I've got a 2001 6n2 polo with a 1.4 mpi (AUD) engine.

It runs a bit lumpy at times and I decided a couple of weeks ago to get it sorted out. I was first told to change the coil pack, did that, 70 quid later nothing changed. Then spark plugs, did that, 35 quid later, nothing changed. Checked the HT leads, fine. Then came the electronic diagnostic machine.. showed misfire on cylinder 2.

So the next port of call was to remove the cylinder head and change the valves. The job was expensive, new valves, timing belt, head bolts, head gasket and labour. on inspection One of the valves on Cylinder 2 had a chunk ripped out of it (like a triangular nick in the metal) so problem solved?.... NO. The valves were changed the engine put back together and the misfire remained. The cause of the damage to the valve remains a mystery.!!

Does anyone know what is causing this misfire? Has something been missed? I've been searching the forum for people with the same issue, and many have misfires but have seemed to be able to solve them well before this point.

I've considered that it could be a warped cylinder head, but there have never been any leaks and all the other cylinders are running perfectly.. any help will earn you a mars bar in reward :-)

Cheers!

If this was an exhaust valve, it is likely that the triangular bit that you mention had burned out, but it is just possible that it was due to fatigue or something being trapped under the valve.

If it is due to fatigue, where has the broken bit gone? Did you check the piston crowns whilst the head was off?

Also, what condition was the valve seat in?

Suggest that, before you do anything else, you carry out a compression test, followed by a cylinder leakage test.

Once the head is off, you have destroyed the evidence, so it is important to carry out the above tests first.

I am a little surprised that there was no improvement, once all of the valves were seating, which makes me wonder if in fact they are, or if there is something stopping a valve from seating. If it was an exhaust valve that was causing the problem, lack of seating can cause them to burn.

If you find that you are low on compression on one cylinder, it would be worth checking that the valve is returning to its seat, and is not being prevented from doing so (for example, by a faulty hydraulic tappet.) - DO NOT remove the cylinder head until you have eliminated this.

Also, DO NOT use the car until you have eliminated that - if it is the cause, you might just get away without removing the head again, but if you drive it, you are likely to burn another valve.

Regards.

Who knows where the broken part has gone, i didnt do the job myself but i was told it wasnt present when the head came off. Since posting this the head has been sent for a leakage test to a company which does this kind of thing and apparently it was showing with some kind of leak, which allegedly was subject to some kind of repair, however when it went back on the misfire was still present.

There is an improvement in the performance of the car but i can still feel a slight misfire and the diagnostic showed it to be the case too.

Oh, and the pistons were checked when the head came off and they were fine.

Unfortunately I cant afford any more cylinder head work now and it seems I'm left to live with it or buy a new head/engine if i want it gone. So i spent an hour yesterday, armed with some torx screw drivers getting into the speedo to tape over that yellow light bulb. I'm assuming the problem will only get worse in time, so ill have to put some cash aside and keep an eye out for a new engine.

As i say, its not all that bad. Its only slightly lumpy at low revs which only really annoys me when I'm taking off in first gear as i can feel a slight shudder as the clutch engages.

My advice to anyone buying a 1.4 mpi engine would be do a compression test before considering parting with your cash!

Thanks for the advice mate, I owe you a mars bar!!!

Edited 30 March, 201313 yr by mp2988

It is still well worth checking the hydraulic tappets, particularly if a compression test points to the same cylinder being the culprit.

These things tend to rattle rather than seize, but for the time that it would take to check, it would at the very least, eliminate this as a possibility.

Should a tappet be the problem, they are not too expensive, and you do not need to remove the head to fit one.

Still advise prompt action though, as if this IS the fault, you will quickly burn out another valve if you continue to drive it.

Regards.

cheers, ill have it looked at when payday comes around again!

on another note, the oil light is now flashing since the head has been off. The oil filter has been replaced today and its had an oil change but its still there, flashing away.. this must be related to the cylinder head having been off - its too coincidental not to be.. I'm researching it now..

