Omega Owners Forum
Chat Area => General Car Chat => Topic started by: al brown on 23 February 2015, 16:18:16
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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
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Most likely through the knock sensors and/or crankshaft sensor.
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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.
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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
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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.
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Is it not misfiring at idle and goes away when driving?
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Edit...
Is it only misfiring at idle?
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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.
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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.
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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.
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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?
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I wouldn't say knock sensor cos the knocks they pick up are high frequency.
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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¿
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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.
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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.
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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. :-\
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What engine? Leads would only exist on older 2.0/2.5/3.0, and most of those can't store misfire codes.
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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.
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What are the trims doing?
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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?
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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. :y
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 ::) ;D
Anyways, soz Al, you may continue ;D
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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.
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:y
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 :y
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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.