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Porsche IMS Bearing Explained

If you’re a water-cooled flat 6 Porsche owner, you will have heard of the dreaded IMS. Forums are full of horror stories and we have known people who have not bought one of these beautiful cars because if the fear of IMS failure.

It is true that the IMS bearing is a known weakness of these engines. However, failure is quite rare and, ironically, more likely to happen on cosseted low milers than hard driven high milers. The internet focuses on what is wrong with cars and creates an impression that all flat 6 water-cooled Porsches blow up. This is not the case, with only about 5% of affected engines thought to fail for this reason. However, as an owner, you will want to have a basic understanding of what the IMS issue is all about which is why we have written this blog.

What is an IMS bearing?

IMS stands for ‘Intermediate Shaft Bearing’, which is the supporting bearing of the intermediate shaft, on the flywheel end of the engine. The role of the intermediate shaft is to drive the camshafts indirectly off the crankshaft. By using an intermediate shaft, the speeds of the chains are reduced, which is better for the chain life. This basic design was used through the entire lifespan of air-cooled six-cylinder engines used through to 1998. You may be surprised to hear that the inclusion of an intermediate shaft, which drives the camshafts indirectly off the crankshaft, has long been a feature of the flat 6 engine used by Porsche as well as the water-cooled Turbo, GT2 and GT3 models as their engines are based on the same engine case and internals as the earlier air-cooled engines. This intermediate shaft features plain bearings that are pressure fed engine oil for lubrication and never fail.

Which models are affected?

All 986 Boxster models.
987 Cayman & Boxster models up to engine number 61504715
All 996 models (not including: GT and Turbo models)
997 3.6 with M96/05 up to engine number 6950745

What causes them to go wrong?

There was a fundamental change in philosophy with the water-cooled engines in that the bearing is a sealed bearing – in other words it is not lubricated by the engine oil circulating around the engine. All bearings wear eventually but the loads carried by the IMS and the lack of lubrication mean that they run hot and can overheat. This eventually cooks the metals in the bearing and causes too much friction, overloading the steel and fracturing it, leaving behind a jagged depression. Once this begins, wear accelerates and you have something of a ticking time bomb.

What is the worst that can happen?

In the event of a complete failure, the cam timing can be lost, causing valve to piston contact which is terminal and your engine will require a full rebuild.

What gives the IMS bearing its fearsome reputation is that it can result in total engine failure with very little warning. Metallic debris in the oil filter whilst carrying out an oil change should make you suspicious. You may find that there’s an oil leak located at rear of engine, or you might have started to hear knocking and metallic sounds coming from the rear of the motor. Clearly any of these warnings warrant further investigation and you should not drive the car at all until it has been investigated.

What can be done to solve the problem?

If you are buying a car and have a choice of one with an IMS upgrade then it is worth paying more for that car. If you already own the car, then options are limited. Don’t treat the car with kid gloves – low mileage and lack of hard use make the risk higher, ironically.

Many people believe that Porsche should have made the IMS bearing a service item. To replace the bearing, you have to remove the gearbox, clutch and flywheel, which is what makes it such a big job. Our advice is to routinely upgrade the bearing if you ever have to remove the gearbox. With clutches often only lasting c 50k miles, a logical thing to do is to do the IMS bearing if you have a new clutch fitted.

Porsche do not sell a bearing on its own – they just sell the whole shaft, which is only fitted as part of an engine rebuild. Fortunately, the after-market is well served and there are upgraded bearings which can be fitted without stripping down the engine. The one that we use is the EPS Eternal Fix bearing kit. This has a patented roller bearing design which can handle loads 12 times higher than the standard ball bearing unit.

We have done a number of IMS bearing replacements. We prefer to take the car back to our workshop than do it via our mobile workshop. You should not live in fear of Porsche IMS bearing failure and can drive your car as it was designed to be driven. Stop driving and seek advice if you do see any of the warning signs but do not be paranoid. Then plan on a routine upgrade if your gearbox has to come out and you will not need to worry again.

 

 

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