‘UN TRAIN PEUT EN CACHER UN AUTRE…’

That phrase, probably familiar to anyone who has traversed any railway level-crossings in France, translates literally as ‘One train might be hiding another’. Do not assume, in other words, that just because one has passed it is safe to drive on without looking very carefully for another travelling in the opposite direction. Sound advice, of course, and no less pertinent to fault-finding in cars – and especially complex modern ones.

I have had an interesting e-mail conversation with a 911 & Porsche World reader over the last few months, about a persistent engine-oil leak from his 996 Turbo. Significantly – at least as far as this tale is concerned – it is one of the last built, just before the introduction of the 997 variant. And it’s a classic case of allowing one problem to blind you to the real cause of the situation. Or in this case, perhaps, of ‘fixing’ something that wasn’t even broken in the first place, with predictable results.

‘I have an oil leak from somewhere at the top of my engine,’ began our man’s opening e-mail. ‘There is only a tiny amount coming out, but eventually it runs down the right-hand front corner of the power unit, and then back onto the exhaust. The engine has been removed no fewer five times in an attempt to find and fix it.

‘On each occasion, all of the relevant ‘O’-rings have been replaced. A new oilcooler and body beneath it have been installed (£960 for that alone!), together with two threaded inserts, again with new ‘O’-rings. The correct Loctite sealant has been used between all surfaces. My garage has checked other obvious areas, like the oilfilter housing, the oil-pressure switch ‘O’-ring, the oil-level sender, and even the oil filler tube. All relevant components have been carefully pressuretested, and all of the oil pipes checked, too.

‘One obvious difficulty in all of this is that as a very late 996 Turbo – November 2004, to be precise – my car seems to incorporate a number of detail changes to the engine that are more commonly associated with the first of the 997-model cars, and so the Porsche PET system for ordering spare parts has been confusing, to say the least.’

Our first suggestion was to make sure that the leaking ‘oil’ really was that – from the engine, in other words, and not either transmission fluid, Pentosin from the power steering, or possibly even brake fluid. And not least to consider taking the car to someone who genuinely knows what they are doing – because, frankly, any garage that has already removed and refitted an engine that many times would most likely not get it right within the next five attempts.

We exchanged several more e-mails, and then eventually came the news that the likely source of the leak had been located. ‘I think the initial problem was, indeed, just an ‘O’-ring on the oil-cooler. Unfortunately, however, and completely unknown to me, the garage also replaced an adjacent press-fit core plug, which on the PET page for my car is shown as a threaded plug