In this part of the article – nuances of the performance of the VANOS hub. Very consecutive, just for the essential things to be clear to analyze this performance problem.
The VANOS hub consists of:
a. VANOS mechanism, which is returned/held in the reference position by the spring but turned from it (reference position) – by oil pressure. A pin is also possible as an additional option to fix the reference position – it is released when/if the pressure appears in the hub;
b. electromagnetic valve, which changes the amount of oil flowing via it;
c. DME, which manages the electromagnetic valve with the help of PWM;
d. sensor, which determines the position of the camshaft (against the crankshaft).
The operating principle is simple: the larger the average current is supplied to the solenoid, the more it opens. The more open the valve is, the more oil flows thru it. The more oil flows thru it, the larger the pressure in the VANOS mechanism. The larger the pressure, the more the camshaft is turned. This system works in closed-loop mode (read here, what it is).
Directly after the start of the engine:
a. DME keeps the valve closed (no current supplied to it; low PWM value, indicated by INPA/ISTA, is intended for control of the electrical connection);
b. for the first 20 cycles, the shaft is held in the initial/reference position and identifies the position of the shaft (against the crankshaft).
If the identified position of the shaft is correct (typically allowed range is +/-10 .. +/-13 degrees from ideal), DME enables the VANOS hub. If the reference can not be detected (for example, due to a damaged sensor) or is incorrect, the error message is recorded in the DME error message memory and the VANOS hub stays deactivated (the valve is not managed).
Note: so, the reason for the error message regarding incorrect VANOS position could be not only problems with the chain (it has stretched or “jumped”); improper repair (replacement of the chain or “repair or the VANOS mechanism), but also a leaking valve!
In the mode, when DME tries to keep the camshafts in the reference position (for example, idle; outlet valve – if the CO catalytic converter is NOT warmed up), the PWM of the valve can be:
a. till 10% means the valve is completely closed; the < 10% value indicates that the valve is electronically checked, nothing more. 10% PWM is not enough to open the valve even for a bit. Such a situation is observed in the case when/if the detected reference position has reached/is maintained (even) by a completely closed valve;
b. around 30 .. 50%, if it is necessary to turn the camshaft for even a little bit to reach the reference position detected at the moment of the engine start;
c. fluctuates (“shivers”) between < 10% and 30 .. 50% – in case the detected reference position is very close to the ideal/maintainable position.
Why are several possible scenarios formed and even “shivering” of the PWM? The reason is simple – DME detects (and tries to maintain) the positions of the camshafts with high precision: degree tenth. It goes without saying that when the engine warms up, the parameters of the sensors and mechanical parts change. The actual position of the camshafts also is slightly changing. In addition, the actual working engine vibrates; the timing belt (chain) also vibrates, accordingly – the position of the camshafts is slightly jittering. For these reasons, DME wants periodically slightly correct the position of any camshaft (for some tenth of a degree or full degree). As this correction is close to 0 and depends on many (and very often – individual to the exact engine) parameters, all three scenarios are possible: position of the shaft is ideal; the position of the shaft has to be changed slightly; or: the position jitters around the excellent/ideal value. All three scenarios are CORRECT, BUT by the condition – the real position of the shaft is accurate enough with the required.
So, even if you see such an image of PWM (light green color; the engine runs at idle):
Don’t hurry with the conclusion that the VANOS hub has some kind of problems (because the PWM is as if significantly changing, but the position of the shaft and the mode of the engine performance remains stable). This is a typical situation because:
a. in the moments when PWM drops to < 10%, the valve is completely closed (the camshaft is in a perfect/requested position);
b. in the moments when PWM is increased to 30 .. 50%, DME has given the command to slightly open the valve to adjust the camshaft for some tenths of the degree. In the mode, when PWM is smaller than 30+%, the VANOS mechanism is in the “rest” position. But why does such a “dead” zone appears?
Here is the example of the valve-parameter schedule from Solenoid Solution:
As we see, till the mark of 200 mA (by pressure 100 PSI), the valve is completely closed; only reaching over this border of 200 mA the valve starts to open. Each valve has a “dead” zone, and its “width” depends on the valve’s physical’s parameters.
In the case of the VANOS hub, a significant mechanical power, which should be overcome for the mechanism to turn – the power of the spring, is built into the mechanism itself.
In addition, in the case of VANOS, the amount of oil leaked from the mechanism itself should be kept into the account. The larger the leak, the larger the “additional dose” of oil will be necessary to compensate for it.
As we see from the graphic in the 1st image, in the current case, the PWM of around 30% corresponds to the turning threshold (the moment when the pressure of oil got in the mechanism is larger than the counterforce of the spring) of the mechanism. What does this threshold value say to us? Value around 30% means that the oil leak of the VANOS mechanism itself is small – the valve still has a wide range of adjustment (in reserve/linear areas). Observations of VANOS data confirm that the system can perform correctly if the PWM of the valve reaches 60+%.
Conclusions, short summary:
a. in the modes in which the desired position corresponds to the reference position, the valve can be both closed (its PWM is around/below 10%) or slightly opened (its PWM about 30 .. 50%), or “jumps” between these values, and all 3 options are correct, by the condition – the desired and actual position of the shaft is the same;
b. in the previously mentioned situation, such PWM “jumping” happens due to the nonlinear management curve of the valve and the whole VANOS hub;
c. in any other situation (when/if DME tries to maintain shaft position, different from reference and/or shafts, maintained in the reference positions, are fluctuating) such shivering means serious problems! Obviously, DME periodically tries to completely close the valve – its “linear” management is unsuccessful!
d. if this PWM threshold, by which the mechanisms start to open, is increased (is above 55 .. 60%), obviously – either the valve is significantly clogged; or the oil pressure is significantly reduced; or – the VANOS mechanism itself has an increased oil leak and the possible culprit: sealing rings of the hub.
Note: the last reason in the point d (increased PWM even after installation of correctly working valve) is the reason for the mechanical repair of the VANOS mechanism. But before, you have to make sure that the oil pressure in the system is correct (corresponds to the required).