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I have been trouble shooting this problem for a couple months, and hopefully i am on the path of resolution.
Talked with an indi shop who has seen this problem with no codes, and suggested I go through the crank case vent system. Figured with 123,000 miles, it was probably due for servicing, and it did not do well in the rubber glove test. Replaced the flame trap and hoses that go to the block and head. The long pipe that folks dread since the bango fitting on the right side of the engine (fly wheel end) is hard to get at, was not replaced. Instead, the nipple on the left side of the engine (timing belt end) is replaceable, and this appears to be where this assembly goes bad and gets plugged up. Turns out the nipple is available for T6 engines, but not for XC70 engines of this vintage. However, the part bolts right up. There is a video on fcpeuro.com that shows how to do the repair and the Volvo part #30731007. Repair was quite easy once I got the intake manifold off. Engine now does just fine with the rubber glove test.
However, after about 400 miles, I got more stumbles and hesitation. But, this time I got a code, "ECM-280C Fuel Pressure Sensor - Signal to high". Pooking around various forums, it sounds like the fuel pressure sensor can go bad and give random high signals, which causes the ECM to dramatically drop the PWM duty cycle to the fuel pump. The resulting drop in fuel pressure would cause the performance issues I was experiencing. It could be that the earlier occurrences of the problem did not have a high enough signal to cause a code to be thrown.
Replaced the pressure sensor the other night, and will see what happens over the next few weeks.
Hope this fixes it.
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Gary D
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