Significant MPG decrease

I have a RWB Vfr 800 with ~5700 miles. Regular service has performed on time. Lately my riding has been short commuting between school and home. The air temperature is typically between 40-50F, I usually allow the bike to warm up before riding and also most of the riding is through residential areas where I am in first or second gear. Generally I tend to ride at higher revs. With that established... Since September my MPG has trended from ~40mpg to 16mpg. Does anyone have an idea why?

 

What temp does the engine run at?

 

sounds like T stat is stuck open, bike should run 175-180 degrees, if you're running 150-160 errrr like mine is t stat is stuck.

 

Engine temp is usually ~160-170F

Also, and this might not matter, I do have a PCIII & LV SBK exhaust

 

that doesnt matter, your bike should run at 180
warm bike up to 180 and go for a spin, if it drops under 170 you're replacing the t stat.
I rode my bike for 6 miles up hill at 6500 rpm air temp was 50 degrees and I dropped down to 147 degrees, my bud on a 6th gen was running 180-182 all day.
That is how I confirmed mine was stuck.

 

I will give your suggestion a try, thanks.

 

Hey now!!!! dont make fun of the short bus riders and window lickers.

 

>

I told you it didn't sound reasonable. Now you have confirmation. Listen to Toe and others. They definitely know what they're talking about. But don't ever allow Toe to touch your bike with jb weld. You're just looking for trouble.

>

 

Their have been 2 occurrences on my 5th Gen that have caused a loss of Power and Higher than expected fuel consumption.

1) Had been degreasing / road grime cleaned my front cylinder bank unbolted to clean my oil cooler and had accidentally unplugged left front spark plug ( Dohhh :redface. Oddly enough although puzzlingly down on power and consumption was up the VFRs V4 was still smooth so took a hundred miles or so before I worked it out.

2) Pair Valve had gummed permanently open causing slightly fluffy mid range throttle, the odd backfire and increased consumption needing removal and clean to return back to normal.

See below UK VFR Thread:
'98 VFR800FiW to '01 VFR800Fi1 Increase in Fuel Consumption and Loss of Power

 

Be ever watchfull for a neighbor who has been seen with a steel can and an 8' length of garden hose.

 

Had same exact issue.... thermostat was bad (explains low temps), and fuel pressure regulator was replaced... problem fixed

 

correct me if I am wrong but I think he is asking why his meleage is so low.

can we say that a slow opening thermostat is the culprit?

I think the pressure regulator would be more than like the issue.

please correct me if I am wrong.

I also noticed that my mileage is horrible in the city but really great on the hwy..

Could this be your issue also?

 

how do we get coolent temp from bad fuel eco? ???

Things to check, if you can, get your stealership to read the computer for you (mine does this free if I arrive with the fairings already off) and they can tell you if your IAT and injector dwell times are right. (read PCIII changes injector dwell times but at 5k rpm it should not be 10ms, it should be around 5ms, this changes with load and demand but at stand still with bike in first gear it should be about 5ms)
googd fuel? if your running that cheap gas from the corner, your vfr might not like it.... I noticed mine likes shell better than esso and esso over petrocan. just a thought
fuel pressure, your stealership can (should be able to) check this in about an hour (requires removing the banjo bolt from the tank and placing a speacial one in place to testing) if its high, then look at fuel pressure regulator
IAT sensor in the air box, reads air temp going into the engine, check the connections and you can also do a quick test with a percison ohm meter and a hair dryer ( use in short bursts as over heating will cause damage)
'silent missing' could be caused by fouled plugs, bad fuel, hard reving, and fast temperature changes( in the cylender) cause deposits on the plugs, these usualy burn off but some times they dont. causing missing that is hard to hear as it is a void in the sound.


Now besides eating fuel like a fat man at a buffet, is there any other symptoms, including any you have dismissed??? are you reving above the 6500rpm mark?? V-tech is a fuel hog at low speeds and slow speed rides tend to eat fuel. if there is a way to get to a higher speed road( even if its a longer way to work) at least it gets you up into 3rd or 4th gear. I generaly ride 50kph (35mph) streets in 3rd with revs around 4k. depending on how close stop signs are I can lope in 4th without luggin the engine.
You are using a PCIII, if its malfunctioning then it can cause you to gulp fuel, Check the o2 sensor plugs, they should have a o2 eliminator on them, if its loose or missing then you need to make a trip to radio shack(if you still have them down there) and get some resistors (or just pm me with your address and I can send some)

just some stuff to look at, hope this helps

 

Thanks a lot for you input Zen, I will check out your suggestions next week.