Fast idle switch and MPG

[dave_jo]

I often find myself coming to a stop sign after several miles of riding my 1150RT and realize I have the fast idle switch "on". I know it isn't a choke but could that be influencing my less than 40mpg mileage?

Purely curious...On a positive note, I have finally learned to automatically turn off the turn signals.

[Boffin]

All the fast idle switch does is open the throttles slightly by way of a cam in the throttle cable junction box (bowden box) under the tank. Once you open the throttle by more than that small amount it does nothing at all until you once again close the throttle.

 

I have found best MPG comes from gentle acceleration and keeping the revs low. If tank range is the aim, then try this, otherwise enjoy the ride

 

Andy

[T__]

I often find myself coming to a stop sign after several miles of riding my 1150RT and realize I have the fast idle switch "on". I know it isn't a choke but could that be influencing my less than 40mpg mileage?

Purely curious...On a positive note, I have finally learned to automatically turn off the turn signals.

 

Dave, in stock form probably not unless you do a lot of idling in neutral with it still on.. As mentioned above all it does is hold the throttle plates slightly open, something you do yourself once you give it some throttle..

 

 

Now if you have a fuel controller like a Techlusion or similar it could hold you above the 1500 RPM cut off point so you are in fuel enrichment even at idle.. It would still take a fair amount of idling time to make much of a measurable difference though..

 

If your fuel economy is lower than it was,, or has recently taken a set back,, first look to the weather.. Cold temps make the engine oil thicker & more difficult to pump & also increase the engine drag.. Cold weather also forces the Fuel controller to stay in cold fuel enrichment longer..

 

Next, look at your air filter element for signs of plugging with dirt or engine oil.. A plugged filter can effect fuel economy..

 

Next, make sure your 02 sensor is functional.. If that thing quits working or gets lazy it will allow open loop operation..

 

Next look at the air inlet temp sender (on top of the air box).. Disconnected won’t hurt to much except starting but corroded terminals or poor connection can drive the cold & open loop operation to a richer state..

 

Are you idling the bike for long periods before riding or after starting? If so that can kill fuel economy as you are getting 0 mpg while idling..

 

Even a larger, higher, or wider windshield can lower your fuel economy..

 

How about your tire pressure? Low on air can effect the miles per gallon but they would have to be really low to greatly effect it..

 

How about dragging brakes? Give both wheels a spin to see if they have any drag on them..

 

Fuel type can play a part here also with higher alcohol content lowering economy slightly..

 

Then you have riding style (or lack there of) .. High speeds (especially above 95 mph really lower the fuel economy)..

 

If you have an aftermarket fuel controller installed make sure that is set correctly.. Those things can really kill the economy if set too rich..

 

Twisty

[RFW]

All the fast idle switch does is open the throttles slightly by way of a cam in the throttle cable junction box (bowden box) under the tank. Once you open the throttle by more than that small amount it does nothing at all until you once again close the throttle.

Exactly!

 

I have found best MPG comes from gentle acceleration and keeping the revs low. If tank range is the aim, then try this, otherwise enjoy the ride

Interestingly, years ago BMW's car division did some tests to see what gave maximum mileage. It turned out that the best way was to accelerate at 3/4 throttle and shift at very low RPM (not much more than 2000 RPM).

 

The 3/4 open throttle reduces pumping losses (losses caused by pumping inlet air around the restriction of the throttle butterfly). The very low shift point reduces friction losses.

 

Of course, the question arises... who would want to drive like that!

[Tipover_Bob]

Next, look at your air filter element for signs of plugging with dirt or engine oil.. A plugged filter can effect fuel economy..

 

Twisty:

I am of the opinion that a clogged air filter will not adversly effect gas mileage, it will only restrict air flow. Now on a carburetor equipped bike, I agree, a dirty air filter would act like a choke and enrichen the mixture, but not on a fuel injected bike. That is why you don't "choke" a fuel injected bike. In fact, because it restricts flow, a clogged air filter would probably increase gas mileage because it would reduce power.

