Causes For Excessive 5Th Gen Mileage

[SteveTheGreen]

Hi i have bough this summer a 99 5th gen VFR800Fi and the fuel consumption is through the roof,

roughly 28 mpg in town and 37 mpg on the highway and this is as good as i could get it and driving like that is like watching paint dry...

I'm interested in what could cause (on it's own or combined) such high mileage because someone with a 4th gen VFR told me he gets about 47 out of it, now i know this has a bigger engine... but only slightly and his bike uses carburetors (i may be wrong but i was told fuel injection is more efficient) also the "internet" says that the average fuel consumption for this bike should be about 40 mpg.

It's my 1st fuel injected bike so i don't know much yet, but i'm thinking these could be some causes:

- air filter -> richer mix

- injector nozzles

- tampering with the ECU for higher power/ richer mix

- engine valves

- I've read that this model VFR has some sort of secondary exhaust circuit that could cause this,

i'm out of my field here but there's an "exhaust air inlet solenoid valve" in the wiring diagram

[TomP]

Yes, that's way too low. First thing I'd look for is a stuck thermostat.

[iamzombie]

What on earth would a stuck thermostat have to do with poor mileage?

For the record, I'm not trying to be confrontational, I'm interested to hear if this is actually a cause for poor mileage.

[checksix]

Thermostat stuck open ==> engine never reaches proper operating temperature ==> engine control unit adds extra fuel trying to warm it up.

[raoufhakam]

What on earth would a stuck thermostat have to do with poor mileage?

For the record, I'm not trying to be confrontational, I'm interested to hear if this is actually a cause for poor mileage.

I may be wrong, but i guess if the thermostat is stuck open, ur engine will always be colder than average and hence, burning more fuel

P.s. i dont know where you live, but I have seen a noticeable drop in my fuel efficiency withwinter gas mixture over the years in my VFR and other bikes too

[Cogswell]

There have been some other threads about complaints of low fuel mileage - though mostly pertaining to 6th gens. Regardless, the principles are the same as on your 5th gen.

http://www.vfrdiscussion.com/forum/index.php/topic/76673-poor-fuel-economy-2009-vfr-800/

http://www.vfrdiscussion.com/forum/index.php/topic/71139-crazy-bad-gas-mileage/

http://www.vfrdiscussion.com/forum/index.php/topic/76863-28-mpg-sound-right/

[BusyLittleShop]

If the engine lacks power then check the condition of the chain and
sprockets... new chain and sprockets sap 2% of available power and
steadily sap more with wear... you maybe losing between 4% to 10% of
your engines power... if the engine doesn't lack power then I'd air up
your tires to the recommend in the manual 36psi F and 42psi R... other
items that help increase mileage are change the oil to a freer flowing
30 weight synthetic... there's a 4 to 6 hp gain between dropping a 50
weight for a 30 weight... check front brake pad drag or sticky caliper
pistons... regrease the rear axle needle bearing...

[Terry]

Steve I've been looking for a similar reason for poor mileage since I bought my '99 VFR this year, although mine (at about 33 mpg) is not quite as bad as yours, but others report much better. I have cleaned the air filter, synched the starter valves, checked the valve clearances, replaced the fuel pressure regulator (not obviously faulty), replaced the thermostat (had failed open), checked that wheels/brakes/chain work freely, but none of these made any noticeable difference. The one thing I haven't done is to get the injectors cleaned, and I may work my way up to that but for now I have decided to just accept the mileage and enjoy the ride. I do tend to use the bike for nothing but spirited weekend playing, so my riding style could well be a factor in this.

Apparently if the injectors are dirty they can fail to shut fully, meaning they continue to leak a little gas when they should be off, and could drip into the engine when it is shut off; I found out when changing the FPR, that fuel will happily gravity feed from the tank without the pump running. So it might pay to look at and smell your oil, is the level slowly rising, does it smell of gas?

Others here recommended using Sea Foam fuel additive to clean the injectors but this made no difference for me.

YMMV indeed!

Regarding the "secondary exhaust circuit" I think you are referring to the PAIR valve system. All this does is bleed fresh filtered air from the airbox into the exhaust headers, to help burn off remaining hydrocarbons and reduce emissions. This can't affect fuel consumption, and you can remove the system to rid the bike of unnecessary hoses!

[Ranger77]

I got horrible mileage when my t-stat was stuck open. Also the smell of running really rich.

