Vanos Rebuild Start Hesitation

[Shonky]

This is another thing I can't find any info on. Diagrams just show that the piston is moved via oil pressure difference either side of the piston, I don't know if there is anything physically mechanical that returns these pistons to their default on shutdown which is why I sort of assumed accumulator pressure was used here when the engine isn't running. Like, what would happen if I unplugged the solenoid pack and ran without VANOS, other than be unhappy that it can't move the cams.

It's almost like the oil pressure that is there isn't always enough to get the pistons in the right place, maybe because the accumulator doesn't hold pressure anymore, or the new seals are stiff and the (potentially non-existent) pressure the accumulator has can't move them quick enough / far enough when the engine is turned off.

Thinking about it

To rotate the cam relative to the sprocket, the oil pressure pushes in and out i.e. along the axis of the camshaft. The helical gears then cause the cams to rotate relative to the sprocket.

However as the cams are turning the helical gears won't really work in reverse i.e if there was nothing holding the pistons in place. Presumably after a lot of rotations they eventually would move/return to a default position but definitely not in the short term or whilst cranking I would say. They need something to actively move the helical gear forwards/backwards to have any reasonable response time.

So this all comes back to the "set timing at shutdown" which is perfectly feasible even after the engine has stopped rotating the cam - just pushing the piston in/out as desired with the pressure in the system will rotate the cam relative to to sprocket. Precise control isn't even required since you just want the cams at one extreme of their adjustment or the other. i.e. just open one valve per cam to force the piston all the way in one direciton.

[jamesfoley]

Thinking about it

To rotate the cam relative to the sprocket, the oil pressure pushes in and out i.e. along the axis of the camshaft. The helical gears then cause the cams to rotate relative to the sprocket.

However as the cams are turning the helical gears won't really work in reverse i.e if there was nothing holding the pistons in place. Presumably after a lot of rotations they eventually would move/return to a default position but definitely not in the short term or whilst cranking I would say. They need something to actively move the helical gear forwards/backwards to have any reasonable response time.

So this all comes back to the "set timing at shutdown" which is perfectly feasible even after the engine has stopped rotating the cam - just pushing the piston in/out as desired with the pressure in the system will rotate the cam relative to to sprocket. Precise control isn't even required since you just want the cams at one extreme of their adjustment or the other. i.e. just open one valve per cam to force the piston all the way in one direciton.

The S54B32 has the exhaust piston extended (fully advanced), and the intake piston retracted (fully retarded), so some solenoid action would presumably be needed to achieve this and it would need that oil pressure.

If there are any recommendations of a decent tool that can log VANOS commanded and actual positions without me farting about with a laptop every time that'd be awesome 😅

[sapote]

“Pulled from the reference manual it says:

The “default” mechanical stop position without VANOS influence is:
Intake Camshaft = Retarded (130º spread angle)
Exhaust Camshaft = Advance (83º spread angle)
The volume of pressurized oil is stored in the accumulator supplying both adjustment pistons. Both pistons are held in the default position by the high pressure oil.“

Could you post this Reference Manual?

[jamesfoley]

“Pulled from the reference manual it says:

The “default” mechanical stop position without VANOS influence is:
Intake Camshaft = Retarded (130º spread angle)
Exhaust Camshaft = Advance (83º spread angle)
The volume of pressurized oil is stored in the accumulator supplying both adjustment pistons. Both pistons are held in the default position by the high pressure oil.“

Could you post this Reference Manual?

Sure, this is what I've been using: https://bmwtechinfo.bmwgroup.com/tec...%20Vehicle.pdf

[Slideways]

OP, did you ever remove the VANOS unit to verify the timing yourself? If the intake splined shaft was put in the wrong position during assembly, then the timing is off. The DME can only adapt to a certain amount of error. Did your shop completely remove the VANOS and reset the timing on the second go around? Cause if they just took the valve cover off and checked timing with the timing bridge, that is only half the procedure.

[jamesfoley]

OP, did you ever remove the VANOS unit to verify the timing yourself? If the intake splined shaft was put in the wrong position during assembly, then the timing is off. The DME can only adapt to a certain amount of error. Did your shop completely remove the VANOS and reset the timing on the second go around? Cause if they just took the valve cover off and checked timing with the timing bridge, that is only half the procedure.

