Porsche M96 flat-six common problems

Common problems and what to look for on the M96

The known issues most worth checking when you view a car with this engine. None of them means a car is a bad buy, but they're worth knowing about before you buy, and a good history often shows they've been dealt with.

1. The intermediate shaft (IMS) bearing

   What to look for: Ask for the engine number before you travel, and look for paperwork showing whether the bearing has been upgraded, usually at a clutch change, and when. Listen as the engine starts from cold: a rattle at the back on start-up is the thing to note.

   Why it matters: The bearing supports the shaft that drives the camshaft chains, and if it fails the timing can be lost and the engine wrecked, often with little warning. Engines from 1997 to 1999 have a double-row bearing, and figures from the US class action put its failure rate below 1 per cent. 2002 to 2005 engines have a smaller single-row bearing, put at about 8 per cent. 2000 and 2001 were changeover years, and the engine number shows which: on the 3.4 Carrera, double-row up to M 661 14164; on the 2.7 Boxster up to M 651 12851; on the 3.2 Boxster S up to M 671 11237. From the 2006 model year a larger bearing arrived (on the 997 3.6 after engine 69507475), which is stronger but not immune, and can't be replaced without splitting the engine. The earlier bearings can be upgraded without taking the engine apart, and an upgrade on file is a strong plus.

2. Cylinder bore scoring

   What to look for: Before starting, wipe inside each tailpipe with a white cloth. On a 911, a sooty left-hand pipe next to a clean right-hand one is the thing to note. Start from cold and listen at the back for a rhythmic tick or knock at idle. Ask how much oil the car uses and whether that has been rising.

   Why it matters: Scoring of the cylinder walls lets oil past the pistons and is progressive. It affects a minority of engines, mainly the 3.6 in the 996 and 997 Carrera; the early 3.4 and the Boxster engines are the least affected. It usually starts on cylinders 4 to 6. A borescope inspection by a specialist settles it, and is worth having on any 3.6.

3. Cracked cylinders or heads and oil in the coolant

   What to look for: With the engine cold, look under the oil filler cap and in the coolant expansion tank. There should be no creamy, milky residue and no oil film on the coolant. Ask whether the car has needed regular coolant top-ups.

   Why it matters: Some engines, mostly early ones, developed a cracked cylinder liner, usually the centre cylinder on a bank, or a cracked cylinder head, letting oil and coolant mix. Rising coolant use is often the only early warning. Specialists also suggest renewing the water pump every four to six years, since debris from a failed impeller is linked to head damage. A documented replacement or rebuilt engine isn't a concern in itself; what matters is that oil and coolant aren't mixing on the car you're viewing.