Another thread about a misfire on cylinder 2 (1.4 AUA engine)

mowog

mowog

Member
Hi all,

Apologies if this is poor form, as I've already mentioned this issue on another thread. I just wanted to create one specific to this problem in the hopes that someone reading/searching has more of a clue than me. This seems to be a regularly occurring fault with the AUA engine, as there are numerous mentions across the web.

The issue is as follows: the engine starts and idles OK, but if I leave it idling for a few minutes it starts to misfire on cylinder 2.
  • I know it's happening on number 2 because if I remove the plug lead or injector connection there is no change.
  • There is a strong spark being delivered - it shoots out of the lead when I unplug it. This makes me think the issue is not the coil pack.
  • Swapping over plug and leads from cylinder 1 makes no difference - the fault stays on 2. This suggests that the plug and lead are fine.
  • No fault codes are being raised (which seems really odd to me).
  • When I rev the engine it sounds/feels like the misfire goes away.
Having read this thread, I tried swapping out the knock sensor, but to no avail. In recent days I've also replaced the MAP sensor, cleaned the throttle body and EGR valve, and checked for air leaks.

The fact that the engine has to idle for some time before the fault occurs is curious, and suggests that the issue is related to the engine warming up.

Has anyone ever heard of a fuel injector that stops working when warm? I haven't, and I'll be honest that it seems unlikely to me. Is it easy to swap injectors over to test if that is the culprit? I've never tried.

As I say, this exact issue seems to be not uncommon, and I'm sure someone must have worked out what the issue is by now. What I'm struggling with is the fact that it only effects the one cylinder, and only when left idling for a while.
 
A lazy or not quite sealing valve perhaps? Not such an issue with richer mixture when cold or above idling revs, but as the mixture gets thinned out on warming up, causes the misfire at idle.
 
I don't have an answer I'm afraid.

However, I seem to remember the Owner's Manual that came with my own AUA A2 specifically instructing that the engine should not be allowed to idle for any length of time after start up, but rather that you should drive off without delay. I'd never previously seen such an instruction with any other car, and I wonder if your issue is related.

The later Owner's Manual that I have on my desk doesn't match my memory at all, however. I'll get out to the car later today and check exactly what the earlier manual says
 
Proghound said:
A lazy or not quite sealing valve perhaps? Not such an issue with richer mixture when cold or above idling revs, but as the mixture gets thinned out on warming up, causes the misfire at idle.
Click to expand...

Somewhat upsettingly, that does make good sense. Though I'd have expected the misfire to be less consistent in that case, rather than it appearing to simply not fire at all. Is the engine clever enough to stop supplying fuel to cylinder 2 if it thinks it won't combust properly?

For obvious reasons I'm less than eager to start taking the head off without knowing for certain what the issue is. Do you think an old fashioned compression tester would work to show that issue in this case? Sorry - modern cars aren't really my thing! I have the kind of compression tester you shove into the spark plug hole as you churn the engine with the throttle wide open. Would that work on the A2, or will I need something else?
 
We have these exact symptoms on our black A2, and it only happens when warm, it was found to be a sticky valve, so yesterday and today I've been replacing the head with a re conditioned one.
 
Kleynie said:
We have these exact symptoms on our black A2, and it only happens when warm, it was found to be a sticky valve, so yesterday and today I've been replacing the head with a re conditioned one.
Click to expand...

Can I ask how you diagnosed the sticky valve? And also where you acquired a recon head from, and how much it cost? Was there no way that the valve could be freed up with the head on?
 
mowog said:
Is the engine clever enough to stop supplying fuel to cylinder 2 if it thinks it won't combust properly?
Click to expand...
The later BBY engine certainly does, @Bargepower had that happen. After the ECU counts a certain number of misfires, it shuts that cylinder off (no injection) in order to protect the catalytic convertor from unburnt fuel, and it runs on the remaining cylinders until you stop and restart the engine. The BBY has individual coil packs for each cylinder though, whereas the AUA has one common coil, this might mean that it can monitor each cylinder more effectively.

