Misfire.

richyb66 · #1 22nd January 2012, 09:49 PM

Ok, not strictly D2 but as it's on my 40V S6 it's close enough for cash.

I've noticed some rough running lately on start-up which is more noticeable when I select Drive. As the car runs on lpg, it only runs on petrol fow maybe a couple of minutes before it switches to lpg and then it runs as smooth as anything.

Last week, as it was still running on petrol, it felt as flat as a fart and the check engine light came on and was flashing. Suspecting it had gone into limp home mode, I switched off, re-started and at almost immediately switched to lpg and ran fine.

Yesterday I used VCDS to get the fault codes and got:

16690 Cyl. 6 burn misfires recognized
16691 Cyl. 7 burn misfires recognized.

which I've now cleared and it's been ok so far. However when driving on lpg, if I switch to petrol, it's still missing slightly and when I switch back to lpg it's fine again.

Based on the symptoms I'm thinking it's fuel injection related. First off I'll probably swap the fuel filter because I'm not sure when it was last done but I'm not sure what to look at next. I can check the resistance of the injectors (if I can get to them) which might indicate a problem and the fact that it runs fine on lpg is steering me away from coil packs or ignition related issues but I don't understand how it can flag a fault code for a fuel induced misfire. Also, is a misfire on two cylinders an indication of something else?

Nollywood · #2 22nd January 2012, 11:22 PM

Leaking / faulty injectors? Despite it running fine on LPG, I wouldn't rule out coilpacks. A partially operating coilpack may run fine on gas, and be bad on petrol. Different burn rates.

I'd swap the coil packs round, drive her on petrol, and check for codes once more.

richyb66 · #3 23rd January 2012, 03:07 PM

At least the coil pack swap is easy compared to getting to the injectors. I can get new coil packs from Audi for £25 a piece inc VAT so I'm just going to replace a pair and see how I get on and I've got a fuel filter on order too.

The trouble with swapping multiple items is you tend never to get to exact cause of the issue.

Nollywood · #4 23rd January 2012, 07:10 PM

^^^ Great idea. Just make sure you replace the ones on the cylinders with misfires.

The injectors are easy enough to remove and replace, and I'm sure there's help to be had from other members. Where abouts are you?

richyb66 · #5 24th January 2012, 07:01 AM

I'm in Tamworth. tbh I've not looked for the injectors yet but if I can get the connectors off them I can at least do a resistance check as a starter.

My coil packs and filter should arrived tomorrow so I'll have a proper look at it on Friday - I did my cambelt and waterpump myself before Christmas so I'm not bothered by a bit of spanner work.

richyb66 · #6 29th January 2012, 08:24 PM

Coil packs on 6 and 7 noaw swapped, together with the fuel filter. No big hassles, just some spilled fuel from the filer (which I was expecting) and all done in around an hour.

I scanned for codes again before restarting and it hadn't logged any misfires since last week so now I'll run it for a bit and see how it goes.

richyb66 · #7 8th February 2012, 07:55 PM

Right, my misfire persists despite 2 new coil packs on 6 and 7 and a new fuel filter..

On initial start up (on petrol) it doesn't run smoothly - not very bad but lumpy and it feel down on power. So much so that I nurse it for the first mile or so until it switches to lpg. If I try and accelerate in anything other than a very moderate fashion, it's like it starts to go into limp home mode and on one occasion I think it did so I had to stop, switch off and then re-start. It was ok then and switched to lpg almost immediately and drove fine.

This morning after driving for 5 minutes on lpg it felt like it was starting to miss at about 60 on the motorway. After a minute or so it cleared and it drove fine. This is the first time it's missed on lpg so now I'm steering away from an injector issue and thinking more of ignition problems.

Tonight I did another scan and guess what, misfires on 5, 6 and 7! I've read a few threads on here and wondering if this could be an ignition amplifier issue (the cars done 151k miles) and if one amplifier was on the way out it could account for 3 cylinders starting to miss.

With this in mind I had a look tonight with a view to swapping the 2 amps over and seeing if the missing swapped to bank 1. Trouble is, I can find them. I had a good look around and traced the wiring back from the coil packs but it just disappears into the plenum with no sign of any amps.

The question is, have I got them. The car is a 2000, 40V, ANK engine code and the coil packs have 4 wires. The 4 wire coil packs are tending to make me thing that it doesn't have amps and a quick look on ELSAWIN didn't make any mention so I just don't know.

This weekend I plan on having a better investigation but I could do with a few pointers. On other thing, I swapped the plugs about 6 months ago from almost new genuine ones to some NGK jobbies that were supposed to be especially for lpg. They're still 4 electrode but I did notice that the centre electrode doesn't actually protrude from the ceramic core, it's about 0,5mm inside it (on all 8) unlike the genuine plugs where the centre electrode protrudes from the ceramic core slightly.

As it ran fine on the old plugs I'm tempted to stick them back in and see what happens but I don't want to swap too many things at once or I won't know what's made a difference.

Nollywood · #8 8th February 2012, 08:16 PM

40V motors don't have ignition amplifiers, they're built into the coilpacks themselves.

richyb66 · #9 8th February 2012, 09:55 PM

Cheers for that - not really the answer I was after but at least it eliminates something.

Now I need to look at what's likely to be causing a missfire on 3 cylinders, two of which have had new coil packs.

I guess it could be as simple as the plugs, which I'll swap when I've got a couple of hours spare. While I'm in there, I'll do a resistance check on the injectors just to eliminate those.

I think eleminating likely causes might be a better option rather than looking for a less obvious issue.

IT · #10 8th February 2012, 10:04 PM

As a wild thought, fuel pump? Perhaps its spinning, but not generating the pressure, or maybe crap in the tank ?

The ECU's do weird things when there are misfires, causing multiple cyclinders to go off, when only one if faulty for example. If you set off rough on petrol, it wouldnt surprise me if that lingered and caused the ECU to do strange things whilst on LPG, but clearing after a few moments of fuel being ok ?

How rough does it drive ? - Down on power is one or two cylinders off at the most. Any more than that, and depending on how they are balanced things get really quite rough and shakey very quickly. If you were driving on 5 cyclinders, you'd know about it.