So my '88 240Dl started doing this a couple days ago and it's really stumping me. The car runs/drives fine for a bit, then it will start making a loud hissing noise which seems to be coming from the passenger side of the engine bay when I apply the throttle. The more throttle I give it the louder the hissing gets and the worse the power loss is. This problem comes and goes rapidly.

From searching all I can find is it may be the cat, but would that cause the hissing sound from the pass side engine bay?[/align]

 

the hissing may be a small leak in your coolant tank. im not sure for a 240, but in a 940 the coolant tank is along the passenger fender, and when the engine is running the tank has considerable pressure

 

Check for leaks around your waterpump. Free guide to waterpump replacement at www.stepbystepvolvo.com.

 

The hissing could be caused by a leaky exhaust manifold, either cracked manifold or blown gasket. I've had a cracked exhaust manifold that made a hissing sound. Or the O2 sensor might be loose.