I have a 2002 VRS, it has a boost issue where on acceleration it goes to 0.5 bar of boost then just drops to nothing, when running correctly it boosts to 1.5 bar then bleeds and settles at 1.1-1.2 bar, there is no leaks from the turbo to the inlet manifold, the dump valve appears to be ok, also swapped that with a mates one to confirm, there appears to be no splits in the pipe work around where the dumpvalve mounts, have tried unplugging the MAF and was still doing the same, we have plugged in a laptop with vagcom to the car to log what was going on the only strange thing was the boost demand and actual was miles apart.

 

We replaced the cam shaft sensor as that had a intermittent issue, the only other fault the car shows is a water temperature sensor but I doubt that would cause this level of issue, I am wondering if the the N75 valve has gone, is there anyway to prove it before I spend out on a new one?

As you have a fault showing with the coolant temp sensor it makes sense to change it, they can cause running issues in quite weird ways. Other than that n75 is a possibility, although boost often fluctuates when this plays up. Could be a badly leaking actuator. You can test the actuator by logging boost with n75 unplugged and it should hold a steady boost of 5 psi (think it's 5 anyway). If it can't even do that then that could be your issue.

Did you have a smoke test done?

If I were you I'd get a new genuine coolant temp sensor in (which you need anyway), test the actuator and try a genuine diverter valve instead of your dump valve (what kind of dump valve is it?) These are the cheapest things and there may well be a solution in there somewhere. After these I'd probably try n75, which aren't that expensive.

Logging would help, looking at fuel trims, maf and lambda and such but I don't really have the knowledge to help massively with that I'm afraid.

so if i unplugged the N75 and ran the car will it create a fault i.e. put it in limp mode? not at home and dont fancy driving 50 miles home in limp.

 

can the temp sensor really effect the car to this meassure?

CTS failure causes incorrect fueling, but i can't see how that would effect boost. My main suspect would be the N75.

so if i unplugged the N75 and ran the car will it create a fault i.e. put it in limp mode? not at home and dont fancy driving 50 miles home in limp.

 

can the temp sensor really effect the car to this meassure?

 

It just means that your turbo would only run off actuator pressure.  It's cracking pressure is 5 psi so if it can't do that or hold onto it in a flat line (which it should) then there is a problem there.  I wouldn't drive home like it that far, just a quick run and log boost as you do it.  For this test you don't really need WOT 3rd gear or anything, you just need to know what it is doing (take to red in 2nd will do).  More I think about it Paul could well be right, N75 could well be the prime suspect, but this also governs the actuator...

I have just replaced the CTS and the N75, it is exactly the same, I will unplug the N75 later and run it up the road.

Ok ran it up the road with the N75 disconnected, it held boost at approx 0.32ish bar roughly 5psi, plugged it back in same issue, boost up too 0.8 bar then it drops back to 0bar on the gauge. Any more thoughts?

Maybe the diverter V/V is not seating properly intermittently, do you have a spare you could try?

The car is fitted with a Forge 007 Dump valve, which appears to be functioning correct, 

firstly, get a service kit, and service that ^^^^^^^^ that was the cause of my issues when I bought my car.

yes the new forge pistons have extra oring on the bottom in the service kit

the dump valve does have the modified piston with the o ring

I have just tested the wastegate, it started to open at .62 bar, also vacuum tested the line from the N75 to the actuator, no leak there.

All sorted now, believe it or not, the cone filter was blocked, unbelievable really, as it does not looked blocked, but it is a stainless mesh type filter and appears to have corroded enough to block the lot.! :|  

Holy moley. Lesson learned: check basics before panicking, we are all guilty of over thinking things sometimes. Glad it's sorted.

I'll never forget that news story of a guy with a rover that wouldn't run properly... took it to the garage, and the air intake was full of nuts! lol..... a squirrel had been storing them up in there! lol...

There was a big problem in Wales last year with birds nesting in warm cozy engine bays coz it was so freaking cold and wet.