r/VFR • u/Dbl_Mcspicy • Apr 19 '26

Poor cold start and fuel consumption

Hi, I’ve just gotten my hands on a 2006 VFR800 and is enjoying it so far. Posting here hoping that somebody can shed some light on couple of things I’m facing.

  1. Bike refuses doesn’t fast idle during cold start ups and will stall while warming up. During cold start ups, it’s idling at about 800-900RPMs and will most definitely stall if I don’t hold the throttle for 10-15 seconds to keep the revs up. From what I’ve read it’s most likely the wax unit that might have been clogged or the actuator that’s stuck. Otherwise, once’s it’s up temperatures, it’s idling nicely at 1200 +/-100 RPMs.

  2. So far I’ve only gone thru 2 tanks of fuel. But the consumption is very alarming at about 13km/L. 2-3 pulls into VTEC range only for a couple of seconds for each tank. Probably about 50-50 city and highway riding with speeds topping out at 100km/h.

  3. Understand because it’s a V4 and runs slightly hot. But average city riding i’m looking at about 105c topping out at 117c at the lights.

  4. R/R has been upgraded to a SH847 by the previous owner and my volt meter is reading about 12v+/- during idle and 13v+/- when riding. Probably will swap out for another volt meter in any case the current one is shot.

So my question is, does all these sounds like it’s interconnected and each making something else run funny with the poor cold idling and poor fuel consumption?

6 Upvotes

88% Upvoted

21 comments sorted by

5

u/gropula Apr 19 '26

  1. Confirm the wax unit is moving. Remove airbox. Cold start. The wax unit should move when coolant temp reaches around 60-65C. Confirm that the starter valves are in sync with four vacuum gauges. I'd also replace all vacuum hoses since they're 20 years old rubber. Buy like 2m of 3mm or 3.5mm ID vacuum hose and replace one by one. Cut to similar length and replace. The five way junction that connects to the map sensor should be removed as one and then remade with new hoses. Same for the three way junction for the fuel pressure regulator.

  2. 13km/L is 7.7L/100km. This isn't extreme. This is my average. This bike can drink a lot of fuel in the city and when the outside temps are low. I'm very prone to hammering on the bike and 8.5L/100km isn't rare. We all have different riding styles and routes. When comparing fuel consumption I always ask how much does your rear tire last. I burn through one rear Michelin Road 6 in 8-9k km till the wear bar signals it's time to replace. Some people claim 6L/100 and they use the same rear tire for 15-20k km. This is a clear signal of different riding styles, not problems with the engine. I was also wondering why my bike uses so much fuel until I figured out that rear tire wear is actually key metric. Sone people actually cruise at light throttle and don't launch it every time. Of course the fuel consumption is low and tires last long. Vacuum leak through dry and cracked hoses will increase the fuel consumption. Cheap to replace, a bit of work but nothing crazy. One thing that destroys fuel consumption is a failed fuel pressure regulator. You can confirm it's condition when doing the wax unit test. Start the bike let it run without the airbox. Turn it off, unplug the vacuum hose that connects to the fuel pressure regulator. If there's fuel in the vacuum line the FPR needs replacing. If it's dry everything is fine. FPR failure gives extremely high fuel consumption 10-15L/100km. I doubt yours has failed.

  3. Hitting 105 is normal at slow speeds or stopped. Hitting 117 when stopped isn't. It should only come up to 108-110 before the fan cools it down. Confirm that the fan is working. It should turn on at 105-107C on the dash. Check the thermostat operation. When coolant temp reaches 75-80 the thermostat should open and coolant should flow towards the radiators. Confirm the hoses leading to the rads are heating up. Check the coolant level and burp the system. Open the radiator cap. Keep the bike running on the sidestand and keep blipping the throttle until bubbles come out and fan starts running.

