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Yesterday while driving home at a low traffic speed(about 25MPH) I heard a loud whoooooosh, like a tire going flat quick. The first sound was longer than the second time I heard it. I thought I blew a tire. When I got home, all gauges were fine. Temperature was good, no noises etc. No fluids, no flat tire(s). But now my average mileage indicator is way off from what it was before the noise. It was around 21+mpg, now showing and very erractic 16-17mpg. This is the average, not instantaneous. I also "think" that the faint whine of the turbo is gone. Hard to tell power b/c I have not driven on wide open roads. But there seems to be a faint power loss. Could I have lost the turbo? Is there some type of hose that would blow off to make the noise I heard? It was like an air hose with the blow gun nozzle on it releasing air. It almost sounded like it was going round and round, just like a tire losing its air going round and round, albeit briefly the two times I heard it. This is on a 2002 V70XC with 80K miles. Thanks.
 

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Does the boost guage behave mormally? Should get boost from 1500rpm+ with reasonable throttle - crossing an intersection.

There are a number of silicone hose joints on the air tubes from the turbo, through the intercooler and on to the intake. One could have come loose and make the "air rushing" sound. Don't be worried if it's a bit oily inside the hoses - that's normal. Clean ALL the oil off with brake cleaner, slip the joints back together and tighten up the clamps.

A blown turbo would mean an oil seal lets go and you get LOTS of oil smoke out the exhaust. Usually from a zero maintenance program.

FYI, a problem 740Turbos had was wastegate. It is vaccum operated but works kinda in reverse. If the vaccum line broke, it could get brittle in high mileage cars, the wastegate is stuck in fully closed position. It would make amazing W.O.T. power......but just for a little while. Usually ended with a rod out the side of the block. Got to watch the guages!
 
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