WHAT GOES AROUND COMES AROUND

I hesitate to recycle previously published text – in the same way that it’s far better for diesel engines not to ingest their own exhaust gases – but sometimes there is no merit in rewriting something for the sake of it. Shown in bold type below, then, is essentially the (remarkably prescient!) answer that I gave reader Peter Robbins in September 2014, after he wrote to ask why his 40,000-mile Cayenne Diesel might have illuminated its check engine light, or CEL. His local garage suspected a diesel particulate filter (DPF) problem, and tried to carry out a so-called forced regeneration, he told me then, but this was unsuccessful.

“I suspect this is something we are going to be hearing a lot more about, as diesel-powered Porsches become both more commonplace and inevitably older. I spoke to Robin Mckenzie at Bedfordshire-based Auto Umbau, and he kindly offered not only his knowledge and practical experience, but also some apparently faulty hardware for me to have a look at. (And to take to pieces for further investigation and analysis…)

‘We had a car in showing those same symptoms a few months ago,’ Robin told me. ‘We thought it was a DPF fault, and tried giving the vehicle a fast run to get it hot enough to clear itself, but that made no difference. We then tried to do a forced DPF regeneration using our Durametric diagnostic kit, but that didn’t work, either. Likewise our VAG-COM unit – and eventually I discovered that only the official Porsche system tester allows this process to be undertaken. What the fault-code list tell us, though, was that there was a restriction in the flow through the EGR valve – the exhaust-gas recirculation valve – rather than it being anything to do with the DPF.’

Always wanting to do the best job possible, and to fit the best-quality replacement parts, Robin rang his local Porsche Centre. ‘I wanted to see if it is possible to clean out these units, but my contact in the parts department suggested that not only would we need a brand-new one, but also that fitting a new valve alone probably wouldn’t solve the problem. We would need what Porsche calls the heat dissipator to which it is attached, and although you can buy the valve separately, it does routinely come with this dissipator. That seems to be adevi ce linked to the cooling system, and which presumably reduces the temperature of the exhaust gas before it enters the valve.’

Robin bought and subsequently fitted the compete unit, reporting that although it seems relatively accessible, on the top of the engine block, and then back towards the bulkhead, it is not a job for the fainthearted. ‘Porsche allows about four hours’ labour, but it took me nearer eight – although I charged the customer only for six. Part of the problem is reaching in over the top of the engine – you end up almost laying on top of it – but also disconnecting the smalldiameter pipes from the exhaust system, and one of the fixing nuts is an absolute pig to reach.’

Did it work? ‘Yes!’ says Robin, with obvious relief. ‘My customer wasn’t exactly thrilled at the total cost – around £800 plus VAT – and it remains to be seen how long the new assembly might last before it suffers the same fate. But it got the car back on the road both quickly and efficiently, and to Porsche’s original standards. Better that than to risk doing much of the same work in cleaning out the old valve, or replacing that alone, and then finding that you have to repeat the process very soon afterwards.’”

Reassembly is – predictably enough – a reversal of the dismantling procedure, but first make sure that you have (safely) sucked out and disposed of any coolant that had pooled in the valley between the cylinders, and also that you have extracted any components that ended up in the same area – and there will almost certainly be several of those, trust us. Black plastic connector (far right) was the single casualty of battle, but fortunately the replacement assembly came with a new one. Shaved ‘O’-ring, dating from when the previous EGR valve and heat dissipator had been installed in 2014, highlights one of the potential traps during reassembly, but is difficult, if not impossible, to avoid: you simply cannot see the stub as it slides into the engine block – although forewarned is surely forearmed. Either way, luckily this one hadn’t been leaking without also destroying the EGR valve remains to be seen, but the ultrasonic cleaning tank over at Neil Bainbridge’s BS Motorsport will figure in the process. (I would have liked to delve inside the second unit, as well, but the car’s owner wants to carry out his own tests on that.) Either way, this story will have served its purpose if it not only alerts you to what is almost certainly a potential weakness within these cars (and the similarly powered Panamera and now Macan), but also shows you how to address it. Or, of course, why you might be better advised to let someone else, with the necessary experience and facilities, address it for you. Fools rush in where angels fear to tread, and all that.

Suffice it to say – and in a sense this, too, might be a valuable moral of the story – that, like Robin Mckenzie, I do not believe on-thecar cleaning of the EGR valve with some miracle solvent sprayed through the air intake to be a long-term solution. Or necessarily even a short-term one. Likewise any of the cheap and supposedly ‘universal’ Egr-valve blanking plates that pop up with depressing frequency on the internet. One of those might do the job. But they are surely of questionable legality in terms of exhaust emissions (in the same way as ‘deleting’ a diesel particulate filter), and even if they are 100 per cent legitimate you still have to remove the valve in the first place – and then live with the knowledge that, however flawed might have been its original specification, you have necessarily compromised your Porsche.

No, far better in my mind never to get into a situation like this to start with. How to do that? Well, in simple terms – and bearing in mind obvious road-safety considerations and all speed limits – whenever possible drive your diesel-powered Porsche like you stole it. It is not a guaranteed solution, and it will cost you some time and a little extra fuel. But it’s a great start, and it has to be immeasurably more enjoyable than groping about inside the car’s engine compartment because for the last three years it has done nothing more strenuous than the school run. PW