Coil Packs and Head Gaskets

[KRL]

A very interested article I read produced by Revo Ben:

Well, it seems that recently the 2.0TFSi engine has been getting much of my attention but once again, it’s the focus of this article! This isn’t to say it’s a bad engine, just that it’s complexity and stringent emissions control mean that the smallest of issues can cause much bigger problems than on older engines!

Recently I have seen a good few TFSi engine that seem to be experiencing a misfire, usually under load and at higher engine speeds but occasionally they play up at low engine speeds, even, as for one I saw very recently, at idle speed! The first of 2 failings I’m going to cover is to do, indirectly, with the ignition coil packs. Anyone who has had dealings with the older, normally fuelled, 1.8T will I’m sure be aware that coil pack faults are nothing new. Infact, a few years ago it was probably the biggest and most published issue of any Audi engine. The fault being relatively minor but causing a misfire that made it seem as though your engine had gone seriously wrong! When these failed they actually shut the affected cylinder down completely, gave in and the engine ran on 3 cylinders. This fault was addressed to a fair degree and later, replacement coil packs, although not totally cured, were greatly improved and became much less of a problem, failing only rarely. The coils on the 2.0TFSi engine has similar coils and so far they had seemed to be reliable but I have recently seen a few failing. The symptoms though are very different! The cause of this particular fault is not originally the coils themselves. The 2.0TFSi has a different design of cylinder head to the 1.8T, it has a cage assembly that contains the camshaft bearing caps rather than individual caps, as on the 1.8T. This cage means that the bearing surfaces can be smaller than with individual ones as the extra strength gained from the design reduces the load acting on the bearings. This reduced bearing surface size improves efficiency as the frictional losses are reduced. The problem though is that this cage needs to be bolted directly to the cylinder head to ensure correct location and bearing pressure so no rubber or metal gasket is present, that would be too varying in size to achieve the running tolerances required for the camshaft bearings. This means that the cage relies on a small amount of sealant to stop engine oil escaping. This is the weak point! I have seen a good few now on which the sealant is allowing oil seepage, and as the cage is the same external shape as the head itself, this oil is able to accumulate in the spark plug recesses! Oil being present in these orifices is bad for a couple of reasons. Firstly, the oil can cause the coil to earth out on the head itself, causing minor misfires and timing pull back but also the oil transmits a lot of heat to the coil for a prolonged period. The coils are normally insulated from direct heat to a degree by the rubber shrouds around the plugs and the rubber seals further up the coil’s shank. This extended period exposed to heat can slowly degrade the coil and it’s ability to produce the strong spark needed for combustion. This usually identifies itself by a gentle hesitance as you apply load and boost pressure, often feeling like a series of very small misfires, but rarely strong enough to make the engine warning light illuminate. It may also be possible at this point to hear the boost pressure being allowed to escape via the recirculation valve. The engine does this to protect itself as it is aware that an efficient and full burn is not possible. It is relatively simple to identify this fault using diagnostic software. You can read something called Correction Factors. This is the amount of timing pull back that the engine feels it must use to stop any pre detonation that would otherwise cause possible damage. This correction factor usually sits at 0.0, and will rise if the engine measure pre ignition. A safe limit usually is around 3.0 degrees as a maximum but this should only be seen under full load and boost. If the level increases to over 4.5 there’s something a miss! If the figure is only rising on one or two cylinder then that would indicate that either the plug or coil on that particular cylinder was on it’s way out! On the last few TFSi’s I’ve seen with the oil leakage then this correction factor was in excess of 5.5degrees – at idle!!! That is a huge sign of an ignition system failure! Usually cleaning out the plug ports and replacing the coils at this point will rectify the fault but the oil leak must ultimately be rectified to stop the fault returning and more coils having a hard time!

Another, slightly worrying recurring issue now that these engines have been around for about 5years is that on some of the older versions and especially the 240bhp+ versions, I have seen several with failed head gaskets! It becomes apparent that the engine is losing coolant somewhere but with no external signs of leakage. The car seems to run ok but when hot, again the correction factors can give away the fact that a full burn just isn’t happening. This slowly gets worse and is due to coolant being present in the combustion chambers. After a while it may be that you can physically see the red stain left by the antifreeze additive that remains even after the coolant evaporates at the gearbox end of the head. This water leak seems as though it’s coming from a coolant hose joint that is bolted to the head as this obscures the head to block joint. It is easy to mistake this coolant build up for a split hose. As the coolant loss is very slow it would be a while after replacing this hose that you would realise that it hadn’t cured the fault. In most cases the head ends up being replaced if it’s in under warranty, presumably due to face damage caused by the leak but I haven’t seen evidence of this as yet.

Discuss.