Since 1992, BMW has used a variable valve timing unit, called VANOS (an abbreviation for the German, variable Nockenwellensteuerung), which advances or retards the timing based on readings from the ECU in order to optimise performance. Initially, a single VANOS setup adjusted only the intake timing, but BMW introduced a double VANOS setup in 1996 to adjust both intake and exhaust timing. The unit sits at the front of the cylinder head, and the readings from the ECU send a signal to the VANOS solenoid, which then adjusts oil pressure to operate two pistons inside the unit. The pistons in turn adjust the timing by advancing or retarding the camshafts.
Here is a short demonstration of how VANOS works.
The seals that BMW used on the pistons inside the VANOS unit are known to break down over time. As they wear out, oil pressure to the unit drops, and the timing is negatively affected. This results in a few symptoms, particularly a loss of power at lower RPM, a stumble at idle when the engine is cold, and decreased fuel economy. At the extreme end of wear, the pistons will rattle audibly inside the unit.
In the last year, I have started to notice some of these symptoms appearing on my E39 528i (M52TU engine), especially a stumble at cold idle. The seals are known to start wearing out by about 30,000 miles, and with mine currently reading 74,000 miles, this was not unexpected. Repair kits are inexpensive – the seals can be purchased from companies like i6 Automotive for £20, and a new rocker gasket is about the same – and it is a job that can easily be done in a day. There are a number of excellent tutorials online, such as this one from Bavarian Autosport (the tutorial applies to the M52TU, M54, and M56 engines). I won’t bother with a how-to here, as the BavAuto tutorial is more than sufficient. I’ll just add a few notes and let the photos below speak for themselves.
It still surprises me how much room there is to work in the E39’s engine bay once you remove the fan and shroud. That is in some ways the biggest part of this job. Remove these, and you’re only looking at a couple of hours to do the VANOS seals. All you need is basic hand tools.
Getting the rocker cover off requires some caution, as the rubber becomes very brittle with age. In my case, the outside came off without an issue, but the inner gasket sitting over the spark plug wells broke in a few places, and in one instance a piece of the gasket fell into the middle of the cylinder head. I was able to fish it out easily, but you’ll need to be careful here. You can see that my gasket was more than due for replacement, as it was leaking in various places. Once the gasket is off, removing the VANOS unit itself is just a few nuts and bolts.
Replacing the VANOS seals themselves is easy. A few bolts come off and the pistons are exposed. Simply pull them off and replace the seals. There are two things to be careful of here: first, when cutting the old seals, take your time so as not to mar the surface of the piston; second, when replacing the rubber O-ring that sits underneath the teflon seal, ensure it is not twisted. Be patient – you want these to seat properly.
One of the things I noticed after dissembling things was some sludge that had built up under the rocker cover. Before I acquired the car, the previous owner had not driven it a great deal, so that was not altogether surprising. It is easy enough to clean up, and worth doing when you have the cover off – on the inside of the rocker cover and outside of the VANOS unit, just attack it with some degreaser.
Everything went back together as easily as it came off. That said, given the fact that you are dealing with engine internals and timing, this is probably not a job for the inexperienced. Actually messing up the timing here would take a pretty serious error, but nonetheless, anytime you have camshafts and timing gear exposed, you want to exercise caution. But if you have some experience with a spanner, grab your tools and BavAuto’s excellent tutorial, and go for it.
If you read the forums, there is some debate about whether the VANOS seals need a break-in period. Some people advocate 200 miles of city driving, while others argue that there is no reason for the seals to be broken in if you’ve assembled everything properly, and you can just drive as normal. It seems to me there is merit to both sides of the argument, although I think the argument for a break-in period really is less about breaking in the seals than it is about allowing the ECU to re-adjust to a properly functioning VANOS unit.
That all said, I have now driven a couple of hundred miles following the repair and do notice the car driving a little better. In particular, the idle when the engine is cold has smoothed right out. For £40 and a day in the driveway, I’ll take that.