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High fuel consumption


gropula

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Hi, everyone! Two months ago I bought a 2002 6th gen VFR, 57000km. My fuel consumption seems high when I compare it to other’s on the internet. It consumes around 8L/100km +\- 0.5L. When the engine is cold I get soot spots on the license plate which I presume happens when unburnt fuel flies out the exhaust, picks up soot along  

the way then deposits on the license plate. The previous owner said the pair/flapper mod has been done and the stock exhaust has been gutted. I’m a new rider so I don’t really ride it that hard to justify 8L/100km. I can’t hear the engine backfiring when it’s hot. What could be the cause?

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8L/100km is definitely excessive!

 

You might have an issue with injectors not closing properly, needing a professional clean and flow balance.

 

You could have a leaky Fuel Pressure Regulator diaphragm which can dump excess fuel into cylinders 3 and 4 via the vacuum hoses.

 

Have you inspected the spark plugs? Do you have any Fi fault codes? What is the state of your air filter?

 

If you don't already have it you can download the Service Manual from this sight 

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Is there an aftermarket module under the seat of some sort?  Also, if it were mine I'd plug the flapper hose back in place - make the airbox work as it should. 

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Thanks for the insights. The FI light on the dash doesn’t light up when the engine is running, don’t know about the codes yet, haven’t seen a mechanic. I will check the filter and spark plugs and reconnect the flapper hose and see if it makes any difference.

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Ok, here is the update, I’ve opened the air box and inside, around the throttle bodies of cylinders 3 and 4 was alot of fuel residue. From that I concluded that Fuel Pressure Regulator diaphragm is leaking, as Grum suggested. I didn’t reconnect the vacuum hose to the air restrictor solenoid, because I concluded that’s not the root of the problem. I might get back to it later when I fix this first. So, the Fuel Pressure Regulator diaphragm, any tips on fixing it? Can it be done by me or is it more of a pro’s job. I’m no stranger to fixing things, but my experience with motorcycles is limited.

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Fuel Pressure Regulator should be easy to replace, see attached pages from the Service Manual.

The part number is 16740-MCW-013. Do a Google search.

Glad you found your issue.

 

IMG_1120.PNG

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You'll need a large end wrench to remove the FPR,  I believe it's 24mm - 15/16 sould also work,  or a suitable adjustable wrench. After unfastening each of the airhorns' screws, you can probably set the airbox base off to the side for access. If insufficient and you want to remove the base, the inlet temp sensor is underneath and will need to have its electrical connector removed.  Be sure to keep the air horns in their original locations and pay attention to the marks that show orientation. 

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Did you plug end of air-restrictor solenoid hose and solenoid itself? If not, you've introduced vacuum-leak that'll throw off MAP sensor. Effectively fooling it into thinking load is higher than actual and tricking ECU to inject more fuel than necessary.

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40 minutes ago, DannoXYZ said:

Did you plug end of air-restrictor solenoid hose and solenoid itself? If not, you've introduced vacuum-leak that'll work throw off MAP sensor. Effectively fooling it into thinking load is higher than actual and tricking ECU to inject more fuel than necessary.

Hey Danno. The OP has discovered a ruptured FPR which is causing the excessive fuel consumption!

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On 7/19/2021 at 12:05 PM, DannoXYZ said:

oh yeah, that would trump any vacuum leak!

Ah yes sorry Danno, you're right. He does also need to make sure the vac hose for the variable air intake diaphragm is blocked off apart from his stuffed FPR. Just assumed it was, but yes this must be checked.

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I'm gonna check if the vacuum hose is properly sealed during the replacement of the FPR. I might even undo the mod after testing that the replacement of the FPR fixed the fuel consumption because I have my doubts about the usefulness of the flapper mod.

 

Does anyone have any dyno testing data on the gains or losses from doing the PAIR or flapper mod? The pair mod seems alright because it's just for better emissions, but it can mess up the A/F ratio. I have doubts on the flapper mod, since the airbox is a complicated system, and in lower RPMs the speed of the airflow is more important than the amount of air, so the restriction caused by the flapper could be beneficial and I doubt that the engineers would put something on the bike that doesn't serve a purpose.

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PAIR only changes AFR in exhaust after it leaves engine. Wouldn't affect fueling in engine in any way.

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18 hours ago, gropula said:

I'm gonna check if the vacuum hose is properly sealed during the replacement of the FPR. I might even undo the mod after testing that the replacement of the FPR fixed the fuel consumption because I have my doubts about the usefulness of the flapper mod.

 

Does anyone have any dyno testing data on the gains or losses from doing the PAIR or flapper mod? The pair mod seems alright because it's just for better emissions, but it can mess up the A/F ratio. I have doubts on the flapper mod, since the airbox is a complicated system, and in lower RPMs the speed of the airflow is more important than the amount of air, so the restriction caused by the flapper could be beneficial and I doubt that the engineers would put something on the bike that doesn't serve a purpose.

