Boost falling off more than it should at high RPM?

[newsh]

I found out something interesting when I was trying to read what my boost was dropping to at various rpms, basically if I floor it at 3k it will peak at about 26 -27psi, it will then drop gradually to 18 psi at 5k, sometimes less, BUT, if I lift off on the throttle and then floor it again it will go back up to 22psi and feel like its pulling properly again. In fact it will do it again at higher revs, lift off and floor it and you get renewed boost.. So what can this be? Wastegate creeping open? DV leaking? Ignition retarding? Anybody BTDT?

[newsh]

I'm really scratching my head on this one..

Knock sensors?

[s2dude]

Newsh...are you positive it isn't a boost leak??
At those boost levels I would have thought a boost leak test could cause damage (30psi ~) due to the anti vacuum factor of the test.
Would be an idea to check and double check all booost and vacum lines and connections though as although my boost was lower (18psi) a while back I had similair symptoms of a slow leak at high boost and it turned out to be that crappy accordion hose which is now a Samco (it had pinholes in quite a few places!)
Also, what is the boost limit of Samco's guys?? Am I right in thinking 30psi or is it 35psi? It says it somewhere on their site iirc

[newsh]

Well, I really don't think I have a leak but I guess I'll have to do another test

Anyone else got any ideas?

[newsh]

I had some leaking at the injectors, worse than I thought when I got the injectors etc out of the way and I could see the oily seepage. Thats one thing about having the PCV plumbed into the inlet, you get an oily mess wherever you have a leak, makes it easier to find.
The screw in plastic inserts that hold the injectors weren't very tight so I nipped them up and fitted new O rings to the bottom of the injectors, there was no leakage that I could see at the fuel end, just at the bottom and it may well have been the inserts.
Are these inserts meant to be sealed with anything? There's getting on for 2 Bar and just a plastic thread screwed into the ally of the head. Was wondering whether to seal them with a smear of silicon sealant but worried they'll never come out again.


Couldn't fit the catch tank cos only ordered the smaller of the 2 hose sizes, I got the 15mm but not the 19mm, I think I must have measured the spigot with the inch side of the ruler and thought 3/4 of an inch was 15mm, that'll teach me to wear my glasses..


Going out for a bit of a test run later.

[newsh]

Anyone??

Are the injector inserts meant to be sealed with anything? There's getting on for 2 Bar IM pressure and just a plastic thread screwed into the ally of the head. Was wondering whether to seal them with a smear of silicon sealant but worried they'll never come out again.

[Greg_S]

They should be ok because the fuel rail holds them in place once the injectors are in, well thats my take on it anyhow

[S-Express]

Also, wouldn't use silicone any where near fuel system components - if any gets into the combustion chamber, it will knacker your lambda on its way out.

[newsh]

They should be ok because the fuel rail holds them in place once the injectors are in, well thats my take on it anyhow

I agree the injector themselves are held in, but the insert the injector is plugged into is threaded into the head, I'm pretty sure they've been leaking past the thread.

I also found the rail was being pulled away from the head a little when the bolts were in so I've ovalled the holes in the rail so they line up better. Hopefully all this will help.

[newsh]

Also, wouldn't use silicone any where near fuel system components - if any gets into the combustion chamber, it will knacker your lambda on its way out.

Thats a very good point and quite topical, what with recent events with supermarket petrol I was thinking this when I was cleaning the injectors and surrounding area with GT85 and trying not to spray it down the holes into the head.

[Ross]

Would a bit of tape on the thread help stop anything seeping through? Worked on the turbo coolant hoses anyway

John did you get a test-run, was the initial boost problem still there?

[newsh]

Would a bit of tape on the thread help stop anything seeping through? Worked on the turbo coolant hoses anyway

John did you get a test-run, was the initial boost problem still there?

Mmm, white PTFE plumbers tape? That might work, temp resistant too. I'll see if its still leaking first, just about to go out for a run now.

[s2_bo]

Good Luck mate, Hope you get it all sorted

[newsh]

Leaks at the injectors are now fixed
Problem persists however
I thought for a while there it was better and maybe it is a tad but at 20psi boost in 5th at 5,000 RPM, if I lift off and floor it again boost goes up to 25psi. I don't see how this can be a leak really unless its creep at the wastegate or the DV is opening somehow.

[Ross]

Godo thought there. I remember seeing a similar problem, boost was often not quite maxxing out or tailing off early, replaced my dodgy S3 DV with a forge item and problem solved! Guessing you already have a decent DV though?