Sharp drop in MPG

bobbsy

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Hi all,

I have noticed over the last month a sharp drop in MPG recorded by the bikes comp (and also evidenced by range per tank).

I have a 2010 F800. I (sadly) ride the same route day in day out for commute, and have not been out for a blast since last summer. I haven't changed how aggressively/non-aggressively I ride - everything in my mind is exactly the same.

However, for the past 7 years, I have averaged 42MPG. The computer has always read 42MPG unless I have gone out for a longer-non-commuting ride. Now it is averaging 35MPG, and has done so for about the last 6 weeks.

The bike doesn't feel any different. The exhaust collector (around the cat) is getting a little rusty, but there is no hole I can see as yet. The only thing I have changed recently is to replace the headlight unit and add a cylops LED globe.

Any ideas what this could point to? A duff sensor somewhere? Bad fuel? Blocked injectors? Could the extreme cold weather have caused it? Anyone else experienced something similar.

(Please I am not interested in what MPG you get on your bike - it's just the 16/17% fall in fuel efficiency I am looking at).

Thanks!!
 
Have you changed garage for filling up the tank, cheapo fuel always gets less mileage, and another thing is tyres, badly inflated just a little PSI will make a difference, especially in the cold.
 
I’m not going to tell you my commuteing mpg over a 2 year period no my 800 gsa only to say that 42 mpg is very low in the first place. JJH
 
I ride an 800GSA every day to work - 50 mile round commute via country roads, towns, city and motorway - and I average 53mpg as per the display. I do find that if I do not keep the drive chain well lubed that the mpg goes downhill. I had an issue with my Scottoiler recently in that it stopped lubing and this dropped my average MPG readout down to about 48mpg. Got it sorted and went back to 50's.
 
Thanks all for your comments. Tyres always kept optimal, chain lubed well - as I mentioned earlier, nothing has changed on the bike that I'm aware of. I fill up at 1 or two garages, superpremium fuel. I could expect a little variation in mpg for the reasons stated but not 17%. Yes, 42 may be low, but that's the nature of a relatively short, traffic heavy, south london commute.

I guess I'm looking for someone to point me in the direction of valves out of adjustment, exhaust sensor out of whack, other...
 
Has the bike been serviced regularly? I know it sounds odd but a part blocked fuel filter can cause you to use more fuel because you are opening the throttle more to compensate. JJH
 
Serviced regularly and all up to date - last service September 2017.

Doing a bit of googling of engines/mpg in general, it seems to be temperature type sensors that could be to blame - keeping the engine in the open loop system.
 
Perhaps you need to go into live data and check that the temperature sensor is reading correctly. If you don't have the equipment you need to pay someone to do it for you. Check the oxygen sensor as well while you are at it.
 
Thanks Smug - yes that'll be off to the workshop I think. Out of interest what equipment is required for that?
 
Smug said:
Perhaps you need to go into live data and check that the temperature sensor is reading correctly. If you don't have the equipment you need to pay someone to do it for you. Check the oxygen sensor as well while you are at it.
Click to expand...

O2 sensor was my first thought. It could allow the fuelling to be quite rich which you would not necessarily feel on a commute though traffic. Was the drop in mpg sudden or did it tail off?
 
My commute (70 miles each way, mainly motorway at 70-80 but with ten miles of town) gives me 55 mpg...

A sudden drop must be down to a fault somewhere, and it's going to take a lot of trial and error to find out what. Or just plug it into a diagnostic.
 
Cold makes a difference of about 5%. I ride all year the exact same route and in winter the MPG will drop 5% I assume due to the bike running richer for longer.

Your drop is more than that.

I had a more severe drop (55 vs 38 mpg) last year. The fuel pressure regulator had partially failed. It was giving an incorrect (low) reading causing the fuel pump to supply more fuel all the time. I had a super-rich black exhaust and lots of poping an banging on overrun. You will need a GS911 to get the fuel prossure reading. Tooks me weeks to sort the issue as it is counter-intuitve until you find out the fuelling on the 800 is very basic!

By the way 43mpg sucks. I ride 40 miles a day and 30 miles of that is in slowmoving London traffic!
 
Depending on where you are in south London I would recommend dropping the bike into Jap&German in Druid Street as being really good guys and ace BMW techs. While at the same time being an independent workshop that charges half the hourly rate.

And I agree that 43mpg sucks. When I was commuting into Westminster I was getting 58mpg from my 08 F8GS.
 
Old style analysis ........... check the spark plug. Black/sooty/moist & you have a problem. Tail pipe black/sooty confirms she is running rich. Check the air filter for any apparent constriction - crap can get drawn in easily, even rodents looking for a warm nest. If all these check out OK, then you need a dealer or a mate with a GS911 as others have suggested. 42 mpg is ridiculous - these F800s can achieve over 70 mpg ridden frugally, commonly 55-60 in give & take.
 
Thanks all for the advice! Drop was sudden, one tank to the next - I didn't notice it until I had to fill up 20 miles earlier than I usually do. It would be great to achieve 50-something mpg - it's been 42 ever since I bought it new, and had had a Hilltop remap during that time as well (will quite happily get 60mpg when I'm out of town), but even that seems low compared to the figures you are mentioning.

Jap and German is a good call - I didn't know about them.
 
Nippy Normans supply Touratech, Wunderlich, Desert Fox and many other makes of accessories.
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