With the flapper,  I'm not sure it's something a dyno would reveal, tho I  have personal doubts that disabling it does anything to change the engine's output. However,  I do think it possubly has the ability to change the character, or smoothness of the output. G6s are known for their stumbling,  surging or hesitation at lower rpms.  That is,  as you point out just where intake velocity is needed.  I have no proof of that,  but it appeals to my intuition.  And maybe only mine  - I  could be FOS. There are of course fueling issues with that as well.  But, Honda doesn't just throw an airbox together,  the design is carefully considered and I suspect that they tuned it for the max performance they could get from it within the limitations imposed on them. They certainly did not install the flapper valve to lower performance  . . .  Like the air filter - changing to a reusable one nets no meaningful performance benefit as the OEM filter can provide all the air the engine can use at any rpm. Owners have ridden them hard, not knowing there was a mouse house on the filter - and didn't even know the difference until they looked inside.    There  are legitimate reasons for doing so,  but we've never seen anyone put up dyno numbers showing any material performance changes from using a reusable filter.. Same with flapper and intake mods. These are street bikes and make more than enough power for all but the most expert rider.  Fiddling with the airbox just is not, IMHO anything worth spending time doing. 

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+1 Cogswell, well said.

I would also like to add, that on the 2017 8gen the Variable Air Intake flapper system was completely removed by Honda. Given that the air filter part number hasn't changed then you'd suspect there is also no change to the airbox shape.

Based on this I ended up removing the system from my 2014 8gen Only to free up the under tank area from clutter(it can be easily re fitted). I've done almost 40,000kms since its removal and can attest to the bike performance as excellent in all respects from my subjective seat of the pants dyno and post mod fuel economy.

But had Honda not removed the system from the 2017 model, then probably neither would I.

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  • 2 weeks later...

Update on the topic. Replacing the fuel pressure regulator didn’t fix the problem. 
 

I replaced the pressure regulator and started the bike with the air box open and no filter. There was no fuel coming into the air box when idle and reving it lightly. So I thought the problem was fixed.
 

Went for a ride, did 100km with 8.1L. Came home, opened the air box, there was fuel on the right rear side of the air filter, and around the air horns of the two rear cylinders. You can see that on the pictures. I marked the opening where the fuel comes from. It’s obvious as the air filter was wet from fuel at that spot. 
 

Was the fuel pressure regulator replacement useless? It was quite expensive, 130€. With Eastern European standard that’s allot. Got the part from official Honda dealership.


What should I do about it? What’s this hole that spews out fuel connected to?

DA1FBF32-15EB-4C26-8C5C-C0486F6143C3.jpeg

776A6B09-0E48-4E70-9289-E3559BB60B19.jpeg

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Sorry to hear the FPR didn't fix your issue. It is a fairly common fault in older vfr's that can cause anything from excessive fuel consumption to very poor, rough running. As mentioned a ruptured diaphragm will caused excess fuel to enter 3 and 4 throttle bodies (the two right side intakes) via the FPR vac hoses, not just around the intake in the airbox. 

 

You should have checked to see if you had fuel in the vacuum hoses from the FPR or fuel dripping from the FPR vacuum inlet, before replacing it!

 

The hose you are pointing to is the Crankcase Breather hose and this comes in next to number 3 intake.

 

Have you downloaded the Service Manual?

 

So apart from the high fuel consumption. How does the bike perform? Just curious but why does the intake funnel in your photo have broken plastic where the mounting screws go, haven't seen damage to the funnels like that before?

 

As to why you have raw fuel entering at this point I'm not too sure. Hopefully some more experienced members will give you better advice.

 

Good luck.

 

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There is usually a bit of oil mist sent through the breather to the airbox, but it would be unusual to see much volume in the airbox unless the crankcase is overfilled. With the bike on the centrestand and engine off, where is the oil level in the sight glass?

 

Just a note on fuel consumption, this is a subjective thing and will vary a great deal with the use of the bike. If you spend your time running in traffic, stopping and starting a lot, the usage will be much higher that steady speed on a motorway. 8 does seem pretty high for a VFR however. My 5th gen with open road fun riding would normally get around 6L/100km. 

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I might be wrong about the fuel coming from the crankcase breather. Today it didn’t smell like fuel, neither did the oil or the oil cap.
 

My oil level is over the top line so that might be causing the breather to spew more oil vapor. I think my oil might be contaminated, so I will change it. It isn’t too old but it won’t hurt. Then I will monitor the oil level after every ride to check if the fuel is coming in to the oil.

 

I went to all shops that repair Hondas and they were all baffled. So I’m probably wrong about it being fuel. But it did smell like it yesterday. Did 70km today, opened the air box and it smelled  like oil fumes, which is ok. The amount of oil residue might be attributed to the oil level being too high.

 

All in all, thanks for the replies. I’ll change the oil and keep you updated. 

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Update to close the topic. I was quick to jump to conclusion. The fuel consumption is fixed. Did three refuels, 6.2 - 6.3 L per 100km. Problem fixed, it just needed one tank to break in I suppose. There is no fuel in the airbox, it was leftover from the bad FPR and I misdiagnosed.