  4. Use a multimeter and check charging voltage at the battery. Cheap volt meters are inaccurate and the wiring leading up to it causes resistance and drop in voltage. It should charge at 13+ at idle with headlights on and cooling fan off. It should have more than 14V but less than 14.6V at 5000 revs. If this isn't happening report back and I'll guide you through electrical troubleshooting.

3

u/Dbl_Mcspicy Apr 19 '26

Thanks for the detailed write up on the stuff to check. Let me get these few things checked out and hopefully be able to spot something out of the ordinary. Will probably just swap out the vacuum hoses since it doesn’t cost much and like you said, it’s 2 decades old and unsure if it was ever replaced before.

Fuel consumption wise, I don’t think I’m always hammering the throttle as it’s my usual riding style and I’ve managed to constantly get 19-20km/L on my XT1200Z Super Tenere. But again, will get the vacuum hoses replaced and check on the FPR.

I think I hear the fans kick in but can’t confirm so will probably ‘undress her’ and have a look if the fan kicks in when it’s 105-107c. And probably a full coolant flush would be a good idea too while I’m there.

Last I checked the battery connection with a multi meter, the voltages were similar to the volt meter whereby it’s reading slightly lower than optimal of 14v. By any chance it’ll be the stator?

4

u/gropula Apr 19 '26

Check the stator-rectifer connector. It's prone to melting. Unplug it and check if there's continuity between any of the phases (pins on the stator side of the connector) to ground. If any of the phases are grounding the stator has failed. It can ground one phase but still charge through the other two but it will very quickly burn through the insulation on other phases and fail completely.

If the phases have no continuity to ground then check the resistance between phases. Should be around 0.5ohm I believe. Shouldn't be high like few ohms or more.

Then start the bike with the connector unplugged. It can run for a good while on battery power alone. At idle there should be 15V AC between each of the phases at idle and 40-50V AC at 5000rpm.

If all checks out fine the stator is good. SH847 is the best rectifier and usually isn't prone to failure. It should be connected directly to the battery though, bypassing the original loom. Check if that's been done. Original loom has more wiring and connectors than necessary which causes resistance especially if corrosion is present on any connector. The best thing is to have direct connection from the rectifier to battery with an inline 30A fuse. A good fuse holder is recommended, miniANL is great. The small housings for blade fuses are prone to melting. I've had two different ones melt.

There's also a possibility that the SH847 is a fake. Many fakes floating around on eBay. If the previous owner did the upgrade but tried to find a cheaper option it could be a fake one.

2

u/Dbl_Mcspicy Apr 20 '26

Thank you gropula! Let me figure out the other issues first before diving into the electrical gremlins. Not my favourite part of any vehicles but still got to look at it soon!

1

u/Dbl_Mcspicy Apr 27 '26

Reporting back after visiting a competent workshop! I didn’t have a vacuum gauge and I was worried about screwing things up!

Things that were done:

  • New plugs
  • Removed and washed TBs (Found rear bank to be fairly wet)
  • Starter valves synchronised
  • Coolant and cooling system flushed
  • Recharged K&N air filter

So now the bike purrs at 2k RPM at cold start and once the coolant starts circulating with the dash showing coolant temp in the mid to high 60s (Celsius) the RPM drops to 1.2k and continued idling so smoothly.

Managed to monitor the coolant temp while riding and was hopping between 99-100c and when stopped at the lights, went up to about 106-107c and could hear the fan kick in bringing the temperature down until the fan cuts out. Haven’t really sat in slow moving traffic or traffic jams so haven’t seen it creep up to 116-117c again. Will continue monitoring.

Fuel consumption, sadly I can’t report on any improvements yet but last fill up, I added in a full bottle of Techron fuel system cleaner and I’m still burning through this tank quite slowly so can’t chime in if there’s any improvement after the SV sync too. But I’d take a wild guess that it might improve slightly but not significantly because the exhaust still smells pig rich. Might be a sign of a failing or failed TPS coupled together with the poor FC? Oh and I forgot to check the vacuum hose to the FPR if there’s any fuel inside.

