Stuck with misfire at idle

[marks3drtaz]

Hi all I have a very annoying misfire/ lumpy at idle and up to 10% throttle opening thus causing very jerky running at slow speed. I have stripped the top end to check for air leaks and checked valve lift via the rockers, the car is running a stealth cam, plenum extender plate, induction kit, fuel pressure at idle is 1.78 bar and an increase to 2.47 with pressure regulator disconnected, voltage to afm is fine and swapped to another confirmed working afm, all sensors tps, temp, and lamda check out, just ordered a rover gauge to confirm readings, injector circuit resistance is 5.1 and 5.4 ohms so a little high but the injectors all give the same readings individually, fitted new leads, plugs, ran without plug extenders, dizzy and rotor genuine lucas and new, cleaned connector to ecu, confirmed earth to rear of engine ok, compression test is fine. So at a complete loss of where to go next. I am suspecting a dodgy injector but there is no leak down problems and the plugs are evenly brown, a poor spray pattern may cause a miss but the as soon as you go above 10% throttle input the performance is very strong and very smooth with no sign of a problem. I need help as I am completely stumped.

Thanks Mark

[PeteGriff]

It sounds like a fuelling issue, but you have checked all the obvious culprits, though an injector could be playing up on low pressure idling. Might be worth having all the injectors out and getting them cleaned (had all mine ultrasonically cleaned a couple of years back). Have you considered the fuel filter, may be a long shot but if partially blocked could restrict flow (but you do say its ok at higher rev's)? Have you checked compression in each cylinder? Is there a fuel hose squashed anywhere restricting fuel flow? Is the coil and/or connections OK? A coil breaking down might give you mis-firing grief. Lastly, ECU issues, you say you have ordered Rovergauge, this may give you some insight. All the best, Pete

[marks3drtaz]

Does feel like fuel It also surges at cruising speed say 80 plus as this is at light throttle, acceleration is fine, low speed running is a pig, this lumpyness is also present at cold inc fast idle, a compression test was fine, also feels like an air leak would but cannot find one, maybe the valley gasket next but never heard of this failing without oil and water leaking as well.

Mark

[EGB]

Check under bonnet in the dark for short arcing from HT leads including the dizzy to coil. A loose plug lead can cause misfire at idle. Ignition module perhaps.

[dwgriffith]

Check/alter the timing. What is it currently set at? and has this been altered before the onset of running issues?

[marks3drtaz]

just ran with lambda's disconnected and runs nearly perfect so much smoother and no lumpiness at slow speed. will order a valley gasket as I do suspect an air leak and I have checked and double checked all other leak possibility's.