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2 hours ago, gropula said:

Update to close the topic. I was quick to jump to conclusion. The fuel consumption is fixed. Did three refuels, 6.2 - 6.3 L per 100km. Problem fixed, it just needed one tank to break in I suppose. There is no fuel in the airbox, it was leftover from the bad FPR and I misdiagnosed.

That is good news, glad it was only the FPR, and you didn't waste your money thinking it wasn't.

Glad you have it all sorted and running well. Thanks for posting the update.

 

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On 7/20/2021 at 4:49 AM, Cogswell said:

With the flapper,  I'm not sure it's something a dyno would reveal, tho I  have personal doubts that disabling it does anything to change the engine's output. However,  I do think it possubly has the ability to change the character, or smoothness of the output. G6s are known for their stumbling,  surging or hesitation at lower rpms.  That is,  as you point out just where intake velocity is needed.  I have no proof of that,  but it appeals to my intuition.  And maybe only mine  - I  could be FOS. There are of course fueling issues with that as well.  But, Honda doesn't just throw an airbox together,  the design is carefully considered and I suspect that they tuned it for the max performance they could get from it within the limitations imposed on them. They certainly did not install the flapper valve to lower performance  . . .  Like the air filter - changing to a reusable one nets no meaningful performance benefit as the OEM filter can provide all the air the engine can use at any rpm. Owners have ridden them hard, not knowing there was a mouse house on the filter - and didn't even know the difference until they looked inside.    There  are legitimate reasons for doing so,  but we've never seen anyone put up dyno numbers showing any material performance changes from using a reusable filter.. Same with flapper and intake mods. These are street bikes and make more than enough power for all but the most expert rider.  Fiddling with the airbox just is not, IMHO anything worth spending time doing. 

It was done on a dyno many moons ago. Got approx 1.5hp across the mid range to top end on a 5th gen. 

 

6th gen, flapper works on a different cycle to 5th gen. it only closes at specific rpm for  noise control. 
 

As to air flow  moving air is less dense than static air. The only air velocity that matters is that through the throttle bodies. 

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  • 4 months later...
On 7/18/2021 at 9:11 AM, Grum said:

16740-MCW-013

 

Just did the replacement of a FPR on my VFR but seems according to ebay like there are no 16740-MCW-013 left in US.

I used 16740-MCF-013 used by the ST1300 and the problem was solved in my case.

 

For reference my bike is doing according to several different fuel pumps and bike odo and google maps 43mpg (us gallons) doing freeway/b roads 60-75ish on 55 to 70F weather, with or without the side cases, no passenger I weight 170 pounds, which after some math is 5.47lt/100km.

 

if you ride on faster freeways, mpg will go to 37mpg which is 6.3lt/100km.

 

before the fix I'll do whichever way I drove 33mpg or 7.12liter/100km

 

------------------

 

more reference... I still think the VFR should be more efficient, my Goldwing 2004 wich is ~400 pounds heavier and driving with a passenger will do 40mpg.

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Fuel-consumption is affected by many variables:

 

- gearing: taller gearing on Goldwing would lower engine RPMs at cruising speeds compared to VFR

 

- engine-tuning: VFR is tuned for high-RPM efficiency and max-power. GW tuned more for mid-range torque. At cruising speeds, engine-revs on GW would be closer to max-VE range in power-band (more efficient).

 

- weight: only affects acceleration, more from standstill. So yes, more weight on GW may hurt mileage, but it comes down to torque-to-weight ratio. GW has 150% more torque than VFR for only 80% more weight.

 

Weight going up hills may appear at first to hurt GW mileage, but it makes up for it on downhill with less power required. Simple test is to coast both down same hill from same speed at top. GW will probably hit higher coasting top-speed down hill, thus its engine will have to generate less power for same downhill speed.

 

- speed: once you're up to steady-state cruising speed, weight doesn't affect mileage as much as aero-drag. Don't know actual drag-force at 65mph, but I suspect both of them will require similar 16-18hp to hold that speed. Longer smoother bodies tend to be more aero than shorter angular shapes.

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9 hours ago, xlayn said:

Just did the replacement of a FPR on my VFR but seems according to ebay like there are no 16740-MCW-013 left in US.

I used 16740-MCF-013 used by the ST1300 and the problem was solved in my case.

Good news the FPR solved your issue or at least greatly improved things, and that the ST1300 FPR is compatible.

 

With my three previous 6gens the Best Fuel Economy I achieved was 4.78L/100k or 49.21 miles to a US Gallon. Worst was 5.84L/100k or 40.28 miles to a US Gallon. My weight (back in the 6gen days!) was 82kg or 180.7 pounds.

 

Just for info the 8gen has excellent overall economy, got as low as 3.94L/100k, 59.7mpg. Generally seeing below 4.5L/100k on average. I don't ever use Ethanol blended fuels, fortunately we still have a choice here.

 

As per what Danno has mentioned, and also check your air filter for cleanliness, and (believe it or not) ensure the drive chain isn't over tight, and no brake drag! And Ethanol blended fuel will give you the less mileage compared to non Ethanol fuel!

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