ph001 wrote: ↑Thu Jun 20, 2019 9:34 am
There is no way even a fairly major fuel pressure variation would make the revs change like that at idle. This has to be valvetronic motor or cam timing related (VANOS compression rings). Did you change both DISA valves in the end or just the one?
This was my original list right back on page one of this thread....
So I would do in this order:
1)
Observe O2 sensor voltages in INPA
2)
Fit new battery
3)
Replace coil packs and plugs
4)
Replace EGR valve and check pipes to sump for blockages
5) Replace / inspect second DISA valve ???
6) Replace valvetronic motor
7) Have VANOS unit stripped and inspected.
That revving noise is the fuel pump, the engine doesn't rev in tandem with the noise in the video. What does happen though is a slightly audible variation in fuel pump sound when the engine is 'shivering' at idle, which is more like a misfire than drastic rev change, as well as that more obvious noise in the video.
I haven't replaced the large DISA valve, but I've had it out to inspect it and made sure it functions. I by no means think you are wrong regarding the Valvetronic motor or the Cam seals, I just have my reservations, as I do with the other potential problems.
The VVT motor learns positions correctly, and seems to function without issue, no odd noises, seems quick and I didn't detect any shorts in it when poking it with my multimeter. I am going to check the accuracy of it some more this evening when I cold start the car. I have for a long time felt this was timing related because of how mechanically the problem occurs, but this fuel pump noise is just odd and cannot be good and also fits the symptoms, albeit with the usual caveats in that I don't have long cranking issues and I feel like there is a problem when the car jerks when coasting to a stop.
The car is due a service next month and I am wondering whether to put a thicker oil in (5w40, currently 5w30) and see if that makes a difference. If it is the cam seals starting to fail, I'd imagine the thicker oil would make a noticeable difference by maintaining the oil pressure more effectively? I have suspected this was the problem for a while but just can't justify the labour cost for something that is relatively speaking, quite rare.
My plan for this evening
Check rough running and Valvetronic position data from cold start
Check rough running data from cold start with VVT motor unplugged
Drive car with VVT motor unplugged - I am sure I've done this before, but will check again, if problem persists I can rule out a valvetronic fault