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[Kevin Wood]

I don't suppose the code is a stale one that is stored? You've tried clearing it and it comes back, I take it?

If there's an unusual tapping it could just be tripping the knock sensor on that cylinder, perhaps? I would have thought that the absence of any detected shock would trigger a misfire, though, not the presence of one?

Could be that it's retarded the ignition so far on that cylinder that it's now down on power? Then again, if a consistent misfire is logged, I'd expect the ECU to shut down that cylinder. It would on an Omega, at any rate.

[TheBoy]

What engine? Leads would only exist on older 2.0/2.5/3.0, and most of those can't store misfire codes.

[al brown]

It's a 1.4 Audi a2 2001.
Apart from the tapet noise it drives fine with no hint of a misfire other than code. I cleared the code but it comes straight back and puts the dash light on within about 20 seconds.
Would the emissions not be out if it was misfiring that badly, but again, it doesn't drive like it's misfiring
Kevin, I was thinking along route of the knock sensor is, like you said, looking for pre detonation which the ECU well then try and fix by adjusting the timing. As it can't sort it out (because the noise isn't pinking it's the tapet) it has triggered the misfire code on that cylinder because it can't get the cylinder to fire correctly (it thinks).
Just my home spun theory of how the system works and not based on any rear works knowledge, hence the question about how it triggers the code.
Was hoping someone would know more than me and correct me, just doesn't seem like an actual misfire especially as it has had substitute parts fitted com a working car and the code remains.
Cheers for all the info.

[TheBoy]

What are the trims doing?

[al brown]

Haven't looked at trims yet, to busy changing the springs, drop links and CV boots today, as that what it failed the not for and it drives fine. Looked like past owner knew about the misfire as plus and leads are almost new, plugs all looked the same colour when I popped them out to check.
Will look at the trims tomorrow but would it not be that cylinder timing it would adjust if it thought it was pre detonating?

[Webby the Bear]

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.

Fair enough, kev. I stand corrected. 

The reason I said it is cos in school we were messing about with knock sensors and trying to induce a signal voltage by bashing the bench vice we has it clamped in and it wouldn't give a signal. Looked in to it and a bit of research showed that it would only react to the high frequency shock wave produced by combustion.

Wonder if some work like that and others don't. . . . Or I've got it all completely wrong 

Anyways, soz Al, you may continue 

[al brown]

Nothing to apologise for Webby, I know how our sensors at work work, I am not sure about knock sensors though so I am trying to learn. Would you need a scope to check output from a knock sensor or is it a voltage output? Ours at work is all done on a computer and I just look at the graph it produces and then from the frequency work out which bit is trying to fall off.

This stated as I thought the car was reporting a spurious misfire but I had no idea if that was even possible as I didn't know how the car worked out it was misfiring.

[Webby the Bear]

 

From what I remember. . . .

A scope will show a zero volt waveform under normal running and spike when knock is detected.

I think it's too fast for a multimeter to pick up. . . Possibly the reason why we got no readings from smacking on the bench. . . Or again, possibly cos it wouldn't pick up those types of knocks. Not sure. Hopefully kev will be able to confirm 

[Kevin Wood]

No Need to apologise, webby. It's quite possible that there's a degree of "tuning" in the sensor too, of course.

I would expect the signal from it to be quite low in amplitude, so it's possible that your scope wasn't sensitive enough. It may also be sensitive to the direction of the shock wave, i.e. it only works in one plane and you were hitting it on the wrong place! You'd certainly have no chance with a multimeter.

If you can see the knock retard values for each cylinder in live data that might be worth a check. I'd expect knock control only to be active at mid-RPM and a decent amount of load, though, so there might not be much to see at idle.

Some live data shows when knock events are detected too.