996, sudden shaky idle, rough revving and exhaust is loud!

Had my 996 fixed last week with a replacement coolant header tank (cracked) and had all spark plugs replaced. Its a MY2000 model, and i've had the PSE sports exhaust fitted. This was put in only last year so its pretty new. I just had a quick run out to the shops just then in it, and it started to sound VERY loud when revving. I thought nothing of it as i went to the shops and came back to it five minutes later. Now the exhaust sounds INCREDIBLY loud (Like some kind of race car), the car is idling at 600rpm, and seems unsteady, and the engine and car is shuddering. If i rev it, the exhaust makes a burbling/wobbling noise. As i was coming home it seemed to be struggling at low speed as it picked up. Any ideas?!

Have you looked under the car to see if there are any obvious cracks in the whole exhaust system. If it is near a lambda sensor the sensor could be reading wrong that is causing the symptoms at idle. The bolts holding the join between the cat to the silencer get pretty rusty and may have broken. They may have weakened after fitting the PSE last year.

Ahh thanks for the quick reply. Maybe the work done on the car last week did some shifting about of the car that might have weakened the old exhaust components. Not exactly what i need in this current financial climate - work thin on the ground and now everything is going wrong with what was a total hassle free motor up till recently!

if its just the bolts then it's not a big job, though they can be stubborn and need to be pressed out - a labour issue rather than parts. 2 hours tops. Similar running symptoms on my 2000 996 turned out to be a failed MAF plus O2 sensor- altered the exhaust note as well - but not to the extent you're describing -would show on a diagnostic and as engine fault.

Sounds like the headers have cracked or similar. A good welder should be able to repair as temp measure. They will be stainless steel i would have thought?

Nucastle - as mentioned, the steel nuts that secure the exhaust manifold and catalyzer section flanges together often corrode and simply waste away. I'm not saying this is the problem, but could well be. If this is the case, it should be a relatively simple fix that any automotive garage could repair at minimal expense. They may even knock out the existing studs as they are knurled and pressed into the steel flange, and replace with conventional nuts and bolts. I have included a photo of the corroded flange nuts I'm referring to.

I went to start my 996 the other day and like the OP the noise was very loud and a quick poke underneatht the car revealed that the manifold/cat gasket had gone. I have ordered a replacement for a few quid but will need a garage to do it as the nuts look exactly like the photo above and will certainly need drilling out and replacing. Oddly, after 10 mins or so the car goes back to normal which I guess must mean where the gasket has failed it expands in the heat to fill the gaps. The upside is does sound like an unsilenced race-car for a while but can be very embarassing when trying to crawl past the White Horse in Parsons Green without sounding like a chav tit.

How corrosive is engine coolant? .... I'm wondering how much my coolant tank problems have to do with this all, since essentially the cracked tank dumped most if not all of my engine coolant over the back portion of my engine only a few weeks ago. Maybe sitting there after the breakdown finally finished those bolts off.

Although coolant can mark paint its not that corrosive. Remember it has to protect against corrosion in the engine although different formulations protect different types of metals. More likely the bolts were weakened further during the PSE installation, then when the coolant tank was being replace the manifold/cat/silencer was knocked and the bolts just gave up. Mine look exactly like the ones in the picture above and its probably only corrosion holding the whole thing together.

I added the picture in my previous post as there's many a 996 looking like those out there. Whilst a gas tight seal is maintained between the two flanges there's no real issue. Wouldn't you think a motor car costing in excess of three times average car money would have quality stainless fasteners on the exhaust system throughout...... :dont know: