engine misfire only when wet

[percy punto]

i have a punto 1.2 8v 2002 with about 90k on the clock. the car runs fine when weather is dry but when we have had a lot of rain like last night i get the management light come on and a code that is cylinder 4 misfire.
i have changed the plugs, ht leads,crank shaft sensor .have had an emissions test and that come up normal also had a live reading data check on a snap on modus and that doesn't show any problems either.
anybody got any ideas as i'm fast loosing patience with the car.As the fault is only when wet i'm a bit confused to say the least!

Thank you!

 

[fingers99]

Next time it does it, wip open the bonnet, engine running. If at night, just look for blue flashes. If daylight, you can use an old blanket to form a makeshift dark room.

Remember, any filth on the coils will form an easy track for HT leaks, which will cause a misfire.

You might also clean up all the engine earths, coat with vaseline or silicone grease and re-attach.

 

[percy punto]

ok thanks, will try that if there are flashes would that suggest that the new ht leads are no good or is there something else? it's only ever miss fire on 4 and when it's wet.

 

[fingers99]

That or -- if on coil side/end, breakdown of the insulation of the coil itself (but try cleaning, rather than spraying, with something like WD40.

 

[percy punto]

Had a look last night..... no blue flashes it does seem to be using a little water too atm and although i've changed the engine temp sensor and thermostat the fan clicks in quite quickly d you think this is all connected?
Thanks again

 

[fingers99]

Possibly. But how..............

Look for water leak (most likely from water pump or water rail O ring -- often only visible with engine running and up to temp, and then, from underneath the car!)

Mk2 Puntos are said to be especially difficult to bleed, too.

If no leak found, compression test.

If those are OK, I'd suspect breakdown of wiring to one injector, or even a hairine crack in the injector itself. No.4 should be furthest away from the timing belt end.

 

[8valver]

if its not scum on the crank sensor pickup being exagerrated by the wet run car on a dry day
get a spray bottle and saturate the number 4 ignition lead/coil only and see if fault re appears

if so change it with cyl one

repeat and if fault changes to cyl one

change ignition coil.


ive also had a tipo track car years ago that wouldnt start one day
had new crank sensor etc etc
cranked over till the starter burnt out wand was at a loss

replaced starter weeks later and vroom she started

it was a weird one but starter was on its way out in the first place and the magnetic field was playing havoc with the crank sensor coil trigger causing my problem.

so any ancilliaries close by being turned on ( causing a magnetic field) near any unshielded sensors or signal cabling can cause weird and wonderful problems


im maybey going into things a bit deep here lol