7 replies to this topic

[DuncanP]

Hi all, Now at this stage I don’t know if all this is related but her is some background to the current problem. Since I got my VX there has been a very audio able ttsssch when changing gear, so I just assumed that someone in the VX’s past had fitted an atmospheric dump valve and just made a mental note that it might be something to change at a later date. Recently I have had to change the turbo on my car (cracked manifold) since then the car seems to have developed quite a bit more induction roar at 2-3k on partial throttle (don’t know if this is normal) and I seem to be getting compressor stall when coming off boost from partial throttle (not all the time and not from full throttle that I have noticed) now which I hadn’t noticed before. So I thought that some of the vacuum hoses hadn’t been put back correctly when the turbo was done, so off came the undertray today to get a decent look. However what I found wasn’t quite what I expected…. There doesn’t seem to be an aftermarket dump valve, and all the vacuum hoses appear to ok at first glance. So am I right to assume that I almost definitely have a boost leak somewhere? If so does anyone have any tips on finding it?

Edited by DuncanP, 22 November 2009 - 01:17 PM.

[JohnTurbo]

If the recirculation valve is like the bosch one used on VWs etc, its very likely you have a split diaphram. The effect of this is that you have no recirc valve at all, as the thing won't open, and this when you change gear, you get the "pigeon" noise of compressor surge and the pressurized air spins your turbo backwards at supersonic speeds. These are especially likely to go on cars with increased boost (read remap), but only last 40-80k miles anyway.

[DuncanP]

Thanks for the reply John. Im not sure that it is the dump (recirculation) valve as im pretty sure that the new turbo (astra vxr) from Courtenays came with a new dump valve and i can't think why Plans would replace the new one with the old one. Although I cannot be 100% on this so it might be worth replacing just in case. Doing a bit more checking this afternoon, I did turn up one perished vacuum hose coming from the rear BCS. After wasting my time at Halfords (why do i ever go there) I ended up stealing a hose from the kit car (what i should have done to start with) to replace the dodgey one. But this doesn't seem to have fixed the problem. All of the other hoses seem ok. Is it worth trying to bypass the rear BCS incase that is broken??

[JohnTurbo]

Just looked at a picture of a VXR turbo, doesn't look to have a recirc mounted to it? In which case i'd not think you'd get one with it, you're not thinking of the wastegate actuator? In any case the stall (in my mind anyway) can only be caused by the recirc not working properly. - this could be a perished vacuum pipe to it also? - When you change gear you let the accelerator up, this closes the throttle plate...so then you have pressurised air thrapped between the throttle plate and the turbo compressor wheel... All it can do it blow backwards through the turbo, unless you provide an easier route... the recirc 'dumps' this air back into the pre-turbo inlet, but importantly after the MAF sensor. (An atmospheric dump vale is always a bad idea on a car that uses a Maf sensor.) Edit:- Looks like the stock turbo does have an integral recirc. Definately worth checking out its plumbed in correctly in any case. Its also possible you're running so much boost that the recirc can't cope? I've seen this problem before.

Edited by JohnTurbo, 22 November 2009 - 05:42 PM.

[DuncanP]

I was under the impression that the VXR turbo had a very similar recirc valve to the standard one i.e. built into the turbo outlet.....of course i could be talking cobblers  vxr_turbo.jpg   24.65KB   3 downloads The car is standard bar the VXR turbo BTW. I will post up a pic of the vacuum plumming shortly just in case......

[JohnTurbo]

Yes...just opposite the tip of the arrow...that looks like the nipple for the vac pipe which will actuate the recirc valve. That should be connected to the inlet tract somewhere downstream of the throttle plate. - Usually on the inlet manifold.

[DuncanP]

Here is a diagram of the vacuum hoses, could someone please confirm if this looks correct please  vacuumDiagram.jpg   19.69KB   7 downloads

[TheStotts]

Had similar part throttle roar which developed into a scream recently and a distinct tsssst sound on throttle off. Assumed an air leak so purchased a boost gauge to see if I was loosing pressure and found the car was actually overboosting to about 20psi. Tweaked the wastegate actuator to drop the boost to stock levels (12psi overboost dropping to about 10psi) and the scream and tsssst have now gone.

You may well have a different issue with similar symptoms but it might be worth grabbing a boost gauge to see what pressure you’re running.

For advice on adjusting the actuator see this thread. You need to turn both nuts anti-clockwise to reduce boost.

Good luck

Edited by TheStotts, 22 November 2009 - 09:54 PM.