Porsche Intermediate Shaft Bearing issue
Most models of the 996 generation of the Porsche 911 sports car were afflicted with a vulnerability in the intermediate shaft that drove their engines' camshafts. Failure of the ball bearing within the IMS leads to varying degrees of engine failure. In general, after an 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 a rebuild or replacement of the entire engine.
Background
During the development of the M96 engine Porsche realized they could save a significant amount of their production costs by using the same cylinder head casting on both sides of the engine short block. This was a good cost saving exercise, but required that the intermediate shaft, which on air cooled Porsche engines had only previously geared down the crank shaft to drive the single overhead camshaft and drive the oil pump, to now continue through the back of the block and drive a set of dual overhead cams on a reversed cylinder head on the opposite side of the crank case. Due to the engine block design, oiling the far side of the intermediate shaft was difficult, so Porsche chose to ride the intermediate shaft on a sealed ball bearing at the side of the engine block connected to the transmission.The M96/M97 engines in the Porsche 996 as well as Porsche Boxster and Cayman models used such an intermediate shaft design. 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. 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 the completely new 9A1 engine that eliminated the intermediate shaft and drives the camshafts directly from the crankshaft, returning to their previous flat six engine topology of having two separate cylinder head castings and running all the ancillaries and timing chains from the rear facing side of the engine.
Causes of IMS bearing failure
The intermediate shaft and front bearing are submerged in engine oil in normal operation, including at idle and when the engine is shut off, while the rear 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. 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 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. Failure of the IMS bearing can also cause bolt failures in the IMS bearing.Preventive 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%, 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, failure rates could increase. 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 Porsche's air cooled engines. 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 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, 1999, and 2000 M96 units, but the IMS failure rate on these earlier cars was so low that it was decided to exclude them from the action.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 preventive measure were also eligible for compensation. However the schedule of payments was weighted heavily in the favour of cars purchased through the Approved Certified Pre-Owned Program.
| Original owner | ACPO purchaser | Used Not ACPO | |
| Up to 50,000 miles | 100% | 100% | 25% |
| 50,001 to 60,000 miles | 90% | 100% | 25% |
| 60,001 to 70,000 miles | 80% | 100% | 25% |
| 70,001 to 80,000 miles | 70% | 100% | 25% |
| 80,001 to 90,000 miles | 60% | 100% | 25% |
| 90,001 to 100,000 miles | 50% | 100% | 25% |
| 100,001 to 130,000 miles | 40% | 40% | 25% |
A total of 235,152 potential class members were directly mailed notification of the settlement and additionally a website was created detailing the settlement. The total number of claims made was only 1.39%, suggesting an actual failure rate significantly lower than the previously estimated 8%.