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[al brown]

Hi all,

As per the title, I would assume the ECU is picking something up as I am getting a cylinder 2 misfire code on a petrol Audi a2 1.4 petrol. Question is how does it pick up a misfire on a particular cylinder, car doesn't show any symptoms of a misfire and has just  flown through an emissions test. It does have quite a tappety noise from cylinder 2 but other than it drives great. Plugs and leads have  swapped for diagnosis but code remains but without knowing how the code is generated I am at a bit of a loss for way to go with it now.

Cheers

[TheBoy]

Most likely through the knock sensors and/or crankshaft sensor.

[al brown]

OK, cheers.
Now time for me to learn something new by asking questions.
How does it know which cylinder? Does it have more than one knock sensor? Or is it a case of phase relation between the knock and crank sensor?
Is it possible then that the noisy tappet could trigger the code? I didn't think it was to loud, have just put fresh oil and some lifter treatment in it so will see if it quiets down.
Cheers for any info anyone has to share.

[Webby the Bear]

Going back to school notes here. . .

It's measured via the crank sensor. Speed it takes to reach a certain point of the crank. If after cylinder two fires (or is mis firing in this case) the crank sensor pulses will be telling the ECU that it didn't get to x degrees in the correct time frame. There's your specific cylinder detection.

Obviously more complicated than what I've said but you get the gist

[al brown]

Cheers Webby,
Still doesn't run like it's misfiring though and the emissions test was spot on, TBs post makes it seem possible that it is a spurious code if it's using the knock sensor as well. I am not sure how it could be out enough for crank sensor to spot it but still have the cylinder fire well enough not to harm emissions. Just had the code reader on it and you can watch the misfire count go up rapidly while it's idling, it isn't just the one or two counts every now and then, is almost constant.
Well see how it goes after a few days with the lifter treatment and see if the knock gets better, would rather not have to strip the top apart.

[Webby the Bear]

Is it not misfiring at idle and goes away when driving?

[Webby the Bear]

Edit...

Is it only misfiring at idle?

[al brown]

Only had a quick look and I think the misfire detection was disabled as I revved it. There is no hint of a misfire either when driving or at idle other than the code coming up, hence why I was asking if it's possible to get a spurious reading. The misfire count at idle goes up it seems almost in time with the revs, as if it's missing on number 2 every time it fires,  don't understand how if it was missing that badly I can't feel it and how it flew through the mot emissions.

[Webby the Bear]

I don't know if I'm honest. I would say though that if you got a code that keeps coming back even after clearing it something's up.

I think I'd be checking for vacuum leaks first in your case. Also cleaning the throttle body and see if codes clear.

[al brown]

Ta,
There is also another a post in help about how a crank sensor works, it also mentions the knock sensor.
I am inclined to hope it is the noisy tappet triggering the knock sensor which the ecu is then interpreting as a misfire.

[al brown]

Forgot to add, the tester said there was oil in the lambda plug and that would be the cause of the misfire code but I couldn't see how that would cause a specific cylinder misfire rather than giving duff readings and causing all the cylinders to run rough. But then why was the emissions results fine?

[Webby the Bear]

I wouldn't say knock sensor cos the knocks they pick up are high frequency.

[al brown]

Agreed, but we have similar sensors all over our helicopters and they pick up all vibration, is the software that picks the frequency out. Are car ones not similar¿

[Kevin Wood]

Agreed, but we have similar sensors all over our helicopters and they pick up all vibration, is the software that picks the frequency out. Are car ones not similar¿

Yep, very similar. When "knock" occurs, there is a shock wave created by the (much too rapid) flame front which causes the cylinder to "ring" at a frequency determined by its' bore and stroke. The ECU will be tuned to detect this frequency. Old (80's) ECUs used to have a plug-in analogue filter circuit that was tuned for the engine application.

It's possible that the knock sensor is used to detect cylinder firing, or that crank acceleration around the power stroke is detected by the crank sensor output. Using the crank sensor and cam sensor outputs combined, the ECU knows which cylinder is supposed to be on its' power stroke (this is how it knows which injector to open, and which coil to fire) so a consistent misfire can be easily attributed to the offending cylinder.

[al brown]

Cheers again  the info. I am sure I will get it running right. I will clean the lambda plug and see how I get on. I have already swapped the 1&2 plugs and leads over and the fault stays, temporarily tried a new coil pack off another car and none of it has made any difference. Hence my hope it is the noisy lifter triggering the code, you can defiantly feel the knock on cam cover.