Intermittent misfire - injector short to plus

H

henryf

Member
Hello,

Whilst driving this morning after about 10 or 15mins of driving suddenly the car started misfiring. Particularly noticeable at hard acceleration and lower speeds but present at pretty much all rev range and loads. I scanned it and got the below, car is a 1.4 petrol AUA. Any thoughts as to what it might be?

Screenshot 2024-01-10 082940.png


For some background, I have been having misfiring issues with this car ever since I purchased it towards the end of last year. It has had both and EGR and a throttle body from a known working car fitted and adapted in. These solved some but not all of the misfiring issues. Misfires were always far more frequent in the wet and damp and on startup for the first 15 minutes. About a week ago I replaced the ignition module + leads from those from a car that was running fine. This seemed to cure all the issues even with all the wet we've had in the past week I've had no issues. Until suddenly this this morning where whilst driving along at about 40mph it suddenly started misfiring.

Anyone got any thoughts as to what this could be? What usually causes an injector short to plus fault? Any help would be much appreciated!

Thanks!
 
henryf said:
Hello,

Whilst driving this morning after about 10 or 15mins of driving suddenly the car started misfiring. Particularly noticeable at hard acceleration and lower speeds but present at pretty much all rev range and loads. I scanned it and got the below, car is a 1.4 petrol AUA. Any thoughts as to what it might be?

View attachment 117526

For some background, I have been having misfiring issues with this car ever since I purchased it towards the end of last year. It has had both and EGR and a throttle body from a known working car fitted and adapted in. These solved some but not all of the misfiring issues. Misfires were always far more frequent in the wet and damp and on startup for the first 15 minutes. About a week ago I replaced the ignition module + leads from those from a car that was running fine. This seemed to cure all the issues even with all the wet we've had in the past week I've had no issues. Until suddenly this this morning where whilst driving along at about 40mph it suddenly started misfiring.

Anyone got any thoughts as to what this could be? What usually causes an injector short to plus fault? Any help would be much appreciated!

Thanks
Click to expand...
Coil pack, or the injector itself I would think, as it's cylinder specific.
Mac.
 
Possibly, but it confuses me that misfires are detected on cylinder 1 and 3 but fuel injectors 1 and 2 are causing issues (so far). So I would say it doesn't appear cylinder specific at all? Worth noting the engine was clear of faults end of last week and the car was driven for about 15 minutes after the faults started developing today.

With regards to coil packs, this was a working pack, it feels like it suddenly went? I realise it could be chance but it seems odd that this was a known working pack (from a working car), swapped in one week ago and now suddenly for it to go seems a little coincidental? Could the car be eating/burning out packs somehow?

I do have a new bosch coil pack + leads I can swap in at the weekend so will go from there I suppose.

But would the coil pack have anything to do with the injector faults?
 
Ok, so is the ECU shutting down an injector likely to cause a short to plus fault? (rather than the other way around of a short to plus fault making the ECU shut down the injector)....if that makes any sense?
 
I would look very carefully at the coil pack (having been here with an AUA) as well as (separately) the injectors. Unbolt the three screws that secure it and have a look at the back of the casing that you can't normally see - on ours the potting compound had dried out and shrunk leaving gaps, and there were also a couple of fine cracks in the casing itself. Misfires / rough running always got worse on wet days. The injectors were also extremely mucky ; replacing them with a set that had been away to injectortune for a service made a significant improvement.

Suspect that you actually have two separate things going on here and both need to be resolved to get it running smoothly (I ended up replacing damn nearly everything ignition and fuelling-related to get it running properly but that was after several years with no apparent maintenance or TLC prior to our tenure).

You say that you've swapped them from a car "that was running fine" but that's not necessarily the same as "freshly refurbished" or "new", and what works in one context may be upset by the move as well as any other environmental differences between the two cars.
 
I had a look at both coil packs when I changed them, I couldn't see any cracking or gaps in the potting compound (I was specifically looking for these) but there is a chance I could have missed them. I'll change the pack and leads at the weekend.

Suspect it might be worth replacing the injectors with a set that have been sent away to an injector servicing place
 
Remember, an AUA coil pack fires two cylinders at a time. The miss fire is on the twin of the cylinders with injector shorts.
I would swop the coil pack, and check again for miss fires.
Two injectors with the same fault seems odd. "Fuel injector # short to plus ... " may not be the injector itself. An injector with an internal short would, I think, report as "Fuel injector # short ..."
Mac.
 
I didn't realise the pack was wasted spark, it still seems odd to me an issue with fuel injector 3 being related to misfire on cylinder 2, i would assume the ECU would know through the cam sensor where the engine was and the misfire and fuel injector would line up? Or have i missed something?

I don't doubt however that it could well be the coil pack causing the issues here and will change it at the weekend
 
henryf said:
I didn't realise the pack was wasted spark, it still seems odd to me an issue with fuel injector 3 being related to misfire on cylinder 2, i would assume the ECU would know through the cam sensor where the engine was and the misfire and fuel injector would line up? Or have i missed something?

I don't doubt however that it could well be the coil pack causing the issues here and will change it at the weekend
Click to expand...
I think that it's cylinder 1 & 3, and 2 & 4 that fire together. Seems likely that the ECU doesn't worry about cam positioning, as far as sparks are concerned, as there's only two spark trigger wires out of the ECU. The ECU does, obviously differentiate the injectors though.
I think you need to change coil pack or injectors to move the diagnosis forward. I'm very suspicious of that "Short to plus ..." fault description. Sounds more connector/loom, than injector.
Mac.
 
