Cold start rattle 2-3 seconds then gone.

[Andratch]

Another Update -

Got the car back today with a new vanos accumulator, and unfortunately y'all were right - no dice. The shop acknowledges that it's an annoying sound, but that it doesn't appear to be anything that is a "grenade waiting to pop." I asked them if it could be rod bearing trouble since mine were just replaced prior to the sound starting, and they agreed that rod bearing nose typically does not go away after 1 second, and is usually a slower tick (half speed of the top end).

At the end of the day, I do think that this is VANOS related somehow. But I've thrown about $2k at this problem so far, and need to put it on hold for a little bit. The car will be stored for the next couple months, so I'll do somethinking and diagnosing while it its sleeping.

For those with this sound, it seems the next item on the chopping block is rebuilding the vanos unit itself, which I'm told is less than $1k...anyone want to take it on and report back "for the greater good?"

[Andratch]

The aluminum plate underneath, it has more than a dozen bolts attaching it, I would check if all bolts are there. The attachment points are metal, if one is missing the plate could be rattling against the mounting point behind it.

Oh yeah. Unfortunately that’s not it for mine. We had it off while we were doing diagnosis.

[6speedS54]

The aluminum plate underneath, it has more than a dozen bolts attaching it, I would check if all bolts are there. The attachment points are metal, if one is missing the plate could be rattling against the mounting point behind it.

[Andratch]

Do you have all the bolts in the reinforcement plate? I was missing one and had a rattle when starting.

What’s the reinforcement plate?

[6speedS54]

Do you have all the bolts in the reinforcement plate? I was missing one and had a rattle when starting.

[maupineda]

The accumulator wont make a difference, its nitrogen pressure provides a pressure reserve that pushes oil from the accumulator to the VANOS unit, this is to maintain a constant 115bar internal pressure at all times in the VANOS unit, it would be most useful during multiple on and off throttle applications where the pressure provided by the pump would drop as it actuates the piston, the accumulator would provide back up pressure to ensure there is oil at pressure for the next timing adjustment.

You can test your accumulator by hooking a pressure gauge to the VANOS, run the engine, and shut it off. you should see pressure drops from 115bar gradually, as the pressure from the accumulator is sent to the VANOS unit, but after a minute or two it will drop completely. if you do not have a working accumulator, the pressure drops rapidly or instantly. I tested this in my own car as the pressure in the system dropped instantly, that drove me to change the accumulator to have the system in spec, but it never made a noticeable difference in any manner other than pressure drop speed after engine shut off. After changing it, pressure takes time to fully drop to zero.

[Andratch]

Hi Andratch, Unfortunately, when I changed my very old vanos accumulator and it made no real difference to the operation of my vanos. The guy who repressurises them told me my old accumulator was only half full.

The accumulator is actually there to make sure there is always enough pressure for vanos operation rather than to provide a boost.

I don't wish to sound negative but i thought you would appreciate another owner's experience.

Does your vanos setup pass the DIS Vanos test?

All the best with a resolution to your issue.

No worries, we're all in the same crappy boat fixing this problem - it's just getting to be an expensive endeavor! I see mixed reviews on the accumulator - I saw a youtube short (that I can't find again unfortunately) where a guy with a e39 M5 had a similar vanos sound, and changing the accumulator fixed it. Then other posts where it seems like it's almost the vanos version of an appendix....just an added expense uselessly sitting in a corner. We'll see how it turns out, but I'm hopeful.

I don't know if it passed a DIS test - I'll check with the shop. They're a reputable place, I've known the owner for 15 years - they did at least recognize that it wasn't timing chain tensioner

[ac427]

ANOTHER UPDATE:

I took the car to a different shop yesterday who I felt had much more skill around advanced diagnostics. They took a look at the vanos and did some pressure tests - they strongly suspect that the problem could be the Vanos Accumulator. It's nitrogen gas-charged, and over time the nitrogen can leak out (even without the diaphragm tearing). The purpose of the accumulator is to give a boost of oil pressure to the Vanos when the oil pump isn't providing enough. If mine has lost its gas charge, the vanos oil pressure drops down while the car is off and it doesn't have proper oil pressure for the first second of cranking and startup.

