car apart, and its 5:00 pm on Good Friday :wink: ) and as suggested by someone around here, I tried blowing through the old and new filters to see how plugged the old one was ( :p ). Surprise, surprise, the new filter was like blowing through a big straw, while I could hardly blow through the old one (it had 104000 kms and was original to the car, the date stamp was Sept 99 on the filter).

This got me thinking about the measured fuel mileage - would a really plugged filter be the cause for inaccurate measured fuel mileage? I've noticed that the measured mileage is about 1 l/100 km low when compared to my calcs at the gas pump (~2 USmpg) for around town driving (its been a while since I've compared for a highway road trip and I can't remember). I never add gas after it clicks off, so I do get a consistent tank from fill-up to fill-up.

I believe the measured fuel mileage is based on the injector cycle times, and as long as the FPR is not wide open, it should be accurate, but as soon as you get too much pressure drop through a plugged fuel filter, the FPR will end up wide open and you will not get accurate fuel measurement. This will be the worst when driving around town, since there is lots of high fuel usage acceleration going on :) .

Thoughts anyone?
 

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Oh well, nix that idea, first tank of gas measured lower mileage than the trip computer.
 

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the measured fuel milage should not change becuase the fuel pump does not work off of the engine power. the changed fuel filter will increase the life of the fuel pump because it doesn't have to work as hard, and maybe give you more high end power because if the fuel filter was stopped up enought not enought fuel would get to the engine at WOP at higher rpms
 

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Petra Passat said:
Surprise, surprise, the new filter was like blowing through a big straw, while I could hardly blow through the old one (it had 104000 kms and was original to the car, the date stamp was Sept 99 on the filter).
Was the old filter dry when you tried to blow air through it? If it was still soaked in fuel, that might make ot harder to push air through it. Just a thought...
 

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Petra Passat said:
This got me thinking about the measured fuel mileage - would a really plugged filter be the cause for inaccurate measured fuel mileage? I've noticed that the measured mileage is about 1 l/100 km low when compared to my calcs at the gas pump (~2 USmpg) for around town driving (its been a while since I've compared for a highway road trip and I can't remember). I never add gas after it clicks off, so I do get a consistent tank from fill-up to fill-up.
i think ~2 mpg lower is normal. that's what i have on my car too, and other people report similar things. but, the best way is to drive a week or two and see if it makes a difference 8)
 

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I didn't notice any change in my fuel consumption when I changed my fuel filter a couple months ago. The old one was definitely dirty (as I'm sure it would be after 90,000 miles). In fact, I'm getting worse mileage now, but I'd be more likely to attribute that to winter gas.
 

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