Tipover Bob

[Paul Mihalka]

The only thing I might think that affects MPG with the fast idle on, that with it off if you close the throttle completely engine control shuts the gas off completely until you get down to about 2000 rpm. With fast idle on it never gets fully off throttle so the gas does not shut off.

[T__]

Next, look at your air filter element for signs of plugging with dirt or engine oil.. A plugged filter can effect fuel economy..

 

Twisty:

I am of the opinion that a clogged air filter will not adversly effect gas mileage, it will only restrict air flow. Now on a carburetor equipped bike, I agree, a dirty air filter would act like a choke and enrichen the mixture, but not on a fuel injected bike. That is why you don't "choke" a fuel injected bike. In fact, because it restricts flow, a clogged air filter would probably increase gas mileage because it would reduce power.

Tipover Bob

 

Bob, all except the part that a clogged air filter reduces the delta across the throttle plate so to get the same power you need more throttle plate opening.. On an Alpha-N fueling system that means more throttle plate opening for the engine RPM.. The fueling system sees that as more engine load & adds fuel accordingly.. As long as it stays stokeometric on the 02 all is hunky dory but as soon as it gets too much TPS for the RPM it pulls spark & adds fuel as it goes open loop.. There goes your economy..

 

Twisty

[RFW]

I am of the opinion that a clogged air filter will not adversly effect gas mileage, it will only restrict air flow.

It depends on the FI system. On the old K-bikes with their LE-Jetronic's air mass sensor, I agree.... a reduction in the amount of air fed to the motor will be sensed and less fuel will be injected. The result (less air and less fuel)will be much the same as if you hadn't opened the throttle as much.

 

But most new BMWs sense the degree of throttle opening directly, and calculate the amount of incoming air based on throttle opening (and from that, determine how much fuel to inject).

 

If your air cleaner is clogged, the FI system has no way to know that less air is being inhaled into the motor as a result of a restrictive air filter. The result is that a clogged filter will result in a rich mixture, because the amount of fuel injected is based on the angular position of the throttle; the actual air mass being inhaled into the motor is not measured.

 

The O2 sensor will sense this incorrect mixture and try to correct for it, but this only works if the filter is not too clogged; the O2 sensor's feedback system can only correct a bad mixture to a limited degree.

 

So the bottom line is that on any newer BMW, a clogged air filter will definitely affect mixture.

 

That is why you don't "choke" a fuel injected bike.

The reason there is no choke on a fuel injected motor is simply because there is no need for one! The FI system "knows" the temperature of the motor, and if the motor is started cold, the FI system is programed to inject more fuel (to make a richer than normal mixture) until the motor warms up. This does exactly the same thing that a choke does on a carburetted motor, but does it more accurately.

[dave_jo]

Thanks folks!

 

I don't let it idle more than putting on the gloves.

Brand new air filter.

Just had 12k service and I presume they plugged it in and everything showed fine with O2 sensor etc. This service made no difference to MPG.

Riding style is "medium"? Mostly 50-65mph in 5th gear. I hit the rev-limiter once. Only rarely do I go over 6K rpm.

Perhaps I'll try using 6th gear more and keeping the RPMs lower.

 

I guess it just likes to burn a bit more than others the way I ride it.

 

Thanks again.

[Tipover_Bob]

As long as it stays stokeometric on the 02 all is hunky dory but as soon as it gets too much TPS for the RPM it pulls spark & adds fuel as it goes open loop.. There goes your economy.

 

Twisty:

I learned something. I wonder how much extra TPS it takes to run open loop verses how much extra TPS it takes to run with a dirty air filter? 1%, 2%, 10%, 30%, more? I expect 99% of the reason people change air filters is for maximum power. Of course that would be open loop. But then, what percentage of time do people run high power, the gain in economy with a clean filter might be overcome by the rapidity of rear tire wear, so in the long run, cutting back on power due to a dirty air filter, might reduce power, even though decreasing gas mileage, it may save tire wear and increase reliability of the rear drive bearings by reducing the torque on them, lol.

Tipover Bob