5th Gen Idle Issues

Greetings,

I have been struggling with this 1998 VFR800 since I replaced the thermostat, all the radiator hoses and completed a valve adjustment.

The problem that I am experiencing is this:
Idle is low (would often die at idle if I didn't crack the throttle)
Idle adjustment screw is pretty much non-functional (Figured vacuum leak, can't find one)
Seems to run OK after getting off idle and the motor is spinning above 3k RPM
Poor fuel economy (this has been an issue since I got it, but the thermostat WAS bad)

Condition of the bike:
1998 VFR800 with 34k miles (I've only had it for about 4k)
Adjusted a handful of valve shims, none of them were terribly off
Has a PC2, which is currently unplugged for troubleshooting.
Synced the throttle valves with the PC2 unplugged and it will idle without dying, just really low.
I currently have the valve covers off and I just checked the camshafts to make sure I didn't mess that up somehow, looks good to me.
Two Brothers Full Exhaust
PAIR system disabled (plugged the hoses coming out of the valve cover)

I was listening and also spraying carb cleaner around vacuum areas and only once did I get the idle to change, and I tightened a throttle body boot that was completely loose, no change to performance, however.
Spark plugs look really dark and rich.

Not quite sure what else to look for on this bike. No codes are being thrown.

 

Sounds a bit like a failing fuel pressure regulator.

 

When my idle was kooky (started low, sped up while almost warm, back down when fully warm), it turned out to be a stick fast idle (WAX) valve. I tore it apart, cleaned everything, reassembled with a rebuild kit. Works great now.

Jose

 

If you've had the throttle bodies off to replace the thermostat have all the vacuum hoses gone back on correctly ? Idle problems are usually an air leak somewhere. The service manual has a great diagram of the hose routing.

 

vacuum leak in the intake will cause high idle rather than low idle which is what the original poster in this thread currently has.

also, if the problem (in this particular case) was limited to an intake vacuum leak the original poster wouldn't have the extra problem of "poor fuel economy" listed as one of the symptoms he's having.

an over-rich and abnormally low idle speed (that cannot be corrected with the idle adjustment) is an indication of over-fueling that the ECU isn't aware of (because no FI codes flashing when key-switched on).

I'm not 100% confident that I'm right in this case, but if the fuel pressure regulator is failed/failing there will be more fuel pressure than normal delivered to the injectors. The fuel pump in a fuel injected vehicle (car, motorcycle, truck, etc.) is always much more powerful than the system is designed for. The reason for this over-capacity is that the designers are taking into account the fact that the various parts inside the fuel pump will wear over time and the pump will end up putting out less and less pressure. As long as the fuel pump is putting out more pressure than the pressure setting of the vehicle's fuel pressure regulator this is not a problem. If the fuel pressure regulator fails and allows the full fuel pump pressure to reach the injectors the result will be an over-rich situation because the ECU (which doesn't know there's a problem with the fuel pressure regulator) will still be commanding the normal injector pulse-width (the duration of injectors open time) and because of the abnormally high fuel pressure more fuel will pass through the injectors during each injector pulse.

 

The other part of the FPR equation is that it varies the fuel rail pressure to keep a constant offset to the manifold pressure; thus when there is a high vacuum in the inlet, the fuel pressure is also reduced, again so that the FI system doesn't need to take account of a pressure difference. If the diaphragm fails, or regulator jams, or the vacuum line is compromised, then a higher fuel pressure will result with rich running.