Achilles Heel: Porsche 996 motors
Internet doom mongers will tell you 996 Porsches are guaranteed to implode. Is the risk really that high?
It’s fair to say the water-cooled flat-six family fitted to Porsche’s 996 and early Boxsters has a bad rep. We’ve all heard horror stories about them grenading, but have the blow ups been blown out of proportion? Just how likely are they to go wrong and how much does it cost to fix?
Porsche M96 engine. Yours for £12k
All Porsche 996 engines suffer catastrophic engine woes – from IMS failure, cracked cylinder liners and nasty bore scoring. It all results in one thing: an engine that grenades itself and hands you a five-figure repair bill.
There are several problems with the M96 engine in the 996, but according to Josh Sadler of Autofarm (www.autofarm.co.uk), forget about cracked cylinder block stories - his company hasn't seen a failure in years. Most of the cars that had poorly cast blocks have detonated and been replaced more than a decade ago.
Broken cylinder liner. Big bill on the way
What is an issue is the infamous Intermediate Shaft (IMS) that drives the camshafts indirectly off the crankshaft. Porsche has used one on every flat six-cylinder engine since the 911 was born back in 1965.
It's wear and lack of lubrication that kill the little bearings. If it fails the consequences are devastating – think intimate liaison with the valves and pistons in the fiery furnaces of hell.
Piston, meet valve
Ignoring the reported crack cylinder head problems (caused on early cars by overheating after damage to the front rads) the next issue that can seriously take the edge off the experience is bore-scoring cause by 'hotspots' damaging the Lockasil cylinder lining.
WHAT'S THE INCIDENCE?
Sadler's Autofarm became an expert for all three issues back in 2005 when engines started to fail and the Bicester-based 911 specialist has done more than 100 996 engine rebuilds, demand dropping after 2010 when values of the water-cooled 911 fell through the floor. Common consensus is that just 5% of cars have suffered an IMS failure.
The problems don't only affect 996s, but early Boxsters and Caymans fitted with the M96 engine. The M97 engine introduced with the 997 (only on the Carrera S; the Carrera stuck with the M96) still wasn't trouble free but later 997s got an upgraded IMS – worth knowing if you're in the market for a car.
When does it happen?
One day the IMS will fail due simply to wear and tear, but some cars were failing at just 20,000 miles and some last more than ten times that. According to Sadler, in his experience, low-revs are more punishing on the bearing.
Like a roulette wheel, there's more force transmitted at low speeds. At higher speeds the dynamic tension smooths out the load forces.
Weirdly, early cars with 'dual-row' bearing are more robust than later cars. That's right, Porsche made the problem worse. Later single-row IMS-equipped cars that were introduced in early 2001 were subject to more failures.
Sadler thinks the bore scoring was caused by Porsche's move to a drive-by-wire throttle that is less accurate than the mechanical system at low revs causing the driver to rev too hard when the engine is warming up, causing the damage.
How can I tell if my engine is about to die?
Finding out if your IMS bearings are failing is easy – if there's any evidence of gold shrapnel in the oil filter housing you're in big trouble.
How do you spot a bore score failure? The nearside tailpipes will be darker than the offside as cylinders five and six begin to burn oil.
What's the solution and how much does it cost?
To repair bore scoring or an IMS failure a respected specialist like Autofarm charges at least £10,000 (plus VAT) – but it doesn't stop there. With the engine stripped, most 911 specialists will charge £1000 to overhaul the hydraulic tappets (more for a latter car with Varioram Plus) and refurbish the oil pump. Go for a bigger bore engine for more power and you'll pay the thick end of £15,000.
Now that 996 values are rising it could well be worth changing the IMS bearings as part of an insurance policy to save you needing that rebuild. Autofarm charges £1850 (plus VAT) and it'd be sensible to throw in a new clutch (£450 plus VAT) at the same time. If your car needs a clutch anyway, it'd be a no brainer to get the mod done.
So a case of 'they don't make 'em like the used to'?
'That's what makes me laugh', says Sadler.
'The 911s of the 1960s and '70s were renowned for camshaft tensioner failures that caused horrendous damage - but they happened in the days before social media…'