So on your car, can you confirm that you're just getting the misfires but it doesn't need you to restart it for them to go away ... you just rev it up and it only comes back when the revs return to idle? That's really sounding like a valve to me.
 
mowog said:
Hi all,

Apologies if this is poor form, as I've already mentioned this issue on another thread. I just wanted to create one specific to this problem in the hopes that someone reading/searching has more of a clue than me. This seems to be a regularly occurring fault with the AUA engine, as there are numerous mentions across the web.

The issue is as follows: the engine starts and idles OK, but if I leave it idling for a few minutes it starts to misfire on cylinder 2.
  • I know it's happening on number 2 because if I remove the plug lead or injector connection there is no change.
  • There is a strong spark being delivered - it shoots out of the lead when I unplug it. This makes me think the issue is not the coil pack.
  • Swapping over plug and leads from cylinder 1 makes no difference - the fault stays on 2. This suggests that the plug and lead are fine.
  • No fault codes are being raised (which seems really odd to me).
  • When I rev the engine it sounds/feels like the misfire goes away.
Having read this thread, I tried swapping out the knock sensor, but to no avail. In recent days I've also replaced the MAP sensor, cleaned the throttle body and EGR valve, and checked for air leaks.

The fact that the engine has to idle for some time before the fault occurs is curious, and suggests that the issue is related to the engine warming up.

Has anyone ever heard of a fuel injector that stops working when warm? I haven't, and I'll be honest that it seems unlikely to me. Is it easy to swap injectors over to test if that is the culprit? I've never tried.

As I say, this exact issue seems to be not uncommon, and I'm sure someone must have worked out what the issue is by now. What I'm struggling with is the fact that it only effects the one cylinder, and only when left idling for a while.
Click to expand...
For correct combustion you need the right amount of air, the right amount of fuel (with good fuel atomisation), good compression and a good spark.

As you have checked the spark, that leaves the others. If the other cylinder are behaving themselves then the air might be OK. You would get codes for that if there is a general air supply problem.

That then leaves compression and fuel. As its specific to one cylinder I would do a compression test to check if there is wear on the piston rings or an issue with valve sealing. You may also have a problem with the fuel injector for that cylinder. You could try swapping the fuel injector with that for another cylinder to see if the fault moves.
 
Almost certainly a sticky valve
I chased a mis fire on an AUA engine for hours
Would start from cold on 4 cylinders then drop onto 3 and it was always number 2 that was cut
The ecu monitors the number of misfires and using the knock sensor works out the offending cylinder and then cuts the power to the injector to protect the cat but his also throws an engine mil light. Cycling the ignition would clear the fault and it would run on 4 cylinders for a min or so then drop back to 3
But the mil light would be on until cleared with vcds
On my AUA it was eventually traced to a faulty plug lead I actually heard it arcing just before it dropped on to 3 cylinders

However the far for common issue is a stuck valve and this does not cause a mil light or any fault codes and commonly only occurs when the engine is warm and nearly always affect cylinder 2 but can affect other cylinders
Another cause is a slightly bent valve normally after a cam belt breakage
Worth looking at the long and short term fuel trims as they tell the story of what is going on
That engine is very susceptible to air leaks and the ecu is easily confused into shutting down a cylinder to protect the cat

Paul


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I don’t speak English .. Maybe after watching one of you will explain in understandable language
 
Hi all

Just a little update on this. Having finally solved my mystery air leak, I thought I would do a bit more investigating about this issue, so I got up early and took the fuel rail off before work.

I decided to swap injectors 1 and 2 around, just to see if the misfire moved.

With it all reassembled I started her up and let her idle. Just as it got to the point where I was convinced I'd accidentally fixed the issue, there she goes again dropping cylinder 2.

