Gen 2 Bore Scoring: The Smoking Gun?

Everyone loves a ‘whodunnit’. And in automotive terms they surely don’t get much better than the cylinder-bore scoring that seems to have bedevilled so many of Porsche’s flat-sixes these last 20-odd years, since they famously adopted liquid cooling in the mid-1990s. (With IMS-bearing failure a very close second, of course.) We present, then, the latest nerve-jangling, edge-of-your- seat episode: further graphic evidence that even the gen 2 997 is by no means immune to this distressingly expensive condition – albeit for probably rather different reasons than in earlier instances.

Previously – as they say in all the best TV dramas: two years ago, in the July 2016 edition of 911 & Porsche World, I reported on what then appeared to be an isolated and certainly rather odd case of cylinder-bore scoring in a 2009-model 997 Carrera ‘S’ at Porsche-Torque in Uxbridge, Middlesex. It was a gen 2 car, and thus equipped with the largely redesigned (and by inference significantly improved) type MA1 engine, with its so-called closed-deck cylinder design. (Which pretty massive change to the engine architecture tells its own story about the earlier iteration. Porsche would not have made such a radical and costly alteration without a very good reason.) Remarkably, the story elicited only a deafening silence from the wider Porsche community, although as I recorded almost a year later, in the May 2017 edition, I was soon having a long correspondence about it with Barry Hart at Hartech, who I still believe to be one of the most knowledgeable and experienced specialists in the molecular-level metallurgy of these engines outside of the Porsche factory. My own view of that Porsche-Torque case, based on empirical experience of other engines over many years, and the precise location and nature of the damage to the bore and piston – and cautiously endorsed by Barry Hart – was that this particular problem was caused not by the chronic but essentially very localised overheating that was (and I suspect remains) the most likely culprit in the earlier M96 and M97 units, but by good, old-fashioned partial seizure. Back in the 1970s I ran a 650cc BSA Lightning that suffered pretty much identical damage to both (air-cooled) cylinders, probably due to overheating caused by excessively retarded ignition timing. (I got it running again and then sold it, in case you were wondering…) Initially, Barry Hart believed that this seizure might have been the result of the owner driving his car too hard before the engine had reached full operating temperature, with piston profiles and necessarily minuscule piston-to-bore clearances as significant contributing factors. (And a management system that necessarily insulates the modern driver from the way an older, lower-tech engine would naturally behave during that warm-up process. Think 356, 911S, perhaps even 944.They all have an innate resistance to being ‘woken up’ first thing in the morning, such that it is almost impossible to drive them too hard, too soon.) The pistons were expanding faster than the cylinder bores, basically. Eventually, though, and having forensically examined several other similarly failed gen 2 engines, Barry concluded that it was probably due to stresses almost unavoidably formed within the cylinder-block castings during manufacture, and which eventually caused them microscopically to distort and shrink across the bores in the thrust direction. Whatever, as they say. The jury is still very much out on that one, and with no further reports coming in from anguished gen2 997 (and later Boxster and Cayman) owners, out there in the harsh testing ground of the real world, it seemed reasonable to suppose that any problems of this nature were relatively few and far between; just one of those things. No news is good news, and all that.

Meanwhile I was having conversations with both Steve McHale at JZM Porsche in Kings Langley and Phil Ellis down at Watford-based ASNU about the different problems that were likely to a rise in these later engines (and in the V8s, as well), thanks in part to the natural characteristics of their ultra – lean-burn direct fuel injection, or DFI, but also to the fact that –absurdly–so many high performance cars now spend so much of their time in stop start urban traffic, with their massively powerful engines running at little more than idle. And not least because there appears to be absolutely no provision for their necessarily hard-working fuel injectors ever to be serviced. (Perhaps unsurprisingly, this is a particular concern of Phil Ellisdon.) The result was my five-page Danger in the city story in the June 2017 edition of the magazine. I am pretty sure that you will understand, then, my immediate interest in a batch of photos e-mailed to me by Phil Ellisdon in early May this year. They show the inside of the MA1 engine from a 2010 997 Turbo: one careful owner from new, 23,000miles, full service history, and apparently used mostly for long journeys after being warmed up from cold with all due care and consideration. It ended up at JZM for investigation into a loud knocking sound from the engine, especially after a cold start, and at which point there were found to have been more than 60,000 misfires on cylinder number four. Inspection with a borescope showed the unmistakable signs of scoring inside that same cylinder, and so the engine was removed and partially stripped for further examination. And, by the time you read this, for replacement with a new ‘short’ engine from Porsche. Curiously, there was no obvious sign of any oil smoke in the exhaust. ‘The short block costs around£10,500plusVAT,’ says JZ’s Steve McHale. ‘But we priced up our usual alternative–completely stripping the old engine, sending it away to Capricorn for machining and pistons, and then rebuilding it with all them any other new parts that would be needed – and there was so little difference that it would have been a false economy not to use a brand new one. And although Porsche Cars GB told us that it knows of no other failures of this nature, the fact is that the new block and pistons may well have been superseded by subtly improved components. You just never know with something like that.’ Indeed.

