So in August of 2013 I drove to maine and averaged 31mpg over 500 miles. Mods were 15%, ddm intake, alta catback, BSH catch can, detroit tuned bpv, and a substantial amount of new maintenance parts. This went on until about October when I got a CEL which ended up being a leaky Detroit tuned bpv. Ever since then the mileage dropped to about 25 highway. I drive the exact same way, 70 @ cruise control and I have always filled up at the same gas station locally. Ever since then the car has been terrible. I changed the fuel filter at around 140k thinking it might have been that, nothing. Have new tires at the proper air pressure, recent alignment, I just did brakes and replaced 1 sticky caliper, replaced my primary O2 which threw a phantom code once but since it had 100+K on it I figured why not. Secondary is 2 years and 25k old. Both are oem NTK.

I'm out of ideas. I double checked the plugs and they look fine. I've used injector cleaner and it did nothing, though I do have a set of freshly cleaned Bosch 380's that are going to go in soon. The car doesn't seem to lack any power at all so I can't imagine it's a clogged cat, nor am I throwing a code for anything. Could it possibly be the original coil? It's not misfiring at all but maybe it's getting weak? Idk I have no clue anymore. fwiw not additional mods have changed since that initial drop.

 

Crazy thought: The bad BPV threw off your adaptation values. Try resetting them.
Also, are you calculating mileage based on miles between fill-ups/gallons pumped into tank at a fill-up, or are you relying on what the the OBC indicates?

 

Reset the ecu via the trip button thing. Only reason I was going off the obc is because it's always been consistently .5-1mpg high. My friend told me the ecu needs a good 50-100 miles to relearn the fuel trims so I'll drive it around and see if that makes a difference hoping it was still running off the old adaptations from when the cel was on before. The bpv code was a while ago though.

 

One thought....
Is it a new bypass valve or did you rework it...
Wondering if it is STUCK in the closed position....so you are in boost at lower rpms, preventing the fuel savings the bypass valve is not installed for....
Does you car seem to be running OK otherwise? Any hiccups or oddities?

 

what about ethanol in fuel? It does drop the mpg compare to straight gas in my stock R56s. Cant really find gas station without those in socal

 

When was the last time the DDM air filter was cleaned/oiled ? I have seen dirty filters both foam and stock be the cause of reduced mpg.

 

Just saw you got new tires....
Same size? That can throw off all calculations...and gearing....
Tires (even in the same size) can account for 1-2 mpg new to old....and more if wider, or more stick rubber, or even going from a low rolling resistance tire to a higrip or high mile/high wear tire with lots of rubber....

 

It's a detroit tuned bpv. I got 30.6 with mixed driving and it on the car on a trip to and from ocean city maryland. It's a new unit that they rework. When it failed they changed out the diaphragm with what I assume is a new one. It's not sticking that I can tell.

All stations in connecticut are 10-15%, but they have been since I started driving years ago. I'm very positive I haven't been putting E85 in the car.

Good call, thought of that too but I cleaned mine out along with my girlfriends maybe 5,000 miles ago with Brand name cleaner.

I had unevenly worn 205/45-17 Conti's on my stock s lites when I got those figures. I also achieved 28-30 on 215/45-17 NT01's on rpf1's. I now have 215/45-17's Michelin pilot super sports though I can't imagine a 10mpg decrease from the rolling resistance of tires alone.

 

Took a highway trip to Hartford and back mostly highway w/ 0 traffic and cruise control set at 70. OBC dictated 25.5. I'll top off today and confirm this. Still have no idea what going on, no cel either.

 

Fuel filter clean?

 

The op stated the Fuel Filter was changed at 140K

 

Same Quality/Octane of fuel? Condition of Plugs?

Motor on!

 

Ooops. my bad.

 

Well I've been using the same station locally for a while now. As of this week it seems they switched over to their winter blend which would reflect the low prices, though I'm not sure if that's 87 only. 87 is 2.97/gal 93 runs 3.31. I've only ever used 93 in this car no exceptions. Plugs looked good when I pulled them out to check the gapping, I'll pull one out today and snap a photo to throw up here you guys can tell me what you think.



No worries!

 

Cel for p1498 came back on��. ***** driving me insane. Going to get smoke tested tomorrow for it.

 

Check the Catch can tubes. I read about that code and people usually get it with catch can issues. Like cracked hoses or leaks.

 

Hmmm. I'll check them again for leaks. I put new hose clamps on them, never seemed to give me an issue in the past. Put that guy on with my intake at about 128k. I also changed the check valve on the valve cover for a new one fwiw.


So I bought the car to a local shop that has extensive BMW/MINI knowledge. Tech said he can't put his finger on it without tearing into the car, but his money is on a bad BPV, or a bad crank damper. Either way I ordered both since I had gotten a P1688 code once in the past which is derivative of a torn crank pulley. Hopefully one of the 2 things fixes it. I'll let you guys know what the results are tomorrow.

Oh and its definitely not any kind of leak, he smoke tested it to 20psi and absolutely nothing leaked.

 

So far 250~ miles in and no cel. Replaced the crank pulley and bpv with new oem parts. Gas mileage shot back up to 30~ on the obc as well so I think the issue was honestly my old crank pulley.

Now to figure out the vibration that only happens under load. Guessing it's axles...sigh. It never ends.

 

Yaaaay! MPG is back!! Boooooooo vibration

 

wish I got 30 mpg....24-25 in town 28 highway.