VANOS has been retimed 5 times I think since it was rebuilt. The first two times by the original shop who did the rebuild who found no issues. Third time was by another shop who found it to be a tooth out. Fourth time was again by another shop who again found it to be timed wrong, and then the fifth time was the same shop who did the head gasket. The fourth time I watched them remove it and confirm the timing was wrong, but I've not personally touched any of this as I'm no mechanic. It was at this point we found it leaking through cylinder 5 intake valves so did a head refresh and head gasket, where the VANOS would have been off again.

I've no reason to believe its not timed correctly, no codes, and no drivability issues as far as I'm aware. DIS test passed fine the last time I ran it but I may run it again just to get some fresh numbers.

[Slideways]

VANOS has been retimed 5 times I think since it was rebuilt. The first two times by the original shop who did the rebuild who found no issues. Third time was by another shop who found it to be a tooth out. Fourth time was again by another shop who again found it to be timed wrong, and then the fifth time was the same shop who did the head gasket. The fourth time I watched them remove it and confirm the timing was wrong, but I've not personally touched any of this as I'm no mechanic. It was at this point we found it leaking through cylinder 5 intake valves so did a head refresh and head gasket, where the VANOS would have been off again.

I've no reason to believe its not timed correctly, no codes, and no drivability issues as far as I'm aware. DIS test passed fine the last time I ran it but I may run it again just to get some fresh numbers.

That's right, forgot about the cylinder removal. It's been a long thread and I only went back to the first post.

After the head gasket replacement, did they do a compression and leak down test?

[jamesfoley]

That's right, forgot about the cylinder removal. It's been a long thread and I only went back to the first post.

After the head gasket replacement, did they do a compression and leak down test?

Leak down was done before the head was rebuilt, its the reason we did the head, but I have no idea if a leak down or compression test was done after.

Before the head was done the car would sometimes not start at all, or stall itself, along with the hesitation. After it was done it just left me with the weird hesitation.

[Slideways]

Leak down was done before the head was rebuilt, its the reason we did the head, but I have no idea if a leak down or compression test was done after.

Before the head was done the car would sometimes not start at all, or stall itself, along with the hesitation. After it was done it just left me with the weird hesitation.

Yeah IDK, I would think a startup hesitation would be down to fuel issue, timing issue or compression issue. Last thing (unlikely) is something is acting up with the DME. You could swap in another DME to rule that out.

[sapote]

The fourth time I watched them remove it and confirm the timing was wrong.

How much was the error? I would think a tooth wrong should cause an error code.

[jamesfoley]

How much was the error? I would think a tooth wrong should cause an error code.

I'm pretty sure it was out a tooth again, but honestly not sure. I remember them telling me that the VANOS shafts (ones with the splines) should pull from the cam shafts and sprockets without binding, which mine were. Again, not a mechanic, just a guy that wants his car to work.

But yea, no code then, and no code now. DME appears to think everything is working fine.

[Tomba]

S54 is timed as
intake retard
exhaust retard.

In idle engine runs as
intake retard
exhaust advanced
these settings cause less valve overlap which reduce rest gases and improve idle performance.


Once you timed the engine and run it idle and check timing again after engine shutdown you will notice the exhaust is out of timing and advanced (moved towards rotation direction). You will need to activate VANOS solenoids and move camshaft with a wrench to move it in default position. For sure it is switched off in advanced position.
I personally use the timing tool for reference only. After timing I check the actual values by cranking the engine and activating the solenoids with a separate device and monitor actual values with a diagnostic tool before completing the engine.

It takes a long time before your camshafts start to show values which you present in your graphs, you can actually see the desired value moving once actual position starts to show a non substitute value. For me this is a sign your sensors are not reporting a correct value. Maybe it is short enough to prevent an error code to be saved. I would still try to swap/use OEM replacement exhaust and intake camshaft sensors and run a VANOS test in Tool32 like in attachment. For starting purposes I would expect the exhaust sensor to be more demanding as this camshaft trigger wheel has a missing slot to detect position and is necessary for the ECU to know which cylinder to fire and inject. No position = no injection/ignition = no / long crank.

