Here is the story :
Volvo 740, '89 automatic, 210,000 miles, turbo replaced in 1995.
When accelerating and the turbo gauge gets close to the yellow zone and above the car is loosing power and if pushing the accelerator further black smoke starts comming out of the exchaust. Whistling sound can be heard simultaneously with the above symptom.
I do not know if it is related, but since I got that same "effect" I got another "extra" : when in park and idle if rpm is between 2200 and 2700 the engine goes in some fluctuating uneven regime of idling - if under that average 2500 rpm or above it stays steady...!?!? - weird !!!
Here is what I am thinking I should check :
some of the hoses may have a smale hole which doesn't cause any problems, but when under higher pressure the pressure lost interferes with the self-adjusting system of the fuel supply and causes unnecessary amount of fuel to be injected.
Second : the fuel pressure regulator has a hole.
third: the turbo boost valve( or whatever is the name of the valve which compensates the excessive pressure from the turbo ) is out of adjustment, so it releases pressure in the atmosphere...

I wonder where to start to get any idea of what is the culprit?!?!

and by the way - the loosing power and black does not occur when the car is in place in neutral gear or in general , when theer is no load on teh transmission... i.e.: only driving I can hear the whistling noise, hence - how could I determine from where it is comming - ...

I will appreciate any suggestions and advices!
Thank you in advance !
Good luck to everybody with the volvo driving and maintenance!
MM

I vote for the hole in the turbo plumbing causing your symptoms. They can be difficult to find sometimes. I use a plug the size of the turbo hose with a Schrader valve installed. Simply disconnect the hose at the turbo and insert the plug. Clamp the plug into place. Reduce the air pressure on your compressor to about 15 lbs. Apply pressure to system and listen for leak. Quick and efficient.

You will find that the cap on the master cylinder is the right size to fit inside the turbo hose. If you make a plug out of a master cylinder cap not only will you be able to locate holes in your turbo hoses but you can also use a bicycle pump to pressure bleed your brakes using the cap with Schrader.

Randy