But precisely what caused the problem in the first place? And, no less crucially, what is to stop it happening again, perhaps after just another 23,000 miles? For Phil Ellisdon the classic smoking gun has to be the injectors, combined with the relatively poor lubricity of modern ethanol-based petrol (which also contains a not insignificant amount of water, in part for its anti-detonation properties). ‘When we tested them, we found the flow rates and spray patterns were far from ideal– and you can clearly see that from the different witness marks on the piston crowns. And I believe this is due in no small measure to overheating that has affected their electrical resistance. There is a tiny Teflon seal at the lower end of each injector stem, where it enters the combustion chamber. All six from this engine were showing signs of blow-by, with two particularly bad examples, and I think it is inevitable that the very high temperatures will have travelled up the stems to the delicate electronics inside the body of each unit. ‘That, together with a carbon build-upon the six nozzles themselves–the natural product of the exhaust-gas recirculation system, and the engine stop-start function in traffic–will have adversely affected the spray pattern, and the management system, the so-called fuel trim, will have pushed more fuel through them to compensate for what the oxygen sensor tells it is too weak a mixture. That washes the necessarily thin film of oil off the cylinder walls, and there you have it. The perfect storm. Metal-to-metal contact and, very soon after that, bore scoring. It can surely be no coincidence that replacement injectors now have dark-grey, graphite-based seals, presumably better able to withstand combustion-chamber gases, instead of the original off-white Teflon jobs.’
It’s fair to say that Steve McHale is less certain about the bore-wash theory–the scoring is not in quite the right place for that, he argues–but he agrees that the injectors are probably the underlying source of the problem, and with the situation exacerbated by those supposedly high-tech modern fuels.‘ DFI injectors work in a completely different way to the older Motronic-style units,’ he says. ‘Fuel pressures in these later engines can be anything up to 150 bar, and so while the injectors need only five volts to pulse them on and off as rapidly as necessary at anything up to 7000 rpm, they need 60 volts to open them in the first place. So their electrical resistance is, indeed, critical – and the one from cylinder four in this engine was in effect short-circuited. ’

Quite what might be the longer-term answer to this seemingly new and disturbing scenario is difficult to say. Careful, more considered use of your car, perhaps – not using it for a two-mile trip to the shops or the station, for a start, despite its ability to cope with that in the short term– and certainly constant, eagle eyed vigilance. Turning off the stop-start function – while you are still allowed to, anyway. Regular testing of the injectors’ resistance (which can be done by a specialist such as JZM without any mechanical interference), and possibly a full fuel-injector service (and stem-seal replacement) every 20,000 miles – although since that might by definition require the removal of the engine from the car, and then the removal of the induction system from the engine, it’s hard to see that happening too often. Perhaps even–at the obvious risk of contaminating the catalytic converter – giving the engine an occasional dose of upper cylinder lubricant, just like we used to way back in the 1960s. Plus ça change… Either way, cylinder-bores coring seems to have become an unfortunately random fact of 21st-century Porsche life, an unintended consequence of the industry’s frankly misguided drive toward ever more ‘performance’ from ever smaller quantities of fuel (as a nation, perhaps even as a species, we should surely be looking at more sensible ways of using any of our cars than, say, the twice-daily, perhaps 100-mile commute), and I can’t help feeling that we shall soon be hearing about 991s with the same issues, and in time even the still-to-be-launched 992. Perhaps the all-electric 911 won’t be too heavy a cross to bear, after all.

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