There's not a good way to test the function of the accumulator, and while it is technically serviceable, you have to mail it out to a specialist and that takes time. So we're going to replace that and see if that addresses the concern. The part should arrive tomorrow, they're going to change it out and crank it up on monday and see what happens!

Hi Andratch, Unfortunately, when I changed my very old vanos accumulator and it made no real difference to the operation of my vanos. The guy who repressurises them told me my old accumulator was only half full.

The accumulator is actually there to make sure there is always enough pressure for vanos operation rather than to provide a boost.

I don't wish to sound negative but i thought you would appreciate another owner's experience.

Does your vanos setup pass the DIS Vanos test?

All the best with a resolution to your issue.

[Andratch]

ANOTHER UPDATE:

I took the car to a different shop yesterday who I felt had much more skill around advanced diagnostics. They took a look at the vanos and did some pressure tests - they strongly suspect that the problem could be the Vanos Accumulator. It's nitrogen gas-charged, and over time the nitrogen can leak out (even without the diaphragm tearing). The purpose of the accumulator is to give a boost of oil pressure to the Vanos when the oil pump isn't providing enough. If mine has lost its gas charge, the vanos oil pressure drops down while the car is off and it doesn't have proper oil pressure for the first second of cranking and startup.

There's not a good way to test the function of the accumulator, and while it is technically serviceable, you have to mail it out to a specialist and that takes time. So we're going to replace that and see if that addresses the concern. The part should arrive tomorrow, they're going to change it out and crank it up on monday and see what happens!

[sapote]

I'm not sure if the 'anti-rattle kit' is needed here, but we'll see.

I would not touch it if the shaft has zero axial plays when rocking back/forth by hand. It's not a plug-and-play process but you have to lap the disk or the cylinder spacer to get zero plays and no binding.

[BL92]

Have you tightned the timing chain tensioner bolt hidden behind the thermostat? these back themselves out and may cause timing chain noise along with an oil leak

[Andratch]

UPDATE:

Ok, next phase of the noise diagnosis starts today. I am re-rebuilding my VANOS unit.

When I purchased the car, one of the perks was that the VANOS was already "Done," and I can see evidence of that in the form of Beisan oil pump disc and exhaust side chain guide. But I have no idea what else was done. So, I'm going to throw in all the new gaskets and goodies and see if that helps. I'm not sure if the 'anti-rattle kit' is needed here, but we'll see.

I'll report back in a week or so with news.

[Andratch]

I'm having these exact same symptoms, funnily enough, it only started happening AFTER I completed my top end rebuild. It has been making a rattling, knock sort of noise for 1-2 seconds upon a cold start, it's always in the morning when the car has sat overnight. Vanos has been upgraded to Beisan and had less than 500 miles on it at the time, so I knew it wasn't that but I too searched high and low for the source. Sounds like you found the culprit for me, I appreciate you updating this thread. Going to order an OEM piece and get to work, most likely upgrade the pulleys since I'll be in that general area anyway.

When you say your VANOS was upgraded to Beisan, do you mean it's a totally new unit? Or it was serviced? What all did you do?

[Andratch]

Update: I’m fairly certain this is a VANOS related noise. While I do have a few Beisan parts, my VANOS unit itself is original - and when using a mechanic stethoscope, the noise is the loudest from the VANOS unit itself.

I’ll probably wait until the weather is a little cooler and bite the bullet on a replacement VANOS unit.

[Andratch]

Here is a video of a DINAN car with 40k miles and it revs almost up to 2k RPM while making that cold start rattle sound:

And another car that only revs up to 1.5k RPM and does not have the rattle:

unfortunately this isn’t my issue. I started the car today and it only revved to 1500rpm, still made the noise.