Inlet, exhaust cam position problem, maybe?

[spannerrash]

Sorry guys and gals but I just thought of another question.

I realise that to do this I would have to rebuild and run the engine.

For those of you a bit more savy than me with VCDS. Is there a "desired Vs actual" cam position that I can monitor? It would at least reassure me that the adjuser isn't simply stuck.

Also, are there any output test that I can do on the adjuser while simply cranking the engine. Again to prove one way or another that it isn't stuck?

[MikkiJayne]

Originally Posted by spannerrash

My gut feeling tells me that turning the engine on the starter motor with no plugs in at all, should generate enough pressure to activate the tensioner.

This is correct. When I test engines on the stand I can hear when the oil pressure is coming up because the valvetrain quietens down significantly just before the pressure switch activates and turns on the light in my test box. From a completely cold start it can take a minute or more of cranking to build pressure, but once its done it the first time, subsequent attempts are just a few seconds.

I suspect you may well be chasing a ghost, but it does sound like you've done enough test cycles to be sure about the figures. A hot compression test would be interesting.

There's not really any way to measure the tensioners as they are so crammed in there. The main thing would be to check the marks on the ends of the cams line up with the arrows on the head but that would need cam covers off. The ECU knows the position of the adjusters from the cam position sensors on the inlet cams. In VCDS it will show you how much adjustment its putting in to get the cams where it wants them. Typically I aim for 0-1° after a timing belt replacement.

One thing does spring to mind - how are you preventing the engine from trying to start during the compression test? If the ecu is powered up and attempting a start cycle but, say, has no fuel pressure if you pulled that relay, then it might be messing with the signals to the cam adjusters and inadvertently changing your values. I couldn't explain why it would do one and not the other, but if the ecu is still awake it might be worth disconnecting the adjusters and seeing if that makes any difference to the readings.

I'll disconnect the crank sensor if doing a compression test in the car as then the ecu doesn't know the engine is turning so doesn't try to start.

[spannerrash]

When I first started doing the compression tests it was only a bit of a "while I'm in there" afterthought. I was originally changing plugs and coils. So, plugs removed, coils removed and fuel pump relay removed. Both chain tensioners were still connected electrically.

I did remove the fuel pump relay first while the engine was still running in order to run the pressure out of the system. I can't remember exactly why but at this point I decided to scan the car once again for fault codes. I think I had the idea that checking fuel trims might help me find my misfire. Interestingly I had no faults prior to pulling the relay and multiple, seamingly unrelated faults afterwards.

I'm not sure what effect the removal of the relay has on the Ecu but it does seem like the Ecu doesn't like it and may in fact disable multiple other systems as a result.

I think at this point I need to reverse direction a little before disappearing any further down this rabbit hole. I'll put everything back together. Fit new plugs and coils (because I have them), run the engine up to temperature and then do a running compression test on cylinders 1 & 5. I will simply remove the plug, coil and disconnect the injector and run the engine on 7 cylinders for each test.

Hopefully this will eliminate the oil pressure and Ecu variables.

It will be a couple of days now before I get back to it as I have to work but I will let you know how I get on.

Actually, I think I'll connect an oil pressure gauge before I put it all back together as access is much easier. I'll then measure what oil pressure I generate at cranking speed with no plugs in.

Thanks for the help so far, much appreciated.

[spannerrash]

After doing quite a bit of research I eventually stumbled upon this YouTube video.

https://youtu.be/tC2Klazem3g

It seems to me that a cold cranking compression test (without spark plugs) does give different readings between the two banks of approx 20psi. The only plausible explanation I can find is that the cam phasers start off in different positions and only start to phase when whatever conditions are satisfied.

[MikkiJayne]

Ha. I was reading a thread about this on FB the other day and I was sure it was relevant to something and couldn't place what until now Obviously now, I can't find that discussion to refer back...

You're spot on, based on that other discussion. I'm sure the basis was that one of the tensioners is at zero and the other is at 12, or 24, until they have pressure and then they end up the same.

With oil pressure they must be the same since in the FL1 the solenoids are connected in parallel to a single output from the ECU, so they must both advance and retard together. Its only in the FL2 they are controlled separately.