Stalling when hot

Discussion in '5th Generation 1998-2001' started by Terry Smith, May 6, 2018.

  1. Terry Smith

    Terry Smith Member

    Country:
    New Zealand
    Joined:
    Nov 18, 2013
    Messages:
    1,819
    Likes Received:
    195
    Trophy Points:
    78
    Location:
    Auckland, New Zealand
    Map
    Lately I've noticed that my 5G (98, 102000km, stock barring a K&N air filter, de-PAIRed, Delkevic muffler) wants to occasionally stall when coming to a stop and pulling the clutch in. This only happens when it is pretty hot (around 100C), and does not seem to happen when the temperature is lower, although I do see the idle dip a little. I have the idle set to 1300 rpm and the starter valves are correctly unbalanced according to the manual, and idle is very steady once I am at a standstill. Can anyone suggest a possible cause? There are no FI codes, and the throttle boots are fairly new and tight.
     
  2. squirrelman

    squirrelman Member

    Country:
    United States
    Joined:
    Jul 27, 2006
    Messages:
    7,625
    Likes Received:
    111
    Trophy Points:
    78
    Location:
    Buffalo, NY
    Map
    only thing i can guess is that the clutch isn't releasing fully (air in system?) and the unusual friction/engagement with the rear wheel drags down rpm until it sputters ?????
     
  3. Terry Smith

    Terry Smith Member

    Country:
    New Zealand
    Joined:
    Nov 18, 2013
    Messages:
    1,819
    Likes Received:
    195
    Trophy Points:
    78
    Location:
    Auckland, New Zealand
    Map
    Thanks for that but definitely not the clutch as it idles really well once stopped, in gear, with no drag. The stall seems more connected to decelerating than clutch action.
     
  4. GreginDenver

    GreginDenver New Member

    Country:
    United States
    Joined:
    Mar 27, 2016
    Messages:
    279
    Likes Received:
    78
    Trophy Points:
    28
    When this happens (as you roll to a stop) whether it stalls or not, have you noticed a rich smell (gasoline in the air around you)?

    I ask because the engine is stalling due to momentarily exceeding usable air/fuel ratio limits, either too lean to run or too rich to run. Figuring out which might be a start toward solving the mystery.

    According to Chapter 21 of the Service Manual the VFR800's PGM FI runs in (what is commonly called) "Speed Density" mode at low RPMs, which means it depends on the Manifold Absolute Pressure Sensor to determine the proper fuel pulse-width (in milliseconds). If the 20 year old MAP sensor is giving inaccurate readings that would be a problem.

    I mention this as a possible scenario because I've begun to suspect there's something not-quite-right with the air/fuel ratio that my '99 is getting while at low RPMs (a bit smelly, on the rich side). With the bike not flashing any FI codes I can only guess that while one of the sensors hasn't completely failed, it isn't giving perfectly correct/accurate data to the PGM FI box.

    At higher RPMs the PGM FI switches to (what is commonly called) "Alpha-N" mode which does not use the MAP sensor for the main Volumetric Efficiency decision making, instead it uses Throttle Position sensor data in place of MAP sensor data.
     
    Last edited: May 6, 2018
  5. Terry Smith

    Terry Smith Member

    Country:
    New Zealand
    Joined:
    Nov 18, 2013
    Messages:
    1,819
    Likes Received:
    195
    Trophy Points:
    78
    Location:
    Auckland, New Zealand
    Map
    Thanks for an interesting suggestion Greg. Aside from a failing sensor, maybe some terminal corrosion might be affecting the resistance values? It has been a few years since I had the airbox out, so I might start with a visual inspection. I'd guess that any deterioration in the hoses (and IIRC the MAP sensor is connected by hose to the inlet) might also lead to less than correct MAP operation.
     
  6. BTF/PTM

    BTF/PTM New Member

    Country:
    Germany
    Joined:
    Feb 22, 2015
    Messages:
    48
    Likes Received:
    0
    Trophy Points:
    6
    Another simple check, could the fuel tank breather hose (overflow hose, I think it's called in the manual) be blocked? If fuel vapour can't release when everything is nice n warm it can lead to fuel flow problems that can cause a funky idle.
     
  7. OZ VFR

    OZ VFR Insider

    Country:
    Australia
    Joined:
    Feb 25, 2013
    Messages:
    1,031
    Likes Received:
    257
    Trophy Points:
    128
    Location:
    Old Toongabbie, Australia
    Map
    A faulty TPS would cause what you're experiencing too.
    If you have a PCIII it's easy to check, if not I'm sure the manual has it covered.
    Either way, let us know what you find.
     
  8. Terry Smith

    Terry Smith Member

    Country:
    New Zealand
    Joined:
    Nov 18, 2013
    Messages:
    1,819
    Likes Received:
    195
    Trophy Points:
    78
    Location:
    Auckland, New Zealand
    Map
    I did a quick disassembly tonight just to see if there were any visible faults. All the connectors on sensors were snapped in tight, and the pins were bright and clean. I hit them all with contact cleaner and the dielectric grease. The vacuum hoses all looked OK but I did not delve to deep in these. There was a little residue around the butterflies which I removed with some carb cleaner. The bike fired right up with no issues after reassembly. I am washing and re-oiling my filter and will do a synch when that is done.
     
Related Topics

Share This Page