1. Background

2. Causes of IMS bearing failure

3. Preventative maintenance

4. MY 2006 to MY 2008 cars

5. US Class Action Lawsuit

6. References

7. External links

Most models of the 996 generation of the Porsche 911 sports car were afflicted with a vulnerability in the intermediate shaft (IMS) that drove their engines' camshafts. Failure of the ball bearing of the IMS generally leads to varying degrees of engine failure. Generally, after IMS bearing failure, the engine internals are contaminated with debris from the failure that requires the engine to be stripped and rebuilt. In severe failure modes, cam timing may be affected, leading to valve-piston impact, necessitating replacement of the entire engine. ^[1]

Background

The M96/M97 engines in the Porsche 996 as well as Porsche Boxster and Cayman models used an intermediate shaft (IMS) to transmit power from the crankshaft to the camshafts. One end of the IMS is held in place by a plain bearing, the other by a Ball bearing, with the ball bearing at the front of the engine, behind the transmission. Three types of ball bearing were used over the engine's life: dual-row ball until 1999 and in some 2000 and 2001 cars. Starting in model year 2000, Porsche began phasing out the dual row ball bearing and went to a smaller single row ball bearing, with significantly less load capacity.^[2] From MY 2006 through 2008 the IMS bearing was replaced with a larger single row bearing that cannot be replaced safely without engine disassembly.

In 2009, Porsche introduced a completely new engine that eliminated the IMS and drives the camshafts directly off the crankshaft.

Causes of IMS bearing failure

The intermediate shaft and front (plain) bearing are submerged in engine oil in normal operation, including at idle and when the engine is shut off, while the rear (ball) bearing is separated by an oil seal and has its own grease lubricant. When the car is at speed or under significant G forces, the sump level drops and the shaft operates with partial submersion with oil mist/splash lubrication.^[3] Some engine failures appear to be due to failure of the rear IMS bearing seal, allowing dirty engine oil, especially in engines run on long oil change intervals (OCI) inside the bearing, washing out the grease contained within it. Any contamination in the engine oil or lack of exchange of fresh engine oil that occurs when open ball or roller bearings without seals are employed, can result in increasing wear of the bearing, and ultimately lead to failure.^[4] The bolt retaining the IMS bearing is also known to fail, which also leads to IMS bearing failure.

Preventative maintenance

IMS bearing failure rates at the time of the Eisen class action lawsuit for the single row bearing used from 2000-2005 was estimated at 8%^[5], although this figure is difficult to quantify because not all owners who experienced bearing failure participated in the lawsuit. The earlier dual row and later non-serviceable bearing has a reported failure rate of 1%, however as vehicles age, it is expected failure rates could exceed 10%. To reduce the chance of an IMS bearing failure, it can be replaced as a service item using one of several technologies including but not limited to ceramic hybrid ball bearings or cylindrical roller bearings. These have a finite life and require replacement at frequent intervals that vary depending on the technology employed. A plain bearing conversion is available that backdates the engine to an oil fed IMS bearing like a Mezger engine. With replacement engines or vehicles built in or after model year 2006 through 2008, removal of the grease seal is considered good practice pioneered by Hartech in the UK. Without a grease seal present, the IMS is submerged in engine oil, providing adequate lubrication.

MY 2006 to MY 2008 cars

Cars made in 2006 to 2008 model years (potentially anything manufactured from 1 January 2005 to the end of the 997.1 model life) received an unserviceable bearing type. For these cars it is recommended to remove the bearing seal to allow oil to lubricate the bearing.

US Class Action Lawsuit

In 2013 PCNA settled a class-action lawsuit brought by Bruce Eisen et al., without accepting liability. The lawsuit sought compensation for owners of MY2001 to MY 2005 Boxster and 996 cars that had experienced an IMS failure. The original action was to include the 1998 and 1999 M96 units, but the IMS failure rate on these 3.4L cars was so low that it was decided to exclude them from the action.^[6]

Porsche agreed to compensate owners of cars made between 4 May 2001 and 21 February 2005 who had experienced an IMS bearing failure within the first ten years of the car's life, or 130,000 miles, whichever came first. Owners who had changed the IMS bearing as a preventative measure were also eligible for compensation. Note however the schedule of payments was weighted heavily in the favour of cars purchased through the Approved Certified Pre-Owned Program ("ACPO").

References

External links

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