Tapping after starting (hot or cold) - then goes away

[Kinkstaah]

Hi Folks.

Wondering if anyone has heard anything like this before. Head is a RB25DETNeo and this tapping happens just for a minute or two after starting

Hot OR Cold.

There are no unusual sounds otherwise.
It sounds to me that it's similar to a SR20DET VCT rattle, however while this sounds like it is in the same area, it happens if the VCT solenoid is unplugged.
I don't know if this will actually affect things, if the issue is internally mechanical and not solenoid.
There's not a lot of info on VCT rattle on RB25DET's.
I have blocked off one oil feed to the head to combat blowby in the past.
Note: This happens at about ~1500-2000 rpm. It will rattle once, or twice, and the car can rev past it fine. I.e, if you revved to 4k you may hear 1-2 taps while it revs through, but it always appears at that range.
Note: If you rev the car slowly through the rev range after starting, this does not happen. You don't need a ton of throttle, but if you stab it or stand on it it'll happen, it will not happen if you say hold throttle at 10% and slowly rev through the range.

The sound appears to be coming from the intake cam, from what seems to be just under the throttle body.
Again, hot or cold, doesn't matter. Will hear it for about a minute of driving and it'll fade away pretty quick. Leaving the car idling does help if you leave it idle for ~2 minutes etc.
No weird sounds under load at all. Car otherwise drives fine.

From what I can tell, if you leave the car sitting for maybe 5? minutes, the sound will return on re-start. A little less than it would if the car is left overnight, but not much.

Could be related (maybe not) but VCT has only just actually been switched on/wired in recently. Could be piston slap. Could be wrong oil, (5-40, was 10-60 in the past) could be something else entirely. Sound does not appear to be getting any worse since I noticed it (couple of weeks). May have always been there, because the car was previously auto and any throttle will go well over 3k RPM where there's no sound and no load.

Any hints as to what this could actually be given the above info?
Looking to know where to begin, or whether to begin at all if this is actually a common thing people don't talk about.

[admS15]

Could the noise possibly be the iacv or cold start valve doing its thing on start up? Mine makes some pretty weird noises from there until its up to temp.

 

Edit, after re reading your updated post with additional info. Unlikely to be what i suggested.

 

 

 

[Kinkstaah]

Possibly, because the stock IACV is broken as it died on the track and gave me 2500RPM idle.
I got around this by actually installing a second IACV which actually controls air flow to the now permanently full open Nissan IACV.

The part that broke, was the part that otherwise moves on heat (i.e the wax pellet). So I can see how heat would no longer affect the operation of that, but I also can't see how it'd change, given it is currently absolutely not functional at all and there's no wiring to the motor right now.

[admS15]

This is weird, i would put it down to being a greg issue. Unfortunately, i don't think I'm going to be much help here. Good luck with figuring out what voodoo is happening [emoji106]

[Dobz]

That’s a head scratcher, you might need to just start a process of elimination. The easiest one is to change back to the oil you used to use. Shim bucket clearances would be a good thing to check too.

[Kinkstaah]

Update (kind of)

Changed oil, no diff there other than better oil pressure. (10-60 instead of 5-40)
Nothing bearingish in the oil, so I mean that's good. Same amount came out that went in, also good.

Took cam cover off and engine builder had a look, was able to rotate the cam, and have the VCT gear *not* rotate.
Then if rotated the engine enough, it'd get to the end of it's 'play' then lock up and all rotate as one.

After this, it was actually locked up again, moving the cam would move the gear. Until you gave a the cam a little more forceful tug and the cam/vct gear would unlock.

Unsure if this is 'normal' or not. There's also oil (again lol) in the Solenoid plug. Strange. Had one in the past have an oil leak there, and it was able to be moved by hand, the one currently in the car was locked in place (when outside of the car) leading us to surmise the original one was fked as it was unlocked out of the car while the other wasn't.

Changed both the solenoid AND the gear that day. Maybe issue has re-occurred?

Engine builder friend is going to try with people who also own these cars to perform the same test to see if the gear performs the same way.

[AngryRB]

Any update on this?

Edited February 6, 2018 by AngryRB

[Kinkstaah]

Nope. Noone has been able to identify this sound whatsoever.  It got *quieter* after an exhaust stud was tightened up. However, the sound is there, so it's entirely possible it was just more audible due to a slight exhaust leak.

It does not appear to be VCT, which is now actually (blissfully) finally wired in and working.

[AngryRB]

I have a similar issue but only when cold, does your intake cam gear have any play like inward/outward.. 

My head was reconditioned with new springs, 260 cams etc so doesnt make sense to be anything else..

[Kinkstaah]

Well, after checking about seventy million things this was actually a badly warped 6boost manifold.

It was 0.5mm out at the front, and 1mm out at the back, and quite twisted. I also had 3 broken studs, but as everything was tight I checked everything else under the sun first.

No more ticky ticky now, and my wallet is much lighter given this also resulted in all the turbo lines being re-engineered as well as a lot of melted wiring from offensively hot exhaust gas going where it wasn't supposed to be.

I only really twigged when I put 98 in the car instead of E85 and could immediately smell the fumes.
Start at the simple stuff, future googlers!

[Piggaz]

I hope it’s going back on the dyno now that    all the exhaust gas is making it to the snail and not exiting stage left before it!

The “big turbo” won’t feel as big now. ?