Misfire.

[richyb66]

I've not tried the measuring block before but assume it's fairly straightforward?

I just did another scan aftger my 32 mile drive to work, no engine management light on but I got this:

VAG-COM Version: Release 409.1-US

Control Module Part Number: 4D0 907 559 A
Component and/or Version: 4.2L V8/5V G 0003
Software Coding: 10752
Work Shop Code: WSC 00000
Additional Info: WAUZZZ4BZ1N044499 AUZ7Z0Y1472110
4 Faults Found:
16684 - Random/Multiple Cylinder Misfire Detected
P0300 - 35-00 - -
16689 - Cyl. 5 burn misfires recognized
P0305 - 35-00 - Please Register/Activate
16690 - Cyl. 6 burn misfires recognized
P0306 - 35-00 - Please Register/Activate
17537 - Fuel Trim: Bank 2 (Mult): System too Rich
P1129 - 35-00 - -

So still missing on 2 cyls and Bank 2 too rich so either it's over fuelling or it's an ignition fault thats causing the fuel not to be burnt.

[richyb66]

Update time, I've spent a busy (and cold) afternoon having a damn good look over the car. First off I got Vagcom hooked up and took some measuring block readings as advised earlier. I stuck with the misfire logging and didn't bother with the lambdas for the time being to keep things simple and try and bottom out which cylinders are giving the problem.

Earlier scans pointed to 6 and 7 (which then had the new coilpacks) and then 5, 6 and 7. The measuring blocks showed that it was missing mostly on 5, with the odd miss here and there on 6 and 7 and suprisingly it was slightly worse on lpg than on petrol. It actually logged about 24 misfires on cyl 5 in around half a minute. I'm not sure if it's my free version of Vagcom or operaor error but the count seemed to re-set itself after just over half a minute. Either way, cyl looked a prime culprit. Hopefully I've managed to attach a log file of the readings and I'v highlighted in bold the cylinder numbers in the columns across the top and also the misfire incidents under in the cyl 5 column.

I then pulled to coil packs off, starting on bank 2 and checked the earth continuity on terminal 4 of the connector and then checked for 12v with the ignition on at terminal 1 - all 8 cylinders were ok.

With the coil packs off, I swapped the plugs back to the genuine Audi ones that I put in when I first got the car. They did about 5k miles before I swapped them for some NGK's that were supposed to be better for lpg. I said that I was a bit suprised at the centre electrode on the NGK's and here's a photo with the NGK at the top and the Bosch at the bottom. You need to look closely but on the Bosch, you can see the centre electrode protrude from the ceramic insulator whereas on the NGK it's actually just inside the end of the ceramic. Looks very odd, I know the spark is big voltage but it nothing like the Bosch plug and I can't help thinking it shrouds the spark - I'd welcome views from anyone else who's fitted plugs like these, particularly if they run lpg.



I also did a resistance check on the injectors, it doesn't necessarliy prove anything but it was easier to do while I had it in bits and the resistance range is 13 to 16 ohms, 7 of my injectors were between 15.2 and 16.7 ohms so close enough for cash and one was 27.3 ohms - though this was cyl 2, which has never flagged any codes so I'm not going to get overly worreid about that at the moment.

I had a really close inspection of all the wiring, connectors etc. looking for damage or corrosion and I couldn't see anything obvious. Finally, I swapped the coil pack on cyl 5 for the one I took ofy cyl 7 a week or so ago. The codes I got after putting the new coil packs on suggested that the old ones might have been OK, so I decided to swap out cyl 5 as it had the most misfires and that coil pack could still have been the issue.

I've now just done a road test, both on lpg and petrol and so far so good. Nice and smooth and no noticeable missing. I think it logged one miss when I switched from petrol to lpg but again, I don't think this is an issue.

I'm going to give it a few runs out this weekend and see how it goes. If it's finally OK, I may well never know what the problem was. I've swapped the plugs, swapped one colilpack and disturbed a load of wiring and connectors so it could easily be any of them, or a combination. I'll keep monitoring how it goes and give more feedback as and when. I'm still half thinking on getting the injectors cleaned, given the mileage it can't hurt, but unless it gives me more trouble soon I'll probably leave that until it's more convenient to do (i.e. warmer weather).

