Strange phenomenon under WOT

[s2carlo]

Experienced something strange today and would help / advise. Car is a 3B S2 coupe with MRC chip. Normal turbo pressure 1.3 bar.

On WOT pressure builds to 1.3 the after 10 to 20 secs there is a small "pof-sound" and pressure drops to 0.3, then after about 10 secs pressure builds again to 1.1, again the "pof-sound"and drops again. It keeps doing this as long as I keep my foot on the floor.

After this sometimes pressure does not build over 0.3 bar, sometimes it builds to the 1.4 value it should on "short acceleration".

What is happening, what is to be checked / changed??
The "pof-sound" might be like a pop-of valve but I don't have one (or is it standard also mounted?).
Can this standard pop-off be broken and therefore swinging or sticking in a position??
How is it operated ? (electrical or spring)
Where is it in the enginebay?

Thanks
Carlo

[Greenm3]

I would check it isnt a hose with a split being blown open after a certain amount of boost. Best thing to do is do a boost leak test.
Could also be the dumpvalve has broken spring (?), faulty WGFV. The dump valve (bypass valve) is between the turbo outlet and the intercooler inlet on the left hand side of the engine bay (if you looking at car front on). Its a bit of a pain to reach but much easier with the front bumper off. I had the same thing when i had a split in the hose wher the T coes of for the dumpvalve. Couldn see a split untill i took the hose off and squezzed it opening it up.

Hth
Michael

[UrS4boy]

I agree with GreenM3. Definitely sounds like a split hose somewhere. The hose has enough strength to hold, say, 1.2 bar but not above that. At 1.3 it opens up the split and dumps the boost, so you start again at 0.3 bar. And so on until you find and fix the problem.