Euro BMW E36 M3 Evo Vanos Noise

A frequent complaint from our M3 customers is the infamous "vanos grumble" from the front end of the engine, particularly when the engine revs fall past around 1200rpm. Often misunderstood, & frequently leading to very expensive repairs, we thought we would share our insight into these units, how they work & how we fix them!

The vanos system is designed to improve the power & torque available at different engine loads across the rev range. It does this by advancing or retarding the camshaft timing on both the inlet & exhaust camshafts independently.

How It Works

This adjustment is achieved through high-pressure engine oil acting upon pistons within the vanos unit that in turn extend or retract the vanos gear shaft. The high pressure is derived from a pump within the vanos unit, driven from the exhaust camshaft, & boosts internal oil pressure within the vanos unit to around 1500 PSI.

The vanos gear shafts have a helical cut gear section that locates into a similarly helical cut gear in the camshaft sprocket, & a straight splined section that locates in the camshaft. As the gear shaft is forced forward, the straight cut spline slides longitudinally into the camshaft & has no effect on cam-timing. However, the helical-cut gear section cannot move forward [or backwards] without rotating the camshaft sprocket - this rotating action alters the relationship between the camshaft sprocket & the camshaft lobes & hence alters the camshaft timing.

The movement of the vanos gear shafts is monitored by signals from the camshaft sensor signals & relayed to the ECU where it is compared with the expected position from the ECU map, & then adjusted accordingly by switching the required solenoid until the required position is achieved. Switching the solenoids diverts oil pressure within the vanos body from one side of the vanos piston to extend it, to the other to retract it.

The Noise

For any gear mechanism to work, there has to be a small clearance or the gears would bind & seize. As the teeth on the vanos gear shaft & the internal cam sprocket wear over time, this clearance increases.

Now think about the valve spring forces acting on the camshaft lobes. As the camshaft rotates, the force applied to it from the valve spring will increase up to the point the maximum valve lift occurs (valve fully depressed). At this point all the wear/clearances in the various mechanisms will be loaded in one direction.

As the camshaft passes over maximum lift, the maximum valve spring force is then applied to the other side of the cam-lobe & any wear in the gears/sprockets will suddenly be loaded in the other direction (basically transferring load from one side of the gear tooth to the other). This transfer of load from one side to the other will near instantly take up any clearance between the gears. This sudden load transfer from maximum one side through the clearance to maximum the other causes a knocking sound & is the source of the characteristic vanos grumble. This is then magnified by each valve (24 in this engine) & is exacerbated by the fact that the camshafts on the euro E36 M3 are hollow-drilled, & so act like drums magnifying any noise.

The Cure

Until recently the standard approach was to renew the vanos unit. This was very expensive & depending on the degree of wear, in many cases proved to be a relatively short-lived solution. The reason for this is similar to fitting a new bicycle chain to worn cogs - the new chain wears rapidly to the profile of the old cogs, & vice-versa, so fitting new vanos gear shafts to worn camshaft sprockets has the effect of wearing the new gears to the profile of the old sprocket teeth which in turn leads to excessive clearance & you are back at square-one! The only real way to affect a lasting solution is to renew the vanos unit & both camshaft sprockets, which further increases the cost to beyond what many of these cars are now worth!

Iridium Engineering has tackled this issue two-fold.

First, we have developed a kit to replace the standard nitrile (NBR) hydraulic solenoid seals with a much more resilient Viton version, which helps maintain the internal oil pressure to activate the vanos pistons. The standard seals are not particularly well suited to the temperatures & chemical attack from oil engine oil & tend to harden & lose their sealing effectiveness over time. Our kit also includes uprated high-tensile bolts to replace the standard solenoid cover bolts which have been known to fail.

Second, we have found a workshop solution that involves refitting the original vanos unit in a slightly different gear-to-gear orientation that, in the vast majority of cases, eliminates or significantly reduces the noise. As you are reusing your existing vanos unit that has already had the gear teeth work-hardened through use, this solution appears to be longer lasting than fitting an expensive new unit.

Special tools & diagnostics equipment will be needed to ensure correct cam-timing & operation is achieved, but many good BMW independent garages should be able to undertake this work in around 6 hours.


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Posted in: BMW