Thanks for the assistance so far, it's much appreciated.

[47p2]

My guess is the car didn't like the NGK plugs. I've had this problem a few times and when original plugs were used the problem was solved

[richyb66]

As a follow up to my last post a week or so ago, a couple of days later I still had issues with a misfire and un-even running. I did some more data logging and cyl 5 was still registering the most misfires with the occasional ones on 6 and 7 as well.

Obviously I hadn't fully sorted the issue so I decided to take a new tack. As the lpg injectors feed the mainfold via a rubber hose, it would be an easy matter to swap lpg injector 5 so it fed 6 and vice versa by swapping the electrical connector and the runbber hose over. All told, less than 5 minutes, followed by another road test.

The results were interesting and basically eliminated ignition issues - the misfire was now on cyl 6, so it looked like an lpg injector was playing up. Fortunately the injectors are simple in design and can be overhauled and £90 and a few days later I had a kit to rebuild all 8. Once the injector rail is removed, it's a simple strip down and replacement of the pistion in the injector, a small spring and an O ring. So simple in fact that although I did the first rail off the car, I did the second rail in-situ and the whole job was about an hour start to finish.

Aside from some wear on the pistons everything looked ok although injector 5 piston was quite gummed up (possibly sticking) with slight gumming on 6, 7 and 8, though none of the springs were obviouly damaged. The pistons have a rubber buffer build in that stops them clattering as they're pulled open against the spring and these looked worn on the old pistons compared to the new ones.

Once assembled it seemed to run much better, a smoother drive, no missing (even when logged) and the injectors are much quieter in operation. I've now put another 500 miles on the car and so far, so good - it even passed the MOT today first time so maybe it's a sign I can look forward to a trouble free year of motoring (though I very much doubt it - I already need to swap an output shaft seal on the front that's weeping).

I'm keeping my fingers crossed that it'll be ok, the inital misfire was mostly on petrol, it then missed mostly on lpg and 2 coil packs, 8 plugs and re-built lpg injectors now seem to have sorted it out and it's pretty much back to normal. Whether a fault in one ECU was affecting the other hasn't really been answered - maybe there was a combination of faults but I can't see an obvious way that a sticking lpg injector would cause it to go into limp mode when running on petrol.

The problem I have now is that a few weeks of bad running have knocked my confidence in my daily driver and I now feel I'm hyper sensitive every second that I'm driving it and probably anticipating issues that aren't actually there.

In a week or so's time I'll have had the car for exactly 12 months, I've kept every bill I've spent on it and I can account for every litre of fuel (lpg and petrol) I've put in it. I'll be very interested to see just how much the last 25,000 have cost me!!

[johnds8]

My old car was LPG and I swapped to LPG specific plugs. Next time I took it in for it's LPG service the guy said to get them out and put the standards back in.

JOhn

[richyb66]

Originally Posted by johnds8

My old car was LPG and I swapped to LPG specific plugs. Next time I took it in for it's LPG service the guy said to get them out and put the standards back in.

JOhn

You'd think that the lpg would need a hotter plug but most people seem to be thinking it's a waste of time (and money).

[sloss]

Interesting thread, you have done a lot of work and are not frightened to get stuck in

It looks to me like you did one or two changes at the same intervention.

As a Scientist/Engineer I'd advise you to follow good practice and only change one parameter at a time to eliminate problems. I think your 'need for speed' clouded things by sometimes changing a few things at one time.

All in all it looks like you got there anyway

[richyb66]

Originally Posted by sloss

I'd advise you to follow good practice and only change one parameter at a time to eliminate problems. I think your 'need for speed' clouded things by sometimes changing a few things at one time.

This is what I'd normally do but as the car is my daily driver and I only get a limited amount of time to look at it I had to deviate from my normal plan. It's a bit annoying because, as you say, it's then impossible to establish the exact cause and avoid similar issues in the future.

I do like to get stuck in, I did the cambelt and water pump just before Christmas.

[sloss]

You got there, that was the aim

[richyb66]

Almost there, it's still hesitating a bit on petrol, especially first thing in the morning but it's fine when is switched to lpg about 3 minutes later. It's obviously not ignition related so my next step will probably be to get the petrol injectors cleaned and serviced.

As long as it's ok on lpg I can at least afford to drive it while I sort it out!