Porsche IMS bearing problem explained
TWG theory as to why the bearing fails on low millage cars. (exclusive to TWG Sport)
Well, ever owned a classic car you have stored for few months at a time with a wheel not turned? Then when you re-used your classic a wheel bearing noise has developed?
The cause is a pin point pressure from the surface of the ball race onto the bearing journal this can be measured with a brinell test and I can confirm this is one of the major causes for bearing failure on M96/97 engines.
This theory also explains why cars can cover 150k plus miles without any problems.
When the engine is standing the pressure is directed IMS then on to the bearing from the cam chains that remain tight due to the hydraulic chain tensioner and the non return valve.
The bearing journal suffers microscopic indentations on its surface thus resulting in a uneven surface for the ball race to run on and then after some time it begins to degrade the case hardening and eventually the bearing starts to break up.
I guess when Porsche bench tested the engines for reliability they wouldn't have left them standing for long periods of time therefore not discovering the problem.
Also 986 2.5 engine Boxsters have less problems why? well they are fitted with a dual row bearing to start and the hydraulic tensioner on this model usually leaks residual pressure which keeps the tension of the IMS when standing, so if you have a early 2.5 986 that the chain rattles on start up don't worry its less likely to damage the IMS bearing. The 2.5 engine has a chain driven IMS by the crankshaft which also relives pressure on the IMS, later engines have a gear driven IMS with little free play.
Porsche replaced the bearing in 2000/2001 with a single row ball race proven to be weak and later replaced it again in 2006 with a bigger single row ball race but all pre 2008 cars have the same issue.
Other failure reasons published are rubber seal breaking up with overheating and engine oil ingress mixing with the LM grease- not good.
If a engine has been overheated because of a blown radiator (common) the bearing must be replaced.
Upgrades are available if in doubt.
I do not advise anybody uses oil spray to lubricate the bearing because LM Grease has different lubrication properties to engine oil, LM Grease is designed to deal with the pin point pressure.
see our next blog IMS prevention