Engine light on, misfire, knocking
 

Just took my 8 up to the shops, noticed a misfire via putting it in drive and it not being a smooth "bite" - more of a jerking movement. Got to shop (2 miles), noticed it again when reversing. Drove back to work, idling down the car park with the window open, there's a light knocking sound and now the engine light's come on.

I pipped the bonnet and had a listen, I can feel the knock on the top of the engine, the right side near the oil filler, but it doesn't get louder when I take the filler cap off. Sounds like someone tapping a margerine tub. I'm gonna get someone to give me a lift home to pick up my laptop and use vagcom for the first time.

Questions:- what would it sound like if it was a simple misfire (electrical/fuel)?
Anybody know what it would sound like if there was a sticky valve?

More importantly, does anyone know what this is likely to be or have you heard a noise like it? Does it come across as serious? I'm panicking now as I have to tow my caravan 25 miles up the road this weekend.

2002 S8. Any help/insight/advice appreciated - I'll also update in a bit with the vagcom results...

Could be something really simple, but no point hypothesising - The good thing is that the warning light is on, which means the engine ECU will have logged something. Get Vagcom cranked up and see what the error is and take it from there +++

My money is on an injector........ Any takers?

Cleared the old codes in vagcom (usual stuff, random misfires, misfire on 6, gear selector intermittent - stuff that was on last time it was read over 6 months ago). Started her up again, sat for a minute, no engine light. Switched off, plugged in, no fault codes.

Spoke to Autopool in Iver, at first he said maybe a broken rocker or a bucket (cam belt & cams out), then he had a listen over the phone and reckons it could be a spark plug loose or pushed out. Basically he said in no uncertain terms, don't run it. Get it on the back of a recovery vehicle and bring it in. Mentioned helicoiling and broken injector. I've never worked on the engine yet, so I haven't a clue - I'm just glad it still runs and I didn't drive too far with the noise. I'm also glad I didn't drive straight down to see him about it.

RAC called out, should be here within an hour.

Keep fingers crossed, he's got me a slot for tuesday/wednesday, so should know more then.

Now to find a donor vehicle with a towbar for the weekend...:-(

Hmmmmm spark plug pushed out? Random Misfires, Misfire on 6. Maybe a quick check of the right hand bank as you loook into the engine bay. Whip of the cover (2 x 5mm allen bolts), remove coil pack, 10 or 12mm hex and stick a plug spanner in to see if its tight.

I would do, but I'm at work at the moment, already had 2 hours getting my laptop & doing the vagcom. I've got my entire tool kit in the boot too after getting my brother's mini engine running last weekend - wish mine was as simple!

Might have a word with the boss, see if he'll let me have a bit of time to look it over before RAC get here with the flatbed.

Well, I've sussed out what it is. Whoever put the spark plugs in last time may have cross-threaded no.6 (I'm presuming it's no.6 - the 2nd one from the front of the right side of the right bank looking from the front of the car, next to the oil filler). I took the coil pack off, looked down the recess and it was rather black and sooty compared with it's neighbour.

I tried a couple of deep sockets on it at first, either they spun it or they didn't fit, but anyway, I got the long-nose plyers on the electrode tip and sure enough, lifted the spark plug straight out - no threads in the recess.

This means it will need helicoiling, but does that mean head off or can it be done in-situ? What happens with the spark plugs then, do I have to buy one odd one every time or do they insert a sleeve and thread it? I've never seen helicoiling done before so not sure of the procedure.

I'm a little happier now I've seen what's wrong, put my mind at rest a little.

Helicoiling is basically putting a tube with a thread on the outside and a thread on the inside to replce the knackerd thread. It means removing metal so that the inner thread is the same size. If I were doing it I would have the head off. There may be ways the a pro would do it with the head on. Be interesting to see. Sorry to hear about that.

Have you tried threading a good plug back into no 6? It may be that the plugs thread is Fubared and the head is ok. If you can get a plug in and torqued down then there is no need to helicoil.

Mike

The plug thread is fine, just a few wisps of alloy in it. I did think about thread-sealing it in, but the way it came out so easily (and wiggled in the hole too) suggests that won't be an option.

Just had the RAC van come, he offered to tow it to the garage, I point-blank refused. He showed me on the laptop "max 10 miles at 30mph with solid bar", I said if he'd just spent £1700 having the gearbox sorted, would he risk it?

He phoned through for the flat bed :rolleyes:

Autopool just called, it's running again +++

Managed to do it without taking head off. I'll get the run-down in the morning as they've got to put it back together yet, and they want to check the other plugs too.

Gutted in one way, as I was gonna get them to do the water pump & ancillary belt while they were off with the cambelt, check it all over while it's off. Never mind, I'll have to use that money for something else I suppose...:rolleyes:

But seriously, I'll try and get some info off them for how they did it and I'll see how she drives - big test coming this weekend if she can tow my van for me. He did say he couldn't guarantee it was 100% as they did it in situ, but it all tightened up fine and runs ok.

:D Excellent.

Always nice to see a solid answer to any problem that develops... Lets hope she tows alright for you, but something tells me even on 7 cylinders she'd do alright :D

Well, she towed just fine, no problems at all. Managed to get my van (which weighs about the same as the unladen car when loaded) towed off the muddy field it was in and on to my usual pitch closer to work.

Incidentally, I was impressed that Autopool were a bit vague with the fault diagnosis until I held the phone near to the source of the knocking sound, and straight away he said "Turn her off, don't start her up again, don't drive her or you'll break something - it sounds like a spark plug's gone through the hole, get it transported down here and we'll confirm it". Sure enough, that's exactly what it was, as I confirmed myself. Either phone quality has moved on a lot or that guy knows these engines very well!!

I didn't get much chance to have the craic with the guys as I got a lift off someone from work who I had to guide there, he wanted to get back promptly, but from what I could gather it was a bit tricky for them to do the helicoil in situ as the spark plug recess is so deep, but it went fine. The shavings were blown out of the cylinder afterwards with compressed air. I'm not entirely sure if this got ALL the shavings out (I hope so), but they seemed happy enough that it was a successful operation and the engine runs slightly smoother now (it's cured my slightly wavering tickover too).

He checked all the other plugs, found no.3 was loose, so that's been tightened up too.

I think the moral of this tale is 2-fold, firstly, change your own spark plugs then you know it's been done properly, if possible try and use a rag & some cleaner on the threads a little before screwing the new plug in to specified torque. Secondly, if you develop any sort of misfire (audible or via vag-com) or wavering tickover, or there's a juddering when in drive and braked stationary, get in there and make sure all the spark plugs are tightened down to the specified torque.

I think I'll be tempted to use something on the threads when I do the plugs, like copper grease or a dab of engine oil, as he commented that it can just as easily be the other way, where the spark plug is seized into it's recess (there was a dark green/blue A8 on an "N" plate with this problem).

Any thoughts on this? Perhaps thread seal, as it's supposed to lubricate on loosening as well as grip when tightening apparently?

Having just read through this one again, DON'T USE THREADLOCK or anything like it. A bit of anti-seize is ok as is a high temp graphite grease.