Surely it'll be 1&4 and 3&2 that spark together on such a system? I know sometimes injectors can get timed together in either pairs or all together, not sure this is how audi do it if they have control of shutting down an individual injector?

Yes the short to plus fault description is what's concerning me (hence the thread title!), mainly trying to see what else it could be. I know the pack is not new but it was changed and running flawlessly for a week. Guess i'll have to wait until new pack has been fitted
 
henryf said:
Surely it'll be 1&4 and 3&2 that spark together on such a system? I know sometimes injectors can get timed together in either pairs or all together, not sure this is how audi do it if they have control of shutting down an individual injector?

Yes the short to plus fault description is what's concerning me (hence the thread title!), mainly trying to see what else it could be. I know the pack is not new but it was changed and running flawlessly for a week. Guess i'll have to wait until new pack has been fitted
Click to expand...
You could well be right about the cylinder pairing, I wasn't sure, whichever cylinder is on the exhaust stroke I guess.
It would be worth looking at the electrical schematic for the AUA, to see what's what in the loom to the coil pack, and injectors. My schematic doesn't include AUA, so can't help there.
Mac.
 
Probably worth a look at the ECU itself, check and clean connectors, and make sure it's nice and dry in the floor compartment.
Looking at the very similar, (except that the BBY has individual coil packs), BBY schematic, it looks like the coil pack wiring is on the same ECU connector as the injector wiring.
Mac.
 
Sounds like it's definitely worth a look at! I assume the ECU lives under the passenger footwell....never looked on these cars!

With regards to pairing of cylinders, on a standard inline 4 with a standard firing order of 1-3-4-2 you can only pair cylinders 1&4 and 2&3 as these are paired/in sync on the crank. Anything else would mean you fire during a compression stroke. At least that's the only way I've ever paired them with wasted sparks and in my head wouldn't work any other way - but then i've been proven wrong many a time before!
 
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Reference from the German site in confirmation of the AUA sparking pairs -

The AUA ignition always ignite 2 plugs at the same time (because the module has only 2 amplifiers) , 1+4 and 2+3
 
henryf said:
Sounds like it's definitely worth a look at! I assume the ECU lives under the passenger footwell....never looked on these cars!

With regards to pairing of cylinders, on a standard inline 4 with a standard firing order of 1-3-4-2 you can only pair cylinders 1&4 and 2&3 as these are paired/in sync on the crank. Anything else would mean you fire during a compression stroke. At least that's the only way I've ever paired them with wasted sparks and in my head wouldn't work any other way - but then i've been proven wrong many a time before!
Click to expand...
I think you're right in the firing sequence.
That underfloor compartment is known to get damp/wet, (leaky screen, in my case), and there's a lot of electronics down there ECU, CCU relays etc.
Mac.
 
Robin_Cox said:
Reference from the German site in confirmation of the AUA sparking pairs -

The AUA ignition always ignite 2 plugs at the same time (because the module has only 2 amplifiers) , 1+4 and 2+3
Click to expand...
Was the two sparks at the same time common in late 90s?
Seems very odd, but I suppose ECUs were first generation.
Mac.
 
PlasticMac said:
Was the two sparks at the same time common in late 90s?
Seems very odd, but I suppose ECUs were first generation.
Mac.
Click to expand...
hasn't that Bosch design of coil or something very similar been around for a very long time (pre-80s?) - that could explain why it had been carried over up until the early 00s until the individual coils were introduced, particularly if originally it was driven by points or a distributor cap?

edited soon after.

Nope. Talking rubbish. Earliest I can see similar is the 6N Polo 4 cylinder petrol engines in mid to late 90s using something very similar.
 
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PlasticMac said:
Was the two sparks at the same time common in late 90s?
Seems very odd, but I suppose ECUs were first generation.
Mac.
Click to expand...
Two sparks, one from each end of the coil, was a very common system from mid-90s to late 2000s. It is known as "Wasted Spark" because one spark occurs shortly before TDC for ignition, and the other occurs in a cyliinder that is coming to the end of the exhaust stroke. The systems eliminates all the moving parts of a distributor from the ignition system, and is very cost-effective compared to individual coil per plug systems.

The energy required to produce the spark goes up with cylinder pressure (increased dielectric resistance), so that most of the energy goes into the spark plug on the cylinder that is on compression. A side-effect of this arrangement is that the polarity of the voltage applied to the spark plugs is different, according to which end of the coil they're attached to.

On most four cylinder engines the firing order is 1-3-4-2, so the pairing of cylinders on the coils would be 1 and 4, and 2 and 3.

Further reading here:

Wasted Spark System
 
Mark Adams said:
Two sparks, one from each end of the coil, was a very common system from mid-90s to late 2000s. It is known as "Wasted Spark" because one spark occurs shortly before TDC for ignition, and the other occurs in a cyliinder that is coming to the end of the exhaust stroke. The systems eliminates all the moving parts of a distributor from the ignition system, and is very cost-effective compared to individual coil per plug systems.

The energy required to produce the spark goes up with cylinder pressure (increased dielectric resistance), so that most of the energy goes into the spark plug on the cylinder that is on compression. A side-effect of this arrangement is that the polarity of the voltage applied to the spark plugs is different, according to which end of the coil they're attached to.

On most four cylinder engines the firing order is 1-3-4-2, so the pairing of cylinders on the coils would be 1 and 4, and 2 and 3.

Further reading here:

Wasted Spark System
Click to expand...
the 2CV of the 80s had a similar wasted spark system albeit only on 2 cylinders. Not sure how far back that went (as the electrics were uprated over the years) although I assume at least the 70s.
 
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