I wonder if anyone here can help or experienced similar. The problem is that the engine splutters a bit at about 2000 and 3000 RPM. There are no fault codes shown and no warning lights appear when it happens. Unfortunately these are the revs it passes through when leaving or pulling off from roundabouts and junctions. I believe it is 1 faulty or dirty fuel injector and at them particular frequencies (circa 2k and 3k) the injector fails to delivery fuel (it's a petrol 2l). Other than that it is the DI (direct ignition) coil on the plug. Mazda changed all the spark plugs last week to eliminate a faulty plug, but to no avail. But it seems that if there is no fault code to tell them what is wrong they do nothing (under warranty) or don't know what to do. If it was my car and not under warranty I would buy 1 injector and swap out 1 injector at a time every weekend until the problem was come. After that I would do the same with a DI coil if all injectors proved to be okay. If it was a faulty injector wouldn't it play up all the time? Therefore must be 'dirty'.
It loses power and a few times now I've had to put on the hazard lights and pull over until it 'clears' and runs on all 4 cylinders again.
Anyone else had this? I've had it 2 months now and it can leave us exposed to an accident while making maneuvers in this rev range.

 

These kind of intermittent faults are very difficult to diagnose. It's unlikely to be a faulty injector. They either work or they don't. If it was dirty it could leak causing over fuelling which would be picked up by EOBD as a fuel trim/lamda fault. Or it could be blocked but that would be unlikely to cause an intermittent misfire. An injector is not effected by engine load, so you should be able to replicate the fault by revving the engine.
It could be a wiring fault. I've had two recently, one VW and a Merc that had poor connections in the loom feeding the injectors. However, both of these had been picked up by EOBD as injector faults. Try moving the loom very slightly and see if there's any change to engine note.
If it's a misfire on just one cylinder that should be picked up by EOBD.
It could be a coil but again that should be picked up by EOBD. Misfire detection is very good on modern cars to help prevent damage to catalysers.
You can do some tests with an oscilloscope on coils, which might detect a weak one, but the dealer may not have the kit. I do .
Rather than one cylinder dropping out, could it be all cylinders losing power? In which case it could be low fuel pressure or an air monitoring fault.
I don't know whether the 2.0l petrol has a Manifold Absolute Pressure (MAP) sensor or a Mass Air Flow sensor (MAF) or both. Either way they could be dirty or failing which could cause the symptoms. If you know where they are and are competent you could take them out and clean them with switch cleaner, but be careful as they're expensive and very delicate.
Could be the fuel pump, reasonably easy to diagnose but it takes time to do the tests.
Check all the vacuum and breather pipes for leaks. You may have a pipe that's slightly loose or cracked that leaks when the engine moves under load.
Good luck with it....keep us informed on progress.

 

I had a similar problem with my old 626. Changing the plugs helped but the main cause was the air inlet manifold gasket leaking. As soon as it was replaced it was like the car had a new engine. Your engine will be a bit young to have this problem but it may be worth getting it checked. Good luck.

 

Thank you both for your replies with input and suggestions. Some extra information here about my Mazda 5, the Mazda mechanic at my local garage said he will have to consult mazda technical. also, when I first took it in for it's 25000 service and listing this problem which did have engine fault codes. They said there were lots of fault codes and yet did not rectify any fault. they just cleared the codes and asked my to bring it back when faults start appearing again with the fault light. That seemed a bit lazy to them, but I suppose while it's under warranty they don't want to spend any money on it unless absolutely necessary. However, I bet if it was out of warranty they would have recommended that I pay for all the fault code repairs. When I next talk to him I will ask for the list of codes they pulled from car, maybe it will shed light on the problem for me or you. It's odd that the fault only appears at such low revs (2000 to 3200). But it feels like only running on 3 cylinders and 2 at it's worse when I have to pull over until I can get it to rev over 3200RPM. It could be that something is loose and at higher engine speeds is 'sucks' it shut (for example a crack in a corrugated air hose). The car is now only 14 months old (28,000 miles). I'll update when new info available.

 

It would be interesting to see the fault codes. However, sometimes a fault can trigger more than one fault code and it may not be obvious from the codes where the fault lies. For instance an air leak could give an Oxygen Sensor fault code together with a multiple misfire fault code. In that situation it's sometimes too tempting for a technician to fit an Oxygen Sensor and off the customer goes just to come back the next day with exactly the same fault. So, it's not unusual when you have multiple codes and a fault that's not that easy to replicate, to clear the faults and ask the customer to come back as soon as the light comes on again.
It's very difficult to know how much time to allow for these kind of fault diagnostics in a busy workshop, but I'm sure they'll get to the bottom of it.

 

That is probably right. Sometimes it's very hard to see or understand actions or non actions without the explanation. If they explained these things, always, it would be much more helpful but I thing they treat all customers as idiots sometimes. I'll update when it's been in again.

 

The engine light came on again on Saturday while driving on snow covered road (UK) over the next few days it came and went about 5 times. When I managed to get to the Mazda garage they found the fault code was p0131 (and also a 'sun sensor' fault which we ignored). That indicated Oxygen sensor. So booked in for Jan 5 for diagnostic and or replacement. Now does anyone with experience of faulty O sensor think that this would cause poor engine performance at low engine speed (up to 38000RPM)? I slutted poorly going up a hill toward Kettering on Monday, then the light came on (4th time this week). Only getting 27.6MPG where before this issue earlier this year I got about 30.1MPG (long term monitoring).

 

It certainly could cause poor performance. P0131 indicates the oxygen sensor has low voltage. I don't know off hand what type of O2 sensor is on the 5 but the principles the same. Most oxygen sensors fluctuate between .2 and .8v. When the engine is hot the O2 sensor will have .2v for a lean mixture and then the ECU will inject more fuel until it sees .8v for a rich mixture. If your O2 sensor is stuck at say .1v then the engine will run very rich as the ECU tries to correct the mixture.

 

Mmmmm! Then I wonder if my car has been running rich all the time in the last 6 months then? As the consumption is poorer than it used (now says 27.5 compared to 30.1MPG) to be and there is a strong smell of unburnt fuel at rear when stopped and ticking over. Very much like very old cars with carburettors and choke left on. So my fingers are crossed that they replace it and it cures my poor performance. Running rich is probably fine when I'm high up in the rev range 3800 - 6000RPM.
MERRY CHRISTMAS Everyone!

 

Right, the Mazda 5 was in yesterday and the sensor itself was fine but they found a 'corroded' connector in the wiring from road salt. They have cleaned and 'cured the fault'. I find it a little poor that, at the time of initial failing (middle of 2009), the car was only 10 months old. Anyway the car seems to be running better. Though it is really hard to tell as all the roads I've been on since collecting it have been snow or ice covered and been unable to give it some stick without wheel spinning, in a controlled and deliberate manner of course ;.). But the geometry on the sport (17") versions doesn't help either in this matter, what with it being too negative a Camber.