So just to be clear, the fault occurs as follows...
  • If I drive the car normally and happen to not sit idling, the car never seems to develop the misfire. That said, it only seems to need to sit idling for a few seconds when fully warm before the misfire kicks in.
  • I don't think it can be injector related (see above)
  • I don't think it's coil-pack related as there is a strong spark if I pull off the lead
  • If I clear the fault code (with the engine running), cylinder 2 immediately springs back to life. This seems especially weird.
Apologies to @bono05 - I did try to watch your video, but YouTube's auto-translated subtitles weren't fantastically helpful, or at least not unless the problem really is that the engine is 'cowardly'! :) I will have another go at some point.

Bearing in mind what @depronman says above, next I might try clearing the code and switching leads 1 & 2 around, and see if that makes any difference. I've a feeling that when I tried that first, I didn't clear the code, and therefore the misfire would have persisted on 2 anyway.

On the plus side I've never even had a close look at a fuel injector before, so what a fabulous learning experience this broken little car is! ?
 
mowog said:
Can I ask how you diagnosed the sticky valve? And also where you acquired a recon head from, and how much it cost? Was there no way that the valve could be freed up with the head on?
Click to expand...
The sticky valve was diagnosed by our mechanic, Gary, who is a well trusted and recognised A2 mechanic in West London. The head came from Autotech Cylinder heads, £400 inc VAT and delivery, then you get £120 back once the old head is returned.

Ian
 
The AUA definitely does the 'three strikes then you're out' trick with a misfiring cylinder (it's not necessarily 3, but the shutdown does take place once the ECU has seen enough recurrent misfires on any given cylinder to switch off the injector). Resetting the engine light allows that cylinder to run again for as much time as it takes the misfire counter to hit the magic number again. Thus the Lazarus-like return to function of your cylinder 2 when the error is cleared is completely normal.

Even if the coil pack is not the chief suspect, take it off and have a look at it (or try swapping in a known-good alternative), particularly the insulation around the back and the sides - for cracks and / or the potting compound coming away. Our one was clearly working *some of the time* - but intermittently sparks were earthing through the casing to the engine block, particularly once the engine had warmed up, and that, along with dirty injectors was enough to cause the engine to run rough, misfire intermittently and then become undriveable. The new coil resolved the problem instantly.

Secondly - are the leads in good condition? If the lead to 2 has a particularly circuitous route from the coil to the carrier on top of the engine could the insulation be pinched or compromised (I can't remember where 2 plugs in on the AUA coil as the car isn't here at the moment) when in situ and when the engine warms up?
 
Thanks for that - yes, I will take a very close look at both the coil and the leads (which are new) and check for the things you suggest.

Hopefully switching two of the leads will either eliminate or validate them. If I can find a replacement coil pack for cheap then I will certainly give that a go. I suspect it is valve-related, but would be nice to eliminate all the cheaper possibilities first!
 
mowog said:
Thanks for that - yes, I will take a very close look at both the coil and the leads (which are new) and check for the things you suggest.

Hopefully switching two of the leads will either eliminate or validate them. If I can find a replacement coil pack for cheap then I will certainly give that a go. I suspect it is valve-related, but would be nice to eliminate all the cheaper possibilities first!
Click to expand...
I tried the cheap coil option - Amazon overnight delivery - it was immediately much better than the knackered original, but failed completely on one output within a day of purchase. Amazon refunded quickly, and I got a Bosch 2-3 days later that rectified the problem completely.
 
ajsellors said:
For correct combustion you need the right amount of air, the right amount of fuel (with good fuel atomisation), good compression and a good spark.

As you have checked the spark, that leaves the others. If the other cylinder are behaving themselves then the air might be OK. You would get codes for that if there is a general air supply problem.

That then leaves compression and fuel. As its specific to one cylinder I would do a compression test to check if there is wear on the piston rings or an issue with valve sealing. You may also have a problem with the fuel injector for that cylinder. You could try swapping the fuel injector with that for another cylinder to see if the fault moves.
Click to expand...

I have this problem on a different engine, a valve issue you would need a leak down test


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