Engine Management Warning (misfires)

[ben]

Hi,

I have had an Engine Management Warning these last 2 days. It occured both times after driving for an hour or so on the motorway - usually when the car is idling again. The light is only intermittent, and goes off after about 30 seconds.

I took it to the Audi garage near work - and their computer says that all four cylinders have been misfiring. The technician also started to blame me for running the car on regular unleaded (it's a 1.6 FSI model).

Now, I have been using regular unleaded since new (2 and a bit years ago) without any problems up until now. Am I actually doing anything wrong here? The manual clear states that the engine can run on regular unleaded (even though it recommends super 98 octance type guff). The engineer was pretty adamant that I should only be using 98 octane - and that I should expect problems.

I realise these FSI engines work in two 'modes' - and it would seem correct that its when it's in its 'economy mode' that the light comes on.

The explanation sounds like a fob off to me. I have it booked into my local Audi garage (a different one) next week for a proper investigation. Just wanted other people's opinions on this.

Ben.

[img=left]http://www.drn.org/mrbenn-transp.png"[/img=left]

 

[gjp33]

I assume 'economy' means FSI mode - i.e direct injection on the compression stroke.

it is this mode that will be vulnerable to poor combustion due to the lower octane fuel than what is 'recommended' - and this is where the misfires are occuring.

what's probably happening is the O2 sensor is picking up the aftereffects of the misfire as excess HC in the exhaust, and the ECU is getting an out of range signal - hence the light coming on.

the cause of the misfires could be purely the lower octane fuel - or it could be an ignition fault. The former does sound the most likely I must admit as it's rare ignition faults occur across all 4 cylinders in this fashion.

have you tried putting in 98RON to see if it helps yet ? Or have these problems only occurred after the last fill up with regular 95 ?

you might just have a tankful of old fuel - the RON does drop off.

 

[ben]

Thanks gjp for the analysis. Yes - it is only when in the FSI mode that the light was coming on. (I noticed that since the Audi garage reset the error today - it hasn't reappeared.

Maybe the Audi technician was right then? I only filled up on Monday - maybe these fuel protests have caused my local supermarket to get out the old stuff.

I'll monitor it later this week - got to take the other car to the garage tomorrow to have a faulty coolant tank float sensor replaced.

Ben.

[img=left]http://www.drn.org/mrbenn-transp.png"[/img=left]

 

[gjp33]

who knows - if anything, fuel lately ought to be fresher. Still, until you get some more fuel in there all you know is that it's been misfiring on the latest batch under certain conditions.

One thing you might try is get a bottle of octane booster, and if you can isolate a reliably condition where it misfires, put it in and repeat - see if it makes any difference.