With regards to the oil light, could it be a simple matter of the idle speed being lower than it used to be?

The oil light switch is in the cylinder head, could it have been damaged?

Is it possible that the cable to the oil light switch has become trapped/partially earthed?

If the oil light flashes at higher engine speeds than just at idle, you need to take some prompt action, otherwise, it could cost you an engine!

Suggest that you first check the above points and then either fit a new oil pressure switch (10 minute job, including tea break!) or fit either a temporary or permanent oil pressure gauge to find out exactly what the oil pressure is.

Fitting the switch is the simplest/cheapest option, but unless you buy a good quality (genuine VW) switch, you could still end up with the same problem and still not be sure if it were due to a crap switch or low oil pressure.

Regards.

well the oil light basically came on as soon as i drove off and stayed on flashing for the whole drive..

But its now fixed. bought a new one for 11 quid and after an unsuccessful attempt at locating it via an internet search i finally found it hiding under the coil pack on the right hand side of the cylinder head (on a 6n2 model engine). was easy to access and remove the old one and screw in the new one. anyone doing this should pay particular attention to the shape of the plastic connector and match it to the shape of the electrical plug socket which goes on top of it before bending the little connection pin inside.

I had taken the plastic cover off the engine to do this, but later figured out it was totally unnecessary, as you can access it without removing this. Put it all back together and the oil light is gone, i can only assume that after the cylinder head work that the oil pressure switch had lost its correct pressure and stayed that way with some kind of blockage or malfunction when the head was replaced, either way its sorted (fingers crossed!)

Cheers for the advice, it was useful when knowing what risks i should be taking and what to have a dabble in!

I have read your first post and I have exactly the same problem. Like you I bought a coil pack, leads, spark plugs and the 1.4 mpi is still misfiring so after getting the reader involed it read an error which happened to be poor compression in cylinder 1. I have had the exhaust valves replaced and reseated, cam belt, head gasket etc replaced and guess what..... yeah you guessed it, the tick over is slightly better (only slightly). So i took it back to the mechanic who did the work and upon plugging the reader in it's came up with another fault... 'Catalyst defective'! Now I have been told that due to the exhaust valves been worn that this would have caused either 1 of the cats to need replacing or one of the lambda sensors! unfortunately I'm told that there is no way of distinguishing which sensor or cat is defective and the only way to tell is to replace one at a time and have the fault/s cleared to see whether they re-appear which is such a hassle!! But it's either that or pay for sensor and pre cat (which would be my first guess being closest to the manifold/engine) and see whether that helps.

I am currently in the process of doing this so I will be able to comment on whether this has solved the misfire issue. I am suspecting it's the first lambda sensor to be honest as this would surely make the engine jump about a bit if it's unable to measure the oxygen/carbs flowing so we'll see how it goes! Oh and to top it all off I need a new clutch/release bearing! (not related to misfire obv). Oh and mine's the 1.4 8v MPI too!!

The AUD engine (1.4 moo) is a major failure on the polo. But oddly doesn't fail on seats? .

Pretty much everthing about it is stupidly designed and is bound to fail.

I had the same problems as you guys, firstly I changed the head gasket and belt. Got a little better then failed. Changed it again with a skim and failed again.had enough and completely rebuilt it, new everything including the front lambda sensor ( over £100) and decated it (pre cat and rear cat) it ran great and I was over then moon. Then 3-400 miles later on the road it seized up.