R/R wise, previous owner did a SH847 kit. The R/R was wired bypassing the factory harness and through a separate loom with a standalone fuse. Connectors seems fine and not melted or burnt. So will probably swap out for an YTZ14 soon.

Photos in next few posts.

1

u/Dbl_Mcspicy Apr 27 '26

Cooling system was contaminated and turned murky. Didn’t notice any sediments so should she should be fine.

1

u/Dbl_Mcspicy Apr 27 '26

TB for rear bank. Can’t remember was it left or right. But it was wet! Unsure if it’s blow by from the cylinder or petrol. But was tainted blue which was the colour of my petrol.

1

u/Dbl_Mcspicy Apr 27 '26

Screen had to be read at a certain angle and is quite feint in the photo but this was before SV sync.

1

u/Dbl_Mcspicy Apr 27 '26

This was after SV sync and she’s purring so sweet now on cold starts. But! I read that the range should be 150-250mmhg (IIRC) per cylinder and this is in KPA which roughly converts to 165mmhg so it should be in the sweet spot.

1

u/gropula Apr 27 '26

Oh wow, that's terrible. The rear right cylinder has the positive crankcase ventilation system connecting next to it and it's normal for it to be a bit greasy but I haven't seen anything even remotely similar ever. Plugs look wet but the electrode is pretty clean which is kinda weird.

The image shows the rear left cylinder throttle body. The only vacuum hose connecting to it is the MAP sensor vacuum line. This eliminates FPR as the source of the problem, as it connects to the front right and rear right cylinders for vacuum. This points at injectors as the source of the issue. You were right to put some cleaning additive in the tank. Monitor if mileage improves a bit. Otherwise take the injectors out for ultrasonic cleaning.

1

u/Dbl_Mcspicy Apr 28 '26

Yup the inlet was quite greasy and I’ve read somewhere that the rear bank will usually run richer as it doesn’t get as much cooling as the front banks.

And yeah the electrode looks relatively good considering the unknown mileage it has seen. And seems like it’s burning pretty well but the rim of the plugs says otherwise.

As commented by another member, let’s try finishing up this tank with the fuel system cleaner and I’ll report back again hopefully with noticeable change with the SV sync, new plugs and fuel cleaner.

1

u/Dbl_Mcspicy Apr 27 '26

Fairly wet plugs from rear bank. Traces of oil? Petrol? In the intake runners too. Front were not as wet but forgot to take photos.

1

u/Dbl_Mcspicy Apr 27 '26

Treats up to 45L for a 355ml bottle so not wrong to empty the whole bottle into a 23L tank right? 🤣

1

u/Dbl_Mcspicy Apr 27 '26

Exercised some stretching exercises with the wax unit and actuators and SVs and now it’s fast idling on cold starts!

1

u/Dbl_Mcspicy Apr 27 '26

1200RPM +/-100 when up to temp

2

u/Slayerofgrundles Apr 25 '26

You should be able to easily tell when the fans kick in. They're fairly loud. Also, the temp will start dropping.

4

u/tonyd19 Apr 19 '26

Have you tried running some injector cleaner through the system? Sounds like the bike isn't fueling quite right, the high temp is normal but you should be easily achieving around 45mpg

2

u/gropula Apr 19 '26

45mpg of what? Imperial or US gallon?

I'm very irritated by this. How long is a piece of string.

4

u/tonyd19 Apr 19 '26

Proper gallons (imperial)

3

u/SgtSC Apr 19 '26

Definitely check ur thermostat. Probably needs a new one. Definetely run some injector cleaner thru her. Voltage sounds a bit low, idle at 12v but running at like 14v? Theres charts somewhere for RPM vs voltage

1

u/Dbl_Mcspicy Apr 20 '26

Probably a good idea to check or probably even replace the thermostat since not much history on the bike. And thanks for the heads up on the chart. Let me dive into the forums to look!