Originally Posted by boostmiser
I just started getting a noise: It only happens on the first turnover in the morning. It lasts about two seconds. If I shut off then restart, it doesn't happen. It also doesn't happen any other times in the day. Just the first start of the morning. Vanos?
If a 2002 M5 rattles for a few seconds only when started cold, I would definitely suspect the Vanos. If you only get Vanos noise at cold startup, you should be happy.
Function of the Vanos ***************
The Vanos units each handle two camshafts, one intake and one exhaust.
Each Vanos has an oil pump producing 100 bar hydraulic pressure.
Each Vanos has four magnetic valves controlling the flow of pressurized oil into and out of cylinders moving piston back and forth. One piston for each camshaft. The pistons move a shaft which has one part with staright splines, and one part with helical (twisted) splines back and forth inside a sprocketwheel and inside the camshaft respectively. The shaft connects the rotation of the sprocket-wheel with the rotation of the camshaft. The sprocket wheels, one for each camshaft, are driven by the cam chains, and are thus driven from the crankshaft.
Vanos High-Pressure oil circuit ******************
The Vanos pumps are fed the engine oil, and the high-pressure sides are tied together with a reinforced hoose. When the engine is switched off, the magnetic valves are closed, and the 100 bar oil pressure is still kept until the next start.
In theory... in reality the oil leaks by the magnetic valves and the pressure drops in hours or days.
In cars with the oil pressure accumulator installed (from fatcory since December 2000), the pressure accumulator contains compressed gas, a piston separating the gas from the oil, and oil under pressure and a shutoff valve. This valve is open when the engine runs, and keeps a reserve of pressurized oil to release into the Vanos high-pressure oil circuit at startup, thus preventing startup Vanos rattle.
As the Vanos units keep the oil pressure up for a long time, do not ever under any circumstances do anything to the Vanos units in your car yourself.
100 bar oil pressure is insanely much. It can easily cut straigh through your arm. 100 bar is the same pressure as what you have at a depth of 1000 m (3000 ft) of water.
If it rattles at startup anyway ? ***************
1. No oil pressure accumulator installed.
2. No electrical signal to the shutoff valve in the accumulator.
3. Shutoff valve leaks or does not close at all.
If there is no noise when running, guard the actual Vanos units, and be happy. Fixing the oil pressure accumulator is not a problem.