Misfire on light throttle - Zetec

[Ben Powers]

I'm hoping the wise folks of this forum will be able to help me once again.

I have a new, intermittent problem in the form of stuttering/spluttering and a lack of power.

It first happened after the car had been sat in the sun for a morning, and I assumed an issue with fuel vapourisation/delivery, as happened occasionally with my mx5 during auto tests on hot days.  I put it away and hoped the problem would disappear.  It did.  The next driver was uneventful.

Today it wasn't so hot, and after about 20 mins of gentle driving the engine started to struggle and misfire again.

It happens on a light throttle, and/or at low revs.  When it starts I lose power and the engine runs very roughly.  It doesn't die, and if I keep my foot down it struggles on before it seems to pop and then original performance is restored.  It's almost like a blockage and then it is freed and full power is back.

There is no loss of performance at full chat, and it doesn't seem to happen if I drive it hard, it's only at light throttle settings that it comes up.

My totally uneducated guess would be contaminated fuel?  It was sat for nearly two years, not sure it that could cause an issue in fuel delivery?  I did also replace the fuel pipes to/from filter recently, but can't see how that could affect anything.  The filter was replaced on MOT earlier this year.

The fuel pump has always been noisy, but if that was failing I'd expect the power to go at WOT rather than when gently pootling about.

I run it on the premium stuff and always have.  Haven't put any other additives in there.

Where would you look first?

[IanS]

My thoughts are to do with over heating.

WOT equals going at some speed and having good airflow through the radiator.

Pootling about you rely on the fan being effective and this is where I would look first.

I would also check that the water pump is working and has not half failed again engine speed could cover up a partially disintegrated impeller.

 

[nelmo]

I had this on and off for 6 months - finally tracked it down to the wiring to my TPS which had come loose. Found it using the data logger in the Emerald software (if you have an Emerald ECU?).

[Sparepart]

One thing to check would be the PCV valve. If it is sticking in the open position it will cause problems at idle and low revs. Just google "how does a pcv valve work" and you will find loads of info on it, how to check, fix etc.

[DeanRudkin]

What engine is it on?