52 plate Punto 1.2 sporting 16v fault codes P0300 & P0302

[Will H]

Hi
Right at Christmas, I had young girl in trouble with a 52 plate Punto 1.2 sporting 16v. She asked for a service to be done, but in reality it had an intermittent misfire.
I collected the car from her house and was horrified to find that the battery was not fully charged (and I was unprepared for this)
As the engine laboured  due to 'kicking back' whilst starting, the dash dials went wild, slamming down into the rest position??
A bump start down a slight incline and I was underway back to the workshop.
On the way back, for a time, everything was OK. Then I detected a misfire and the MIL light was flashing. by the time that I arrived at the workshop, it was on permanently.
A Snap-On scan revealed codes P0300 & P0302.
Spark plugs were all burning the same + OK, but as I was doing a service, I replaced them. Plug leads were visibly in good order.
As I had heard that faulty coils can damage the ECU, I thought that I would fit another coil. - No one had a coil in stock, not even the Fiat main agent - approx 10 days due to Christmas!!
The only option was to fit a second hand one from a Brava. (same part number)
The coil pack is a Bosch 0 221503 4 07 (see photo)
Finished service, including new battery - fault codes cleared and test drove - OK
However, I had a phone call yesterday from the girl, stating that the car was exactly the same. After driving for a short while, the car losses power - max 25mph and jolts and jerks violently - the MIL light is flashing.
I have not had the car back to do another code read, but if it is correct that the fault symptoms are identical, then I would expect the codes again to be P0300 & P0302
I had thought that if the ECU had 'blown' then the misfire would have been permanant?? Or does this sound like a typical ECU failure scenario for these cars.
I have now found a source for a new coil & ECU 'kit' at reasonable money, but it will still expensive if I am barking up the wrong tree.
All help gratefully received.
Will H

[Derek Edwards]

Hello Will

I've done a few of these with the misfire coil / ecu fault, but they all had 2 separate coils. The fault on them was no drive to one of the coils so down to 2 cylinders.

All the best
Derek

[Will H]

Hi Dereck
Thanks for immediate responce.
What are your thoughts about the crank sensor??
Will H

[Derek Edwards]

Not sure you'd get the P0302 if it was crank sensor, (unless there is some damage on the phonic ring maybe) but a scope on the coil primary shows the ecu fault clearly as it fails to trigger the coil.

Derek

[TopGun]

Will,

I think you have a Motronic ME7.3 H4?

If so then I do not have a vehicle specific code detail but P0300 as a generic EOBD is "Random misfire"
and P0302 is "Misfire cylinder 2"

On that basis the codes do not point to the CKP. That said we change shed loads of CKP's on Punto's of assorted ages but all show up clearly as faulty on a scope.

Check with the dealer to See if the car has had or needs the software update that is supposed to prevent this problem.

You really need to scope the coil outputs from the PCM to be sure but faulty coils do tend to "spike" the output drivers in these PCMs.

HTH.

[TopGun]

Derek,

"Snap" - If i waffled less and got straight to the point then I might have beaten you to the post!

[Will H]

Hi guys
Thanks once again for info.
I have booked the car in for Sat morning, so will have more part numbers available (ECU on top off throttle housing with 2 x large cable connectors).

'Top' - you said "we change shed loads of CKP's on Punto's of assorted ages but all show up clearly as faulty on a scope." - but what symptoms were they showing??
I'm trying to get my head around why intermittant and jerking so violently (allegedly) - just thought that it may be giving confusing information as when to fire on which cylinder??
Will H

[Angus]

Will

A Fiat with a single-cylinder misfire when it warms up?  My first thought would be [yet another] duff Marelli injector.

A quick resistance check would be worthwhile.

[Will H]

Hi Angus
It would be so much simpler if the fault was present all the time. However it appears as you say when the car has warmed up a bit and under load.
When I test drove it prior to working on it, it appeared to be just failing on one cylinder (OK at idle), but reports of such violent jerking - I'm thinking either total failure (on/off) or failing on two cylinders?? 
Will H

[Angus]

Will

Yes; I noticed the disparity between your experience of the fault and the owner's description.  I just couldn't see how a coilpack, or coil driver, failure could give you the feel [and code] of losing only one cylinder.

I note that the coilpack and injectors have their own fuse; maybe recording the amp flow across that fuse during a road-test might show some irregularity.