new fuel filter, lower mpg - 986 Forum - The Community for Porsche Boxster & Cayman Owners

tonycarreon · 03-21-2010, 06:09 AM

so i just replaced the fuel filter which had never been replaced, to my knowledge at 72k miles.

now the car is getting way worse mpg than before - from 24mpg before the change to 17mpg after the change. ( have filled up once since the change )

has anyone else noticed this? any recommendations? the car doesn't run bad and still seems to have the same acceleration as before. no bad idle and no abnormal "gasoline" smell.

Perfectlap · 03-21-2010, 08:28 AM

do you still have the old one? maybe put it in and see if your mileage goes up.
I just replaced mine and didn't notice any difference. I also replaced all of the 02 sensors.

Adam · 03-21-2010, 08:39 AM

Have your driving habits or conditions changed at all since you swapped out the filter? If not, then I would run through another tank full or two and see if the mpg restores itself. There is no reason a new fuel filter would reduce gas mileage like that so I'm suspecting something else is at play here.

manolo · 03-21-2010, 10:36 AM

I'm just guessing, but if the old one was clogged up, you could have higher fuel pressure with a clean filter. You would be using more fuel/running a little richer until the DME trims itself back eventually. Just a thought.

Adam · 03-21-2010, 10:54 AM

Quote:

Originally Posted by manolo

I'm just guessing, but if the old one was clogged up, you could have higher fuel pressure with a clean filter. You would be using more fuel/running a little richer until the DME trims itself back eventually. Just a thought.

That's a good thought, but I don't think that's the case. The ECU makes hundreds of calculations per second and it wouldn't take very long to figure out the correct mixture. Also, depending on the year there would a return line so any unused fuel would go back to the tank anyway.

Steve Tinker · 03-21-2010, 02:00 PM

Have you checked if there is a fuel leak around the filters snap on connectors. It doesn't need much of a weep while driving to blow out your fuel usage - remember it will only leak when the engine (or ignition) is on or running.
So to confirm a leak, you will have to get under the car with the ignition in the "on" position to pressure up the system.

Lil bastard · 03-21-2010, 08:04 PM

Look for leaks as mentioned.

Disconnect battery for 10 min. to reboot the DME.

Most likely anecdotal, you cannot judge MPG based on one tank, run 5 or 6 and then avg. it out. Also, your baseline is likely skewed, esp. if based only on one tank, or even a few.

Cheers!

tonycarreon · 03-22-2010, 08:36 AM

checking the fuel filter, just a double-check if someone can :

the "arrow" on the filter, which indicates flow, should point towards the rear ( engine ) of the car. correct?

thanks

i see no obvious leaks or seepage, so maybe i'm just going bonkers or the computer needs to reset itself for the easier flow.

--
tony

ARModen · 03-22-2010, 02:08 PM

Check the filter for leaks with the car running. Having the ignition in the ON position does not turn on the fuel pump. The pump only comes on when the engine is cranking or running for safety reasons.

tonycarreon · 03-22-2010, 02:47 PM

Quote:

Originally Posted by ARModen

Check the filter for leaks with the car running. Having the ignition in the ON position does not turn on the fuel pump. The pump only comes on when the engine is cranking or running for safety reasons.

yeah i did, no leaking / seepage. the connections were dry and no gas smells. also the under panel was dry ( dusty even ) and had no gas smell. probably all in my imagination or the computer needs a little time to adjust to a clean filter - like i said the original filter was on there @ 72k miles.

i'll keep track of the mpg and worry about it later if it's still a problem. thanks for the help guys.