SHAKE, RATTLE AND ROLL

These are, perhaps inevitably, not the best-quality pictures ever to grace these pages. But that’s what you get from screen grabs derived from e-mailed smartphone videos – and I think they still convey pretty accurately what an idling 996 clutch and then bare flywheel look like. Best not get too close, anyway…

They – the videos, that is, and from which I have taken the screen shots – were sent to me by a 911 & Porsche World reader and 996 Carrera owner, who wishes to remain anonymous. He is naturally concerned about what can best be described as an intermittent but still fairly regular light tinkling sound when the engine is running.

The video clips originated with his independent Porsche specialist, who our man has entrusted with the task of finding and eliminating the source – and who, as you will have deduced, has already thought commendably laterally, and run the engine sans gearbox in an effort both to eliminate that from enquiries and also physically to examine the clutch and the flywheel.

I shall let our reader himself take up the story. ‘I am really scratching my head here, Chris, with a rattle coming from what I thought could be the dual-mass flywheel in my 1998 996 Carrera 2 with manual transmission. The engine also has a slight but noticeable vibration at tickover.

‘My local independent first suspected an ignition issue – reading the fault codes during the last two services has shown a misfire on all six cylinders at idle – and so he went to the trouble of taking the ECU out of his own almost identical car and, after doing the necessary recoding, temporarily fitting it to mine.

‘That made no difference, so the next step was to take out the gearbox and run the engine to find out if he could see and/or hear what might be going on inside the clutch housing – that was the first video I sent you. That was inconclusive, so then he took off the clutch and the dualmass flywheel, quickly discovering that the latter appeared to have virtually none of the required radial damping action.

‘Not unreasonably assuming that to be the cause of the problem, he ordered and fitted a new DMF, but the noise was still there. And for which reason he subsequently refitted my original flywheel. No point in spending all that money if I don’t need to! Next, he tried a used but good crankshaft position sensor, disconnected both lambda sensors, and even tried new Variocam solenoids and a MAF sensor – but nothing made the slightest difference to either the misfire or the noise.

‘The car has done 103,000 miles, with a full engine rebuild at approximately 100,000 miles in 2012, when it was in the hands of the owner before me. I have the invoice for this. I bought the car about five years ago, and as you can probably tell from those mileage figures I don’t drive it much.’

My own first reaction was to suspect a possible timing-chain issue. This being a printed magazine I obviously can’t let you hear the noise for yourselves, but it is definitely far too light in tone to be the dreaded cylinder-bore scoring, or worn crankshaft bearings – or even a small-end bearing. And if not that, then perhaps – horror of horrors – the intermediate-shaft bearing. Maybe something to do with the clutch friction plate or pressure plate.

Fair point, agreed my correspondent, when we later spoke on the phone. ‘But my specialist also ran the engine without the clutch – that’s the second video – and although the noise was quieter, perhaps suggesting that the combined weight of the cover and the friction plate was exacerbating the DMF issue, it was most definitely still there.

‘IMS bearing? Well, yes, I suppose it could be that, but my specialist assures me that there was no tell-tale swarf in the engine oil at any of the services he has carried out for me over the last five years, so I would doubt it.

‘Either way, there are still fault codes coming up for misfires – two on each cylinder bank, and all in the lower rev range. Needless to say, any help you can offer would be much appreciated!’

Plainly there remains quite a lot more diagnostic work to be done on this one – if it were my engine I would be having the IMS bearing out for close inspection, and I have suggested as much – but in the meantime I am throwing it open to the floor, as it were. I am convinced that someone out there must already have experienced and hopefully cured the same issue.

And there is always the possibility, of course, that there are two entirely separate faults going on here: the misfiring, perhaps because of faulty coil packs (it certainly wouldn’t be the first time for one of these engines, would it?), and whatever mechanical malady is causing the noise. We shall see.

The engine was run again minus the clutch, and in the video clip we have seen – and from which came these two screen grabs – the noise was far less obvious. That tends to point to a clutch problem, but we’d suspect either the IMS bearing or the short chain (and its tensioner) between the crankshaft and the IMS – and as a 1998 unit this engine would have the original duplex chain rather than the later – and supposedly quieter – ‘Hy-vo’ item. More on this in due course, ideally when we know the definitive answer

There is a not inconsiderable amount of work involved in removing a 996 gearbox (see last month’s how-to story) and then enabling the engine to be run such that one can (hopefully!) track down a mysterious noise, but desperate times call for desperate measures. Here, with the clutch still in place, the light tinkling noise sounded to us very much like a timing-chain problem