Help with misfire

[honest phil]

hi all another newbie here hope one of you can help.
my son has a m reg fiat punto 55 1.1cc,quite a little gem but there is a fault i cant pin point.
it start's and ticks over fine no loss of power. the fault being occasionly when travelling between 15 to 30mph and then accelerate it misses(and you have to drop down the gears to stop it from misfiring).new plugs,new leads,new coils,new airfilter.dose any one have an idea what could be wrong PLEASE it's driving me mad.
Phill
ps it dosent do it all the time

 

[CinquePunto]

could be something like the Lambda sensor, or the vacuum thingamybob sensor

lambda sensor is quite common to go on cars that age, try another one out, it may fix the problem, but are pricey new... try getting one from a scrappy if you can (if it doesn't work you've eliminated that)

 

[arc]

does it do it when warm, cold, or no pattern.

lambda and map sensors are where i'd be starting at

 

[honest phil]

warm,cold,damp dry what ever it still dose it

 

[honest phil]

sorry to be thick but what is the map sensor please

 

[arc]

ok, i've not worked a punto55 but my cinq sporting used to have a very similar engine in there.

On the inlet manifold, there should be a thin pipe that goes to a sensor. that is the MAP sensor. It senses pressure in the inlet manifold, and helps the ECU deciede on fueling and timing.
Check that the pipe joining them is free from crap, remove it and blow down it. Check it is not split anywhere. This sensor could be faulty also.

As has been said, it could also be a lambda fault. That sits in the exhaust, monitoring O2 content. It is also used to adjust the fueling (not sure about timing on that one - but i suspect so.)

Is it def a misfire, and not something more like fuel starvation? It could be a clogged fuel filter - has that ever been swapped on your car?

Another possibility, is the crank / TDC sensor. This is on the bottom left end of the engine, it watches a pulley on the end of the crank and notes when the engine is at TDC. This is used for timing. If that sensor is dirty, it can cock things up. It doesnt really sound like this one is at fault - but it is worth giving it a clean to eliminate it.