[jamesfoley]

.
It takes a long time before your camshafts start to show values which you present in your graphs, you can actually see the desired value moving once actual position starts to show a non substitute value. For me this is a sign your sensors are not reporting a correct value. Maybe it is short enough to prevent an error code to be saved. I would still try to swap/use OEM replacement exhaust and intake camshaft sensors and run a VANOS test in Tool32 like in attachment. For starting purposes I would expect the exhaust sensor to be more demanding as this camshaft trigger wheel has a missing slot to detect position and is necessary for the ECU to know which cylinder to fire and inject. No position = no injection/ignition = no / long crank.

Assuming you’re referencing the TestO graphs from way back, I think these are from before my valve replacement and head work, so might be worth double checking this is still happening now.

I think I only logged intake at the time because when I tried exhaust it’s just crash. Perhaps it’s worth me trying to get both inlet and exhaust actual readings on a hesitation.

Sort of assumed that if it was the cam sensors then the hesitation would be a little more frequent, or I’d have actual running issues.

What do you mean by it taking a long time though? In the graph the point that the expected value jumps and the wobble on the actual value starts is when the engine starts cranking.

[sapote]

Pulled from the reference manual it says:
----------------------------------------------------------------------------------------------------------------------
The “default” mechanical stop position without VANOS influence is:
Intake Camshaft = Retarded (130º spread angle)
Exhaust Camshaft = Advance (83º spread angle)

The volume of pressurized oil is stored in the accumulator supplying both adjustment pistons. Both pistons are held in the default position by the high pressure oil.
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------



1. S54B32 intake cam spread is 70º to 130º, so 130º would be fully retarded, and exhaust cam spread is 83º to 128º, so 83º would be fully advanced.


2. So I'm assuming this is the default position it goes back to, and the accumulator holds the pistons there. But I'm not sure the accumulator was designed to hold pressure indefinitely, videos of pressure tests show it does slowly bleed off. S62 actually has a shut off valve on the accumulator to hold pressure for the next start, but looks like that might just be to remove rattles.

Just interested to know if the VANOS is supposed to assume the default position when the engine is switched off, or if it is supposed to quickly move to that position when switched on using store accumulator pressure.

The Reference Training Manual is wrong because:
For intake, 130* is max retarded so the manual is correct on this, but
For ex, 83* should also be max retarded, not advanced as the manual says. EX valves are opened before TDC compression, and so 128* is more advanced than 83* (the same for ignition before TDC compression, that 30* is more advanced than 5*).
So the manual should say EX camshaft = Retarded (83* spread angle) instead.

So both cams should be at their max retarded by the default stop without any vanos oil pressure applied on the pistons.

Here is what happens - I think - during engine first starts up: Both sides of the piston have the same oil pressure, but the front side area is larger and so both pistons are pushed rearward to their max advanced position.

"VANOS adjustment is carried out by the ECM pulsing the inlet and outlet solenoids to allow pressurized oil to the back side of the adjustment pistons. The surface area on this side of the piston is larger so that the oil pressure is greater and the adjustment piston will move causing the valve timing to change."
They call the larger side of the piston as the back side, but it should be the front side of the piston according to the convention.

[jamesfoley]

They call the larger side of the piston as the back side, but it should be the front side of the piston according to the convention.

The front side is the smaller chamber which has a constant pressure applied. The back side which is the larger chamber is where pressure is fed or bled by the solenoids. The larger surface area on the back side allows the same amount of pressure to overcome the pressure in the smaller chamber, moving the piston forward toward the engine. When pressure is bled from the large chamber the pressure in the small chamber pushes the piston backward away from the engine. Large chamber would be the side with the cap on the front of the VANOS body.

At least that’s how the diagrams read for both S50 and S54 and how I understood it.

I don’t really know how it works without VANOS influence as it appears the small chamber always has high pressure applied regardless, and oil in the large chamber should be sealed by the solenoids, so disabling VANOS or unplugging the solenoid pack should mean the pistons are just locked at whatever their last position was.

Also not sure what happens to piston position when pressure eventually bleeds off on the small chamber side as the engine is shut down.