A couple of months ago we sold my wife's W-reg Skoda Octavia SLXi 20VT to my parents at a knock down price as a favour...and I'm beginning to wish we hadn't.

We owned the car for 3 years, was a one-owner car we bought from a skoda main dealer, has FSH and apart from a failed battery early this year had never missed a bit in its 27000 miles.

That is of course until the very day my parents took the car home, and it started badly misfiring!

The misfire is very intermittent. It can run perfectly for ages, then will misfire for 10 minutes before clearing itself. It always starts to manifest itself when accelerating away in a low gear - at higher speeds in top it runs fine.

The car has been into the dealers twice now and they have failed to diagnose the problem, and say the engine reports no fault codes. My father has so far changed both the spark plugs and all the coil packs. After the coil packs were changed it ran fine for 450 miles.

I now feel very guilty selling them the car, even though it was fine when I sold it, so I'd really like to help them get to the bottom of this problem. I'm happy to pick up all the repair bills, and would even take the car back off them, but for the price I sold it to them they'd have to replace it with something much older and far higher mileage.

If anyone can shed some light on what else might be the cause I'd be very grateful. Or alternatively suggest a really good solid independent skoda or 20VT specialist in the north-west (no offence to any decent service staff at main dealers, but it does seem that if they can't plug a computer in and have it tell them what to do, then they don't have a clue).

Thanks in advance.

Gary.

Could it be that your parents are using a different filling station, and that they have got dirty fuel.

This could have blocked the injectors, fuel filter etc.

JD

uh oh exact reasons you dont sell cars to freinds and family.... but we all do lol

Joel

I don't think it's a fuel issue down to a dodgy local filling station. I'd brimmed the tank for them at my regular garage ready for them to drive away, and it started misfiring at the end of their 150 mile journey home. Embarassing huh! It's been like that on and off for the past 3000 miles they've driven since they've had it.

The car started misfiring on my parents driveway recently and by removing the various ignition leads he managed to isolate it down to a consistent misfire on cylinder 1. So you're right in that it could be a fuelling issue - perhaps a dodgy fuel injector on this cylinder. I'm not very mechanically minded, but my only other thought was a sticking valve. I picked up some Redex injector cleaner and some oil additive for clearing sticking valves at the weekend, but no feedback from my father yet as to whether they've done the trick.

MAF sensors seem a common point of failure, but I would have thought that would lead to generally lumpy running, and not a consistent misfire on a single cylinder?

Cheers,

Gary.

you say you have isolated it down to one cylinder...you say you have chnaged the spark plugs and coils... have you tried giving that suspect cylinder a diffrent ignition lead... and the lead from one to say two and see if the miss fire moves with the lead...

just an idea... Redex injector cleaner soudns good idea too...

have you been out in the car and experienced the problem first hand to make sure it is a misfire and nothing else?

Joel

As it's a W reg the first thing I would ask is if it is non-drive by wire, if it is I suspect the fault lies with the ignition stage amplifier which is mounted on the rear of the airbox. These are expensive to replace but the problem is often fixed by simply cleaning the contacts.

Interesting - that is only the 2nd time I have ever seen someone else mention the ignition amplifier in a forum. I used to have a misfire on my 1999 Audi A3 1.8T and took the ignition control unit out and tested it with a battery and voltmeter. I concluded that it seemed to be working ok and didn't replace it. At the time (about 12 or 18 months ago, maybe longer) they cost something like 125 quid (and I don't think that included VAT).

Basically the ECU passes a small charge to the unit, and this effectively triggers a separate circuit (of higher current/voltage) that fires the cylinders.

The accounts I read on the web said that if this unit plays up then you get consistent misfiring on a certain cylinder, but usually it's the coils that are the problem. Many people find that swapping the coils moved the misfire to a different cylinder.

My particular misfire used to happen when changing gears, and sounded like a flat POP, like I'd run over a plank of wood. It actually went away over time, but something I did do was put Optimax in and give the car some decent high speed runs, and heavy foot action, in the hope it might help clean out any crap in the system.

I would agree that if swapping coils didn't move the misfire then your problem could be the ignition amplifier (it has many different names).

I'll dig out some info I found at the time and post links here.

Here you go:

http://forums.audiworld.com/a4/msgs/1594531.phtml

http://www.sjmautotechnik.com/trouble_shooting/S4_Ignition.htm

Hope that's of interest,

B

I know it may sound daft , but I do know we had one in not long ago with a missfire.