[superfunkomatic]

Check the VFRs on Fuelly. 37 mpg on the highway is not terrible, not the best economy, but not terrible. 28 mpg in the city is low but if you are riding stop and start traffic or short burst commutes then your mileage will suffer. I used to ride 3-6 kilometres to work and got horrible fuel economy. Still get about 40mpg consistently, often more, the key seems to be steady speed, taking the 'weight' of the right wrist - i.e. less riding WFO.

[jaydj]

I have also noticed that my mileage goes to shit this time of year, every year since I've had my VFR, as there is an apparent switch to "winter fuel" mixture. Always happens around late Oct here in the northeast (PA) where I find my mileage go from about 180-200/tank to 150-170/tank depending on how I ride. This is topped up to blinking gauge, and it is usually about 4.5 gallons at fill-up. I've found a local ethanol free station and may try that to see if there is a diff. For now, I am steadily at around 160/tank for the last few weeks.

It was such a dramatic drop the first year, but it has happened every year since.

[Mohawk]

Now that wouldn't have anything to do with the weather & general temperatures getting colder, so the engine takes longer to warm up, cools down quicker & generally burns more fuel longer just to get to working temperature. This should in theory be offset by the slower riding we do due to the reduced grip levels !. As far as poor MPG is concerned, it could be any combination of the above.

[SteveTheGreen]

Wow so many people, I've just joined the forum when i started this thread, wasn't expecting that many answers, thanks for the input!

A lot of people mentioned a stuck thermostat, can someone please say what is normal operating temp for the engine? I've looked int the service manual but so far (perhaps I've been careless as it's 400+ pages i haven't found it) in the summer it usually gets warm fast, especially at traffic lights and my fan starts at (and keeps it at) 100*C which from what I've seen so far is normal but now that it's 10-12*C outside it gets nowhere near that, barely reaches 80*C and when riding in avg town speeds (50-60 kph around here) it sits at 70*C tops so i will check that thermostat.

My next question is if anyone knows if the thermostat is tied into the ECU (i.e. is purely mechanical on the coolant loop or also electronic) and generates a signal to it or the ECU uses the same coolant temp sensor that shows up on the dash (that would make more sense to me form an embedded point of view) because i haven't seen big changes from summer to winter in the fuel economy.

I do need to use this bike for work commutes (about 6-7 Km (or 4.3 Miles) in each direction.

Also it does smell quite rich after arriving somewhere.

[Mohawk]

normal temp is 80-84C, lower than that usually means its very cold or the engine is very rich. Both of which can be caused by a stuck thermostat. The simple way to test is to start the bike from cold, put your bare hand inside the right side fairing upper vent & touch the top front of the radiator, as the bike warms, you should NOT feel any heat until the temperature gauge shows 80C+, if you feel heat before the gauge even reads or at less than 60C then your thermostat is probably sticking open.

Unfortunately the only way to access it is to drain the coolant, raise the tank, remove the airbox & then the throttle bodies to get to the thermostat, stupid design by Honda.

[Terry]

The thermostat is purely mechanical with no connection to the ECU. Section 1-6 in the manual states that the tstat begins to open at 80C. The ECU gets its info from the Engine Coolant Temperature unit mounted on the back left of the front head; that's also where the gauge readout comes from. The fan switch is independent of this and plugs into the front of the left rad, should come on at 98-102C and off again at 93-97C (section 1-12)..

Sent from my iPhone using Tapatalk

[spud786]

The ECU has the bike in a richer state until normal water temp is reached, The thermostat just aides with that happening quicker. However if the thermo is stuck open and water temp falls below 160 degrees , ecu will revert into richer state, which equals worse fuel mileage.

[oyama]

Thermostat stuck open ==> engine never reaches proper operating temperature ==> engine control unit adds extra fuel trying to warm it up.

Yep, bad gas mileage made me realize I had a stuck thermostat.

[keny]

Anyone tuoght of the fuel pump if non mentioned above is wrong? If it runs whit a to low pressure the fuel don´t get to right form as it passes the injectors that can course bad mileage becorse it don´t get burned efficient. Low pressure in the pump can be becores of a glogged fuel filter also

[RolandHTG]

I have had bad mileage issues due to both stuck thermostat and a dirty injector (two separate issues several years apart) The thermostat is a PIA to change.

When my injector got dirty the first symptom I noticed was my oil level increasing due to fuel leaking into the crankcase. One day I noticed the sight glass was full. After checking other posibilities suggested here I sent my injectors off to be tested and cleaned. 3 were ok but the 4th was "dripping". After installing the cleaned and refurbished injectors the fuel mileage is back to normal (40-41 mpg).

[Baileyrock]

Welcome to VFRD Steve!