Cylinders not firing at low RPM - driving me mental!

[r00x]

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.

 

[Anonymous]

As you've already changed the spark plugs, I think I know the answer to this, but I'll mention it anyway.....

No oil in the spark plug wells? Round the spark plugs and around the bottom of the HT leads?
Oil in there is good for running/misfiring issues.

Myself, I'd probably go for coil pack AND HT leads next. Just my opinion though.....

 

[r00x]

Hmm... well, now you mention it, there is a teeny bit it seems. Not exactly a "puddle" around each plug but there is some there...

How does it cause misfiring? By impeding the conductivity to the engine block?

How can I remove any trace of oil there? I'd rather not let any get into the engine by removing the plugs. Pre-plug change there was more oil than this, and I soaked it up as best I could with paper towels, but the engine still ran like crap for half a minute because some inevitably dripped inside.

Can coils/leads cause very specific misfiring at certain RPM? I could replace them anyway and have "spares" I suppose

 

[martinthrapston]

Cam cover gasket needs changing,as well as ht leads and coil pack,oil in plug wells can kill both items causing bad running

 

[Anonymous]

Soak the oil up with some kitchen roll before you take the plugs out.

 

[r00x]

Thanks for the info. Now it's bad weather and I don't have a garage to change the gasket under cover :evil: typical...

So the oil will fugger up the coil pack and HT leads, eh? Yikes.

Well, since my last post I have more information, can you tell me if this confirms or counteracts the possibility of it being the coil pack, etc?

1) If I unplug the throttle position sensor: the car drives smooth as butter! Perhaps a slight "putt", once, but otherwise it drives "like a normal car". :grin: however, engine tone is different, lacks its normal racy tone, and the car runs like poo in traffic (long delay on throttle response). Obviously engine check light as well. Plug it back in, engine is back to hesitating as before.

2) If I unplug the MAF: car drives better, but still exhibits sputtering under acceleration (just less often/severe, or so it feels). Engine tone different to normal and different to the above - sounds like there's an induction kit on the car or something :lol:

3) I tried replacing the TPS (whatever, they're cheap, easy to do) but no real changes.

4) Having said that... as of the last several days, the character of the sputtering has changed. It can now run like crap when accelerating all the way up to 4000RPM (sporadically - it won't if the engine RPM is already ~3000+ but if going from say, 1500 to 2000 and it happens to sputter down there, then it will carry on up the rev range) but meanwhile seems less prone to misbehave down low on gentle throttle. This makes it more driveable in low-speed traffic but more of a pain transitioning to higher speeds This change occurred BEFORE replacing the TPS and seems to have nothing to do with that.

5) Definitely worse on journeys involving the motorway. Will be relatively OK shortly after coming off the motorway. But five, ten minutes later, quite bad. Seems to be as the heat in the engine from the motorway trip starts to heat the engine bay (now there is little airflow) the problem gets worse and worse.

Any of that pointing to other issues do you think? Or are we still targeting the rocker cover gasket & coil pack (I have new HT leads)? I'll change the gasket and coil pack ASAP anyway, but if there's anything else, would be good to know :roll: I thought engine designs from pre-2000 were supposed to be simple!!

 

[Technics1210]

I had a similar issue on my 1.7 puma today, diagnosed P0302 fault code miss fire cylinder 2, i had some of the symptoms as you described, back fire, lumpy acceleration, loss of power, engine getting very hot etc, it's possibly one of 3 things, plugs, ht leads or coil pack unless anything catastrophic had happened, mine was coil pack, accompanied by flashing mil light, coil packs are however cheap on the puma at around £25, easy to fix with basic toolkit & basic knowledge, be careful driving it with the fault as unburnt fuel can destroy your catalytic converter, i would imagine the above will apply to 1.4 & 1.6 variants ;-)

 

[r00x]

Hi! I should have updated this thread!! Mine was also the coil pack. I changed the HT leads as well, just because. I changed the rocker cover gasket, pulled out and cleaned the plugs and their sockets in the head (filthy and oily), cleaned a lot of the engine up actually, and replaced the coil pack and HT leads all at once.

Since then it's been great, although I've recently had an issue where on rare occasions it doesn't want to start (just turns over) - not sure if it's related!

 

[IronD]

everyone seems to have already anwsered your problem but i'll add my little bit anyway!

My wifes car did exactly the same thing, i instantly changed the sparks and leads and straight away the car ran like a dream, no stuttering or lack or power anymore.

might just of been a bit of luck but worth changing the leads, its a cheap enough fix