The wrong spark plugs had been fitted and one of the coil packs was of the old style and had gone down after only a couple of months running.

So could be worth checking that out too.

Hope you get to the bottom of it.

Sarah

Joel, I haven't experienced the misfire first-hand. I've only been out with my father in the car a couple of times since we sold it to them, and of course it ran perfectly! He's much more mechanically minded than I am (has stripped down and rebuilt engines etc in the past), but is somewhat out of his depth with all the electronics on modern cars. If it was an old 2 litre vauxhall with carburettors he'd probably have sorted it by now He didn't mention whether he'd moved the ignition leads to see if that moved the problem. Will need to check that with him.

Eddy, thanks for the tip on the ignition stage amplifier if it's an early car. The car was registered in July 2000. I'll get my father to look for this component and clean the contacts.

Gary.

Mine misfired regularly for 18 months and was never satisfactorily resolved. The coil packs on the non DBW engines are much higher quality and larger units than the DBW engines. I moved things around and my misfire seemed to move with the injector, but it came back even after the injectors were cleaned (and no injector misfires were recorded during the cleaning process). Mine was improved by dosing the petrol with RedeX, but never completely cured. There was a suspicion in some quarters who ought to know, that it might have had a sticking valve, but since that was a head-off job, I never got round to it before part ex'ing the car.

Thanks a lot for the extra info BK1 - definitely very interesting and the next avenue to consider.

Sarah - spark plugs and coil packs were our first thought too. But both have now been changed on all cylinders and the problem has persisted.

Have to say how impressed I am with the speed and quality of responses on this forum!

Gary.

Cheers Nick - sticking valve was one of my thoughts too, with the valves probably being so tiny on these cars. As you say though, lots of hassle to take the cylinder head off, and although my father has done that in the past on older cars, I don't think he's tooled up to do it on the 1.8 20VT unit.

Gary.

Yes this is a great forum.

Hope you get it sorted.

Sarah

my octy misfired tracked it down to the coil pack that sits under the bracket (the awkward one) the dealer fitted re-designed version running perfect.

One thing that worries me is if your car has been misfiring for so long that the cat may have been damaged due to the unburned fuel traveling through it something you should address.

Cataclean is the best on market for cleaning the system out and protecting the cat but its

The car's been behaving itself for the past few hundred miles. But I passed all the info about the likely culprit being the Ignition Control Unit / Ignition Stage Amplifier on to my father. He went down to his local Skoda Main Dealer and had a chat with the service manager who had never even heard of that part...and insisted he must mean the ECU which he said was under the dash and was approx.

The ignition control unit isn't cheap, so be sure to check all other avenues before you buy a new one. Also, although I investigated my part and electrically tested it, etc, I didn't get it replaced and my exhaust pop did disappear.

Bear in mind that the reason the dealer wasn't familiar with the part could well be because they have never seen that part fail...

One thing you might be able to do is try swapping your unit for one off a similar car (be sure to check exact part numbers as there are different types, made by different manufacturers). I think that as long as you did the swap with ignition off then you won't cause error codes in the ECU.

That said, I would hope that the Skoda dealer would let you talk to a mechanic about the part, and perhaps let you try the swap with a part of theirs. Unlikely but maybe worth asking. Your main problem here is that since your problem is intermittent, you're not guaranteed to know for sure that a new part instantly makes a difference.

Good luck, and keep us informed. I'm very interested to know whether you find the ignition control unit to actually be the culprit.

The AGU (ie original non-DBW 1.8T) has a Control Unit Power Output Stage 'LAZ' listed in the ignition components fitted whereas the AUM/ARX/AUQ (DBW VVT) engines don't. The part number is 4D0 905 351 (although it could have been superceded) and the price circa

The AGU (ie original non-DBW 1.8T) has a Control Unit Power Output Stage 'LAZ' listed in the ignition components fitted whereas the AUM/ARX/AUQ (DBW VVT) engines don't. The part number is 4D0 905 351 (although it could have been superceded) and the price circa

I don't know if you've found out any more, but I just read a post on the Audi forum I use, mentioning that misfires could be due to some melting wiring.

See here:

http://www.audi-sport.net/ubbthreads/showflat.php?Cat=0&Number=345249&an=0&page=0#345249

My father broke and remade the connections to the ignition control unit over a week ago now, in an attempt to clean any gunk off the contacts. Since then the car has been faultless. Will keep you posted with any further updates.

Gary.