Replacing M40 Camshaft

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It seems the M40 is prone to camshaft failure, either because of inadequate oil supply or possibly surface hardening of the camshaft or valve train components.

The symptoms of a worn cam is that the engines wont rev over 5 to 5.5k and misfires the first second or so after a gearshift during wide open throttle acceleration. It can also be intermittent, the engine can occasionally rev normally to redline but most often not. Wear can usually be spotted by inspecting the camshaft, but to inspect the rocker arms and pressure discs the camshaft has to be removed from the engine.

I finally got the job done and replaced the cam, cam followers and tappets an my 318i. The cause for the cam to wear out is possibly a blocked oil spray tube as there was a considerate amount of black sludge under the cover, suggesting that oil service according to the service indicator is not frequent enough. To prevent this happening again I will use a slightly cheaper oil and change it at 5000km in the future. The check valve in the top deck was working OK so I just cleaned it and put it back in.

I also changed the valve stem seals and lapped in the valves while I was in there as well. The cylinder bores was within specs with obvious traces of the original cross hatch honing, but piston to cylinder wall clearance was a bit on the high side I think, close to 0,25mm measured at top of bore. Its not mandatory to pull the head but its a good opportunity to decarbonise, grind valves and replace valve stem seals .

1. The pressure discs between the valve stem and cam followers don't come in the cam kits. My dealer said that they seldom wear out, of course I stupidly believed him. This caused my car to be off the road for another week as the dealers don't have anything in stock any more.

The oil spray tube also had to be ordered separately. The Haynes manual doesn't say anything about mandatory cambelt tensioner or idler pulleys replacement. Check them carefully and replace if loose or worn. Mine had some slop in the bearings and they were not particularly expensive.

2. The camshaft template and flywheel locking tools would have to be made up. The locking tool is real easy, I paid equal to 8 USD to have the tool made. The camshaft template though is a lot of work if using a mill. I ended up with cutting 3 strips of 6mm aluminium sheets and then bolting it together with four 6mm bolts using a square and a caliper to get the angles and dimensions right. Worked perfectly.


Fig.1 shows my home made tools and fig.2 the tool dimensions


3. The improvised tensioning method described in the Haynes manual seems to work okay. I used a really cheap (3 USD) 0-4kg spring scale which I calibrated with 2 litres of water.

Make sure to get a scale that is possible to calibrate as they can be way off from factory, and obviously a 10kg range is no good. I cut the groove in the Insex key 85mm from the center of the insex.

4. The gaskets on the Motronic connectors are real easy to drop so be careful to recover them when you unplug the connectors for the injectors, ICV, sensors and so on.

5. The green rubber ring between the head and the thermostat housing/front cover is easy to forget. I almost did.

6. I'm a bit sceptical about using synthetic in the run-in period of new engine parts, take it for what it is just my opinion. I use plain mineral oil without any extra additives. STP has a special run in additive but I really don't think its necessary.

Everything else is pretty straight forward, supplied with this cam kit was a red jelly like oil instead of the usual MoS grease. I rubbed half of it into the parts and poured the remaining half over just prior to initial startup after having cranked up the oil to the head by hand.

Be aware that the Motronic is really rotten after a week without the battery hooked up, the new camshaft should not be allowed below 2500rpm for the first 20 minutes, and have an assistant to spot for oil and water leaks while you are running in the cam (or follow the manufacturers instructions if supplied).

Original article by Grunde Gjertsen, posted on