RPM's dropping @ cruise, please read.

Alright, just purchased an '04 VFR800 and so far I haven't had a whole lot of problems with it.

There is one thing that is really getting on my nerves and it's something I can't explain by any typical mechanical means. Problem: whenever I'm cruising at a stable RPM for more than ~6-10 seconds the engine drops about 400-500rpms on it's own. For example, I was cruising on a flat straight road in 3rd gear @ 5,500rpm's @ 55mph. I grabbed the throttle and the switch housing and held the throttle grip as tightly as I possibly could to ensure it didn't move. After cruising at this speed for about 10 seconds without a single change in direction or speed or elevation, the RPMs dropped 400rpms and my speed reduced to ~52-53mphs.

I can't imagine what it could be, its so consistent all of the time, it's not the throttle grip sliding it's not the throttle cable being pinched, it's something else I can't figure it out.

I ordered new spark plugs and a K&N air filter and I'm going to get that stuff in hopefully by next Friday. Anything else you guys can think of?

 

How many miles on your 04? Maybe the valves need adjusting? Fuel line clogged? Fuel pump losing some pressure? Does it ever return to the previous RPM without extra input from the rider?

 

Sounds like the EFI system going into closed loop to me. Do a search for o2 sensor removal, and you'll likely find that some of the posts describe similar symptoms.

 

Alright, I did some research (found 1 post) about the topic. It appears it's something that the bike is designed to do in the process of it leaning itself out. My question is, does anyone know if it's circumstantial? The engine leaning itself out like it does, is it because the engine is too hot? Maybe because the o2 sensor is dirty? I mean, if I've owned one for 3 weeks now, and put about 800 miles on it, why am I the only one complaining about it (besides the other post I found)?

Does everyone feel it and just get used to it? It's really annoying for me especially on slab it's like the most aggravating thing in the world.

Basically I'm wondering, if I rode a brand new off the press 08 VFR would I be able to notice it doing it? Or is it something that becomes noticeable over time and engine wear?

Does it mean the engine is getting too much fuel? When it leans itself out is it letting more air in the cylinder or is it cutting fuel? Is it possibly that I'm using premium fuel only?

 

After reading some more posts about fuel and then double checking the manual it appears I should be using 87 octane. I'm extremely convinced the o2 sensor thinks the bike is running too rich because of the premium fuel mixture that it isn't used to seeing (11,000 miles with 87, 800 with 93) It might be a long shot but I'm going to drain it and give it a shot.

 

Hi, sorry for the brief reply earlier; I obviously did not provide the correct search criteria... Have a look here:

http://vfrworld.com/forums/mechanics-garage/17334-o2-sensors.html
http://vfrworld.com/forums/sixth-ge...ging-throttle-during-light-accelleration.html
http://vfrworld.com/forums/sixth-generation-2002-2007/8599-06-engine-lag.html


IF it is the EFI system going into closed loop mode, it is not dependent on heat, dirty sensors etc. It seems to happen to a lot of early to mid model VTECs, but appears to have been partly resolved with the ECU update that happened on 2006 and onward bikes. Basically the fuel system thinks that you are cruising and therefore leans out the fuel mix to improve emissions and economy. But unfortunately it leans it out too much, and you lose power. You then apply throttle, and after a second or so the ECU wakes up and feeds more fuel, giving a surge and a jerk.

The cheap partial cure appears to be bypassing the o2 sensors, as shown in this link: High-res Pictoral Of O2 Sensor Eliminators - VFRD

The more complete but expensive route is to do the sensor bypass and add a Power Commander. There are plenty of PC threads here too.

Also, I run 98 octane exclusively. I have tried a lot of different fuels, and they make no difference to the surging.

Hope this helps more.

 

Bleh, I'm sure a 06' ecu swap is completely out of the question. :redface:

Thank you very much for your help Winaje

 

Well nothing is out of the question, but you'll get a better result with less hassle by doing the sensors and PC. The connectors on the new ECU are different, and I think that the security system is all linked up etc too.

 

I've gotten no response in my thread to the same problem on mine. I'll check what you guys have talked about here to see if it helps.

If I have a PCIII, should bypassing the O2 sensor take care of it?