sudden big drop in MPG

[trc.rhubarb]

Bike has ~52.5k miles on it and runs great. It's a bit noisy and has been since I picked it up a year ago. One day I'll get around to ordering new cam chain tensioners.

Anyhow, I've generally gotten in the high 30's and sometimes even mid 40's for mileage but the last 3 tanks of gas (over the last 5 days) have been in the low low 30's. Even the bike knows something is up and shows 31.x mpg on the screen after a reset of the average.

It runs well and accelerates fine, no carbon build up on the bags or muffler.

air filter changed, plugs checked, valves & tb were done and checked good at 48k. FD fluid was changed then and there is no freeplay. No noises in the driveline when rolling in neutral. Checked for drag on front and rear wheels. Brakes aren't hot after riding. New rear tire was put on just before the issues started but the tire looks good and is still at 40psi; front good too. The only other change is that it's been raining.

What's next to check?

[LaurelPerryOnLand]

http://bmwsporttouring.com/ubbthread...555#Post808555

[Rinty]

What's next to check?

Oxygen sensors.

But I would just have the whole system scanned.

[trc.rhubarb]

Thanks, I think too much for it to just be the cold. In fact, since it's been raining, it's not been that cold.
O2 sensors will be interesting to test. I guess I'll see if it gets better first.

[kk3an]

I had the same symptoms at about 38,000 miles, and replacing both oxygen sensors made the mileage pop back up.

Looking back, I'm not sure if just a simple cleaning of the sensor area itself will cure this or not, or if a total replacement is needed.

Dan

[tjt94]

It is because of winter blend fuel. I always lose about the same is California is the winter.

[El Gato]

Originally Posted by tjt94

It is because of winter blend fuel. I always lose about the same is California is the winter.

That was my initial thought as well, but CA switched to winter gas early this year -- about 6 weeks ago on an exemption from the governor due to the Richmond refinery fire.

[tjt94]

Originally Posted by El Gato

That was my initial thought as well, but CA switched to winter gas early this year -- about 6 weeks ago on an exemption from the governor due to the Richmond refinery fire.

That may have been how long it took to deplete the tanks. Regardless, the difference in actual cost between they mileage figures is not that much so keep an eye on it. If it is running fine just ride. Good luck.

[Twhiteside]

Interesting - I just reset the MPG calculator yesterday and noticed I was pulling at best 37-39 MPG doing some simple highway cruising. Definitely recall getting much better than that here in SoCal a few months ago. F'n winter gas indeed..

[trc.rhubarb]

Bike is running well just bad mileage. I don't ride my other so much but I think I'll switch for a week and see if it's getting bad mileage too. That would show for sure it's the gas. Thanks for the input!

[Tmaximusv]

I scanned this thread earlier and didn't have anything to offer...I just remembered something to offer.

If your gas cap isn't sealing properly, you can lose about 40% of your fuel to evaporation (under warm conditions) DAMHIK.

[trc.rhubarb]

Originally Posted by Tmaximusv

I scanned this thread earlier and didn't have anything to offer...I just remembered something to offer.

If your gas cap isn't sealing properly, you can lose about 40% of your fuel to evaporation (under warm conditions) DAMHIK.

O'ya, went through that one befure :) Funny how you can keep smelling gas and 'wonder where it's coming from'

[ukdave]

I had a leaks from fuel pipe connections more than once.

[trc.rhubarb]

Good one to check. Even though I'm not smelling gas, it could be leaking. Hopefully my new shocks arrive this week and I can just take the whole thing apart.

[Multiplicity]

You should also treat the fuel with Techron every 10K miles to keep the injectors clean.

Buy a bottle at the local parts store