Vanos Rebuild Start Hesitation

[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]

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]

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]

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]

“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

[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]

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 😅

[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]

It can't assume stored pressure at crank time. That could be hours/days/weeks/months. The accumulator is only for short term "pressure storage".

It sets it at engine shutdown as I understand it, but I can't find any specifc mention of that in the BMW training documents.

I agree, that makes sense, but I equally can't find any reference to how this is meant to work.

Without any high pressure do the cams naturally set themselves from cranking i.e. to the "default mechanical stop"

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.

[Shonky]

It can't assume stored pressure at crank time. That could be hours/days/weeks/months. The accumulator is only for short term "pressure storage".

It sets it at engine shutdown as I understand it, but I can't find any specifc mention of that in the BMW training documents.

Without any high pressure do the cams naturally set themselves from cranking i.e. to the "default mechanical stop"

[jamesfoley]

During cranking, slow rpm, so I think it's best to advance both IN and EX cams to maximized chamber fill as no reason to retard the IN valve closing at low rpm. You gain more power at high rpm with more retard IN valves because the intake gas still flowing in at high speed. But I don't know if the vanos system can move the IN cam from max retard to max advance during engine shut down.

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.

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.

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.

I honestly think there is something wrong with the VANOS somewhere, but its not hugely complicated, high pressure oil goes in and pistons move back and fourth. With it working and passing tests I don't really know what the issue could be other than it potentially not holding the position it expects when being started.

Even though its had new piston seals, could these be damaged causing a pressure loss? Should bad seals present themselves as a problem consistently, rather than the intermittent issue I have? Could the oil pump disc be bleeding pressure, or not building enough pressure? Is the accumulator not holding any pressure so it can't make adjustments and engine start / stop?

Determined to find the cause of this but do feel I'm clutching at straws a little now.

[sapote]

Anyone able to shed some light on what happens to the VANOS when the car is started and the car is turned off?

From what I've read it should be in the "default" position for next start, which is intake fully retarded and exhaust fully advanced. Does this happen when the engine is switched off? If so, where does the oil pressure come from to do this (oil pump / accumulator)?

Looking back at my post where I log VANOS intake actual vs DME commanded on a rough start you can see that it doesn't start in the same spot the DME is commanding, which makes me think its not always reaching the default position when switched off (https://nam3forum.com/forums/forum/m...060#post208060)

Is it at all possible I'm losing oil pressure within the VANOS that is either causing the exhaust or intake pistons to shift, or for them to not reach the default positions when turned off?

During cranking, slow rpm, so I think it's best to advance both IN and EX cams to maximized chamber fill as no reason to retard the IN valve closing at low rpm. You gain more power at high rpm with more retard IN valves because the intake gas still flowing in at high speed. But I don't know if the vanos system can move the IN cam from max retard to max advance during engine shut down.

[jamesfoley]

Anyone able to shed some light on what happens to the VANOS when the car is started and the car is turned off?

From what I've read it should be in the "default" position for next start, which is intake fully retarded and exhaust fully advanced. Does this happen when the engine is switched off? If so, where does the oil pressure come from to do this (oil pump / accumulator)?

Looking back at my post where I log VANOS intake actual vs DME commanded on a rough start you can see that it doesn't start in the same spot the DME is commanding, which makes me think its not always reaching the default position when switched off (https://nam3forum.com/forums/forum/m...060#post208060)

Is it at all possible I'm losing oil pressure within the VANOS that is either causing the exhaust or intake pistons to shift, or for them to not reach the default positions when turned off?

[sapote]

The VANOS just not functioning correctly after being rebuilt initially still plays on my mind, but I have no idea what I can realistically test other than running the DIS VANOS test. Honestly just feels like cam position is wrong sometimes until enough oil pressure is made to move them, kind of as if the accumulator isn't holding pressure, or the VANOS isn't returning to the correct position every time when the engine is turned off.

for vanos, all you can do is to check the timing and do the DIS test.

[jamesfoley]

They're not that bad once you get the hang of them. Medium sized needle nose pliers (long enough to reach the clamps, but not so big that they can't squeeze through things to reach) are your friend.

I use one of these



Wasn't really the getting them unclipped that was the problem, its that they stick to the rubber coupler and require some persuasion to free up, at which point they get kind of mangled and need to be replaced. Do wish they were just thinner screw style hose clamps.

No, it goes battery -> BST -> fuse box -> terminal by the hood switch -> starter -> alternator

Looks like it splits into two from the positive terminal at the battery. Smaller cable feeds fuse box and is bundled in a larger loom, chunky cable is protected by the pyro fuse and goes through the car and through the firewall to the jump post under the bonnet. Being a UK car my fuse box is on the other side, so I guess the loom for the fuse box just goes through the dash.

All terminals look fine up to the jump post in the bonnet. Will try and find some time to pull the manifold off and check under there.

What is frustrating is I jumped in the car this morning after it sat overnight and it starts up perfectly fine https://www.youtube.com/watch?v=kWFITtBoRDw

I feel like it'd actually be easier to solve if it just hesitated every time. I need to try and catch the hesitation on video.

The VANOS just not functioning correctly after being rebuilt initially still plays on my mind, but I have no idea what I can realistically test other than running the DIS VANOS test. Honestly just feels like cam position is wrong sometimes until enough oil pressure is made to move them, kind of as if the accumulator isn't holding pressure, or the VANOS isn't returning to the correct position every time when the engine is turned off.