It's bow in the skip huge waste of over £500

If you are planing on replacing the engine I would go for the AKK which is exactly the same but is fitted to seats mostly and guess what it doesn't fail so much. Or even better go for an AUA or AUB 16v. Even the AFK or AHW wi ll work(lupo).

good luck with it mate, these misfires are a nightmare to diagnose. When I was initially researching this I found loads of people having issues with lamda sensors and intitally thought it could be that, but the compression test pointed me elsewhere. I'm still living with the misfiring engine, I've never really understood whats going on inside the engine as sometimes you would think the engine was brand new the way it drives then for no good reason its like someone has loosened all the frigging bolts under the bonnet. What I have been told that is although its not perfect its still reliable enough, and to be fair its never not started or stalled on me. Personally I think it could be a combination of issues, the low compression due to a leak in the cylinder head (which I've read is prone to warping on these model engines as apparently the alloy their made of is crap) and possibly another issue such as the throttle body being clagged up or the ECU having an intermittent fault. If my car begins driving badly all i need to do is turn off the engine then turn it back on again and i can see an immediate improvement.. thats not just a leak in the cylinder head!

The AUD engine (1.4 moo) is a major failure on the polo. But oddly doesn't fail on seats? .

Pretty much everthing about it is stupidly designed and is bound to fail.

I had the same problems as you guys, firstly I changed the head gasket and belt. Got a little better then failed. Changed it again with a skim and failed again.had enough and completely rebuilt it, new everything including the front lambda sensor ( over £100) and decated it (pre cat and rear cat) it ran great and I was over then moon. Then 3-400 miles later on the road it seized up.

It's bow in the skip huge waste of over £500

If you are planing on replacing the engine I would go for the AKK which is exactly the same but is fitted to seats mostly and guess what it doesn't fail so much. Or even better go for an AUA or AUB 16v. Even the AFK or AHW wi ll work(lupo).

thanks Dixy, are you definitely sure those engines you listed will fit? I've consdered a new engine and if these options will basically slide straight in with no massive adjustments needed, ill start looking into it!

The AUD engine (1.4 moo) is a major failure on the polo. But oddly doesn't fail on seats? .

Pretty much everthing about it is stupidly designed and is bound to fail.

The only difference is the installation on the Seat it also uses a front mount onto another subframe So the engine is better supported?

Yes it is a stupid design by VAG as they consider the engine to be consumable you can't remove the crank without the block distorting and the cam cover is infact the half the camshaft bearing, so you have to remove the cambelt just to remove the cover

Cylinder 2 injector issue ?????

The AFK will fit and work straight away, as its the same engine. That's not to say that if it is an electronic fault then the engine would probably run the same. Yes there is a front mount ( which wold make the polo better) but I think that bolts into the little eyelet on the fron of the case, and should be removable.

As for the 16v setups, they will physicly bolt in easily if you get the complete thing, so mounts loom manifolds etc, but you nay need to change the ecu. This isn't an extremely difficult thing as its just swapping over boxes but is an added effort. Imo the 16v engine on the 6n2 are miles better then the 8v.

As for the injector issue, that reminded me. After the rebuild and exhaust work it was still lumpy and hard to start. So I got a new Bosch battery in and it's started on the button and idled perfectly, so this nay be a factor as Vw electronics are known to be super fussy about voltages.

Take your plugs out and have a look.... Could be as simple as that to identify it; why you'd rip the head off without doing a compression & leak down test is beyond me though

Update..

There were 2 faults! The injector on cylinder 2 was knackered, and there is some kind of leak on the cylinder (after a compression test). When I took out the injector rail I could see that the 3 tiny holes were almost varnished over on the number 2 injector.. so a trip to the scrap yard and having taken one off a Lupo and sticking it on the polo there was a massive improvement.. that intermittent misfire has long since gone and now only the slight loss of compression remains as a constant. The effect of that is a slight rumble on idle (not bad enough for any action just yet) and the engine sounding slightly like a diesel when its idling - this seems to be a common issue and people complaining about it are likely to have a similar slight loss of compression on one of their cylinders.

Options remain a cylinder head replacement (so I'm keeping my eye out for a second hand one) or as discussed a new engine.. my favorite is the cylinder head so i can recondition it before it goes on.

Thanks all for the advice!

Glad it's kind of sorted, can't believe you'd had the head off etc without checking for fuelling, need air/fuel/ignition for an engine to run, if you've got issues like that it's worth checking them all lol.

Lessons learned!!

Hopefully this thread will help someone else to do it the right way round eh!