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Originally Posted by gsmith329
just what is the IMS, a throwout bearing for the clutch
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The Porsche Boxster 986 Enthusiast Guide: Intermediate Shaft (IMS) Bearing Info and Fixes
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What is an intermediate shaft and what does it do?
In short, the intermediate shaft times the engine. More specifically, it turns the camshafts on the heads and also turns the oil pump. It's necessary because the camshaft sprockets for each head are located on opposite ends of the engine block. I speculate that this is done so that the same head casting can be used for both heads, i.e. the head on one cylinder bank is the same as the head on the other, just rotated 180 degrees about the vertical axis.
The IMS sits just below the crankshaft. There is a chain on the flywheel side of the block that connects the IMS to the crankshaft, another chain on the flywheel side that connects the cams on the passenger side head to the IMS, and then on the accessory belt side of the block there is a chain that connects the cams on the driver's side head and a sprocket that turns the oil pump.
How common are these failures?
It's generally believed to be a small percentage, but this figure is meaningless as it is likely to depend on mileage etc. There are still plenty of low mileage Boxsters out there; these cars could suffer an IMS failure later down the road when they get into the 100k-200k range.
It's not something that predictably affects every single car like VW's 1.8T timing belt issues. There are cars that have 100-200k or more on the original IMS bearing. However, it DOES HAPPEN. Some choose to blow off the failure as if it's not going to happen to them, but there is no reason to believe it won't. In the 3 months or so that I spent looking for a Boxster, I saw at least 3-4 cars with IMS failures in my area.
A used replacement engine runs anywhere from $5,000-7,000 which is approaching the value of the entire car in some cases. Mechanically totaling a car is not something one wants to play the odds on.
So what, exactly, fails?
IMS failures are almost always caused by something involving the bearing on the flywheel side of the IMS (the bearing on the other side is different and doesn't suffer failures).
The bearing is actually located inside the end of the intermediate shaft. The outer race of the bearing is snugly fit into the end of the hollow intermediate shaft. The inner race of the bearing then sits on a flange, which is bolted to the crankcase. A stud that runs through the middle of the bearing holds the bearing in place on this flange.
What causes the bearing to fail?
There's much speculation regarding this. The main issue seems to be that the bearing is a sealed bearing, which is an odd choice for an oily environment. The bearing has seals on either side of it that are supposed to keep it sealed from engine oil. However, these seals don't seals don't do their job very well; engine oil gets inside the bearing, cleans out its grease and often gets trapped inside the hollow intermediate shaft where it slowly deteriorates. With its factory lubrication gone and a pool of slowly rotting synthetic dinosaur soup sloshing around inside it, the bearing starts to wear rapidly.
As far as the stud failures, the stud is fairly small and therefore has a tendency to break.
Can I prevent the failure easily or detect an impending failure?
Some speculate that changing oil more often or removing the seals on the bearing so it gets fresh oil can help prevent the failure. I wouldn't count on either saving an engine from the scrap heap as there's no way to gauge whether the bearing is already deteriorating or how long it will last.
There are no signs of impending failure. Upon failure, many report an obnoxious rattling noise as the bearing disintegrates; at this point, it's too late.
So what kits are out there?
Currently, the only IMS bearing replacement kit on the market in the US is made by LN engineering and consists of an upgraded bearing and new flange.
LN Engineering IMS Retrofits
The LN bearing is superior to the OEM one in that it uses ceramic balls, which are several times more durable (and expensive) than their metal counterparts. Clearly, balls of steel simply aren't good enough to last in a Porsche engine.
The kits cost $520 for the single row bearing version and $600 for the dual row bearing version. Which version is required depends on what bearing is currently installed in the car. The custom bearing extractor (which is an essential for removing a dual row bearing) is another $70. Their page gives some background info about IMS failures etc as well.
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That page is missing a few things. It's generally believed that IMS happens more frequently in low mileage vehicles that have been sitting a long period of time or are not driven on a regular basis.
Also Porsche changed the IMS bearing in the 987, essentially making it what the LN Engineering IMS Retrofit kit is. The switch over happened in late 2005 987 vehicles, there's more information out there with a search if you're interested.
Another thing to remember: the LN Engineering IMS Retrofit is not a cure. IMS can still fail. LN offers what they call a IMS Solution which should never fail
How does the IMS Solution work? | The IMS Solution
but it is expensive, currently runs $1,700
What’s included? | The IMS Solution
If you're really worried about it, get a IMS guardian, which will warn you before failure occurs. Runs about $300
IMS Guardian | IMS Guardian. The first advanced warning system for Intermediate Shaft (IMS) failures!