Hey all - I've been fighting an issue with my 1.7l Puma which I can't seem to fully diagnose.
In short, the car idles fine and drives fine, except at ~1500-2000RPM one or more cylinders will often just refuse to fire under acceleration. It can be either one or two "puffs" or a whole string of incomplete ignition cycles that sap the car's power away.
My observations:
My attempts at fixing have sometimes seemed to reduce the problem to almost nonexistent (no problems for an entire journey) for a day or so but could be coincidence.
If conditions are right and you floor it at low RPM, you can sometimes get a massive BANG out the exhaust. I try to avoid this as it can't be good for anything, I'd guess unburnt fuel igniting in the exhaust.
It doesn't happen from a cold start. Can take a few minutes or longer to appear. Once the engine is approaching running temp it can happen.
It also appears temp-dependent. Either the ambient temp inside the engine bay, or the air intake itself? On starting a journey inlet air temp will be ~14C and it behaves well. By the time we reach ~30C the problem can be reproduced fairly reliably at certain mid-high throttle levels. On hot days/motorway journeys and traffic where the inlet air temp reaches ~50C the problem can be quite bad (very prone to doing it at even light levels of throttle, almost always a few missed ignition cycles when it happens too). I monitor the temp & other engine data with a bluetooth OBD adapter and Torque for Android.
O2 sensors doing what they should? Interestingly at low RPM acceleration the pre-cat O2 sensor still shows oscillating O2 levels (like on idle). At ~2000RPM+ when accelerating hard the O2 sensor seems to "stick" to a high reading which I assume is normal. When coasting the O2 sensor "sticks" to a low reading (this is Volts, by the way). It interests me that the problems I get are happening at the point where the O2 sensor is still showing varying fuel-air richness... but I have no idea what I'm looking at. Just thought I'd mention...
No engine check lights or warnings or anything!
What I've done so far:
New spark plugs
Cleaned MAF (may have reduced a bit after that, but returned)
Airbox replaced with stock (was crappy aftermarket filter. More low end torque now, higher MPG)
Replaced dodgy ground strap (seemed to clear it up for a while, or could be coincidence)
Cleaned ground strap mounts & bolt engine side (seemed to clear up but could be coincidence)
Recon'd injectors (more power, higher MPG now)
ECU reset (no difference)
What I plan to do:
Rocker cover gasket (looks a bit tatty on the edges)
Fuel filter
??? HT leads? Coil pack? I thought these would cause more continual problems than sporadic heat-related low-rpm misfires.
In short, the car idles fine and drives fine, except at ~1500-2000RPM one or more cylinders will often just refuse to fire under acceleration. It can be either one or two "puffs" or a whole string of incomplete ignition cycles that sap the car's power away.
My observations:
My attempts at fixing have sometimes seemed to reduce the problem to almost nonexistent (no problems for an entire journey) for a day or so but could be coincidence.
If conditions are right and you floor it at low RPM, you can sometimes get a massive BANG out the exhaust. I try to avoid this as it can't be good for anything, I'd guess unburnt fuel igniting in the exhaust.
It doesn't happen from a cold start. Can take a few minutes or longer to appear. Once the engine is approaching running temp it can happen.
It also appears temp-dependent. Either the ambient temp inside the engine bay, or the air intake itself? On starting a journey inlet air temp will be ~14C and it behaves well. By the time we reach ~30C the problem can be reproduced fairly reliably at certain mid-high throttle levels. On hot days/motorway journeys and traffic where the inlet air temp reaches ~50C the problem can be quite bad (very prone to doing it at even light levels of throttle, almost always a few missed ignition cycles when it happens too). I monitor the temp & other engine data with a bluetooth OBD adapter and Torque for Android.
O2 sensors doing what they should? Interestingly at low RPM acceleration the pre-cat O2 sensor still shows oscillating O2 levels (like on idle). At ~2000RPM+ when accelerating hard the O2 sensor seems to "stick" to a high reading which I assume is normal. When coasting the O2 sensor "sticks" to a low reading (this is Volts, by the way). It interests me that the problems I get are happening at the point where the O2 sensor is still showing varying fuel-air richness... but I have no idea what I'm looking at. Just thought I'd mention...
No engine check lights or warnings or anything!
What I've done so far:
New spark plugs
Cleaned MAF (may have reduced a bit after that, but returned)
Airbox replaced with stock (was crappy aftermarket filter. More low end torque now, higher MPG)
Replaced dodgy ground strap (seemed to clear it up for a while, or could be coincidence)
Cleaned ground strap mounts & bolt engine side (seemed to clear up but could be coincidence)
Recon'd injectors (more power, higher MPG now)
ECU reset (no difference)
What I plan to do:
Rocker cover gasket (looks a bit tatty on the edges)
Fuel filter
??? HT leads? Coil pack? I thought these would cause more continual problems than sporadic heat-related low-rpm misfires.