[Solved] Misfires on cylinders 2 and 4 - 1.4 Petrol (AUA)

Thalion

Member
Hi ! I hope someone can shed some lights on my misfires problem.

Audi A2 1.4 petrol, AUA engine, 275.000 km

So last week I was driving on the highway, the car is completely ok (but heavy with luggages), when suddenly a heavy rain drops. Within seconds, the car stutters, loss of power, MIL blinks... I manage to get to the emergency stop line before the engine shuts off.
I try to turn the engine on again, but still no power.

After a few minutes, the rain lightens a bit. I finally manage to get the engine started, and while there was very little power, I managed to exit the highway, and reach a proper parking space. The time to reach my computer and VCDS cable, rain has stopped.
So, I do a diag, and find codes P0300, P0302 and P0304 : misfires on cylinders 2 and 4, which explains the loss of power.
I clean the codes, start the engine, it run smooth. So I go to finish my travel. But after a few meters, loss of power again and blinking MIL. However, the road is mostly flat, I was near the end, and I reach home.

Luckily I was just about ready to do the service, so I have new spark plugs, new air, oil and fuel filters - I changed everything. Still misfire on 2 and 4.

I swapped the injectors - no change.

Cleaned the throttle body and EGR while I was at it, adaptation (I'm getting used to this part), no change.

I did a compression test, all cylinders seem alright : pressure between 11.4 and 12 bars.

I checked for sparks by using a spark plug at the end of the wire with contact to the body of the engine, and seen some sparks.

I believe that if there was a problem on the fuel system before injectors, or air intake, the misfire would randomly occur on every cylinders.
So I think it might be the coil or control module, but how to check for sure ? I thought about crossing the wires between the coil and spark plugs to check if I managed to make the misfire switch cylinder, but as the spark is timed for each cylinder, that would not work, right ? So how can I check the coil ?
Or maybe someone has an idea of something else that could cause this, and how to check it ?
 
Compression test would be the best diagnostic to carry out first and avoid mis diagnosis and parts costs, however because it's 2+4 I imagine the coil pack is suspect number one if it was say 2+3 then head gasket failure would be a real possibility
 
As I said, compression test looks ok.

Turns out the ignition coil is not expensive, so I ordered one and will test it when I receive it - as you said, it's indeed the number 1 suspect.

It seems the ignition coil uses the wasted spark system, so cylinders are indeed paired at some point. I haven't found information regarding which are linked by this system, but it could be an explaination, so... worth a try.
 
I would bet on the coil, especially in view of the weather conditions described. I will not be surprised at all if when removed you find that the insulation / potting compound on the back of the casing is cracked or failed. Our car - also around 290,000km when the problem became obvious - needed a new coil which solved most of the problems. A Bosch was the best choice in the end ; a cheap Amazon coil worked better than the dead one, but almost immediately failed on one cylinder (not a cylinder previously affected when the old coil was in situ) so was replaced by a Bosch 2 days later when it could be delivered.
 
I would bet on the coil, especially in view of the weather conditions described. I will not be surprised at all if when removed you find that the insulation / potting compound on the back of the casing is cracked or failed. Our car - also around 290,000km when the problem became obvious - needed a new coil which solved most of the problems. A Bosch was the best choice in the end ; a cheap Amazon coil worked better than the dead one, but almost immediately failed on one cylinder (not a cylinder previously affected when the old coil was in situ) so was replaced by a Bosch 2 days later when it could be delivered.
I would agree with this diagnosis & would confirm with an inspection with bonnet off in damp/ humid conditions as the arcing will almost certainly be seen.
 
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