Misfire on overrun
Discussion
Car is a Peugeot 1.6 16v petrol, TU5JP4 engine, 90k miles.
Has a bit of hesitation/judder on the overrun. Happens on a warm day when trying to hold a lowish speed of say 50mph on a slight downhill. Pulls ok at normal low throttle openings. Doesn't feel like just one cylinder missing, feels more like two or perhaps the throttle valve fluttering about for some reason.
Have previously removed and decarbonised Throttle body & flap and replaced body to manifold seal.
Could this be related in some way to the EVAP Purge valve opening on the overrun? For the moment I have now disconnected this from the manifold and blanked-off the inlet and pipe which seems to have improved matters. Although not completely sure as misfire only happened under certain circumstances on a particular road. However also did other things at the same time so can't be sure which has made a difference: also disconnected the battery to reset the ECU and cleaned the plugs by soaking the tips in redex.
Is it possible that the petrol tank is letting air in somehow and on a hot day when driving downhill the ECU is opening the EVAP purge valve to allow the stored vapours get sucked into the maifold. This causing an air leak and running weak during this time?
Has a bit of hesitation/judder on the overrun. Happens on a warm day when trying to hold a lowish speed of say 50mph on a slight downhill. Pulls ok at normal low throttle openings. Doesn't feel like just one cylinder missing, feels more like two or perhaps the throttle valve fluttering about for some reason.
Have previously removed and decarbonised Throttle body & flap and replaced body to manifold seal.
Could this be related in some way to the EVAP Purge valve opening on the overrun? For the moment I have now disconnected this from the manifold and blanked-off the inlet and pipe which seems to have improved matters. Although not completely sure as misfire only happened under certain circumstances on a particular road. However also did other things at the same time so can't be sure which has made a difference: also disconnected the battery to reset the ECU and cleaned the plugs by soaking the tips in redex.
Is it possible that the petrol tank is letting air in somehow and on a hot day when driving downhill the ECU is opening the EVAP purge valve to allow the stored vapours get sucked into the maifold. This causing an air leak and running weak during this time?
GreenV8S said:
Perhaps it's applying overrun fuel cut-off but not doing it consistently. For example if you'd lost a little throttle travel the throttle may be tripping open just far enough to remove the fuel cut for a moment.
Could well be some sort of fuel cut-off triggering and then a fraction of a second later stopping, perhaps because it detects a change in manifold pressure.What might cause that, MAP sensor? I cleaned that a while back, however considering the Chinese ones are only £10 might be worth changing that anyway.
Is it possible this could be caused by spark plugs of too hot a rating?
Just been reviewing the history and am reminded that I had NGK Iridium ILFR6B plugs in for six months but eventually concluded that these were causing
a slight misfie at low throttle openings under load. Then changing to slightly hotter rated Denso KH16TT plugs which solved that particular problem. However could these new set of {hotter} plugs be causing the overrun misfire as not sure I ever noticed it previously.
Just been reviewing the history and am reminded that I had NGK Iridium ILFR6B plugs in for six months but eventually concluded that these were causing
a slight misfie at low throttle openings under load. Then changing to slightly hotter rated Denso KH16TT plugs which solved that particular problem. However could these new set of {hotter} plugs be causing the overrun misfire as not sure I ever noticed it previously.
Gassing Station | Home Mechanics | Top of Page | What's New | My Stuff