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my 1999 110 hdi appears to be losing boost from the turbo ,all the intercooler pipes are ok with no leaks at all, but when you drive it you
can hear it leaking. if you rev it up it sounds like its coming from the metal pipe that runs from the turbo (about 1 inch in dia)
any ideas or is this normal

not normal id say mate, the pipe could be cracked.

You can test for a boost leak by getting a plastic plumbing cap from a diy store thats the diameter of the air intake pipework, drill a hole in it to fit a schrader tyre valve (buy that from kwikshit) and glue the valve into the cap, then remove the intake pipe from after the maf (or at least after the air filter) and slide the plumbing cap into the pipe end, securing with a jubilee clip....then using a footpump, pump up to 10 - 15 psi and see if it holds?

A leak will produce a nice hissing sound to find and a spray of soapy fairy liquid solution will blow visible bubbles where the leak is.

Obviously dont run the engine lol but you will have silence otherwise to enable you to track down the leak. Even if some inlet valves are open, the exhaust valves for the same cylinder would be shut and vice versa so it cant falsely leak thru the cyls so its a good test.