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Lucid,
You do very well explaining what Haynes fails so miserably at. I am thinking you won't mind me adding to your post a few thought that arise in the hope it will be helpful.
1- "First problem, we turned the key to ON not running to listen for the "whirring" sound which we didn't hear."
Your '88 has LH2.2 Fuel Injections. The pumps run only during cranking and running. The Key On "whirr" came with LH2.4 in the '89 240 (typical Haynes "tip"). And this whirr only lasts a second or so in any volvo that does so, which as you say, does not happen in the 88.
2- "Well, even without the fuse it kept idling for 15 minutes so we gave up"
Two problems here:
a) Fuse 6 is NOT the pump fuse for U.S. LH2.2 fuel injected cars. Only for K-jetonic in foreign markets. The cover label is misleading. Removing fuse 6 would have no effect by itself for sure.
b) There is no dedicated pump fuse, per se, only the Main FI System fuse (25 amp near the coil on your car). And when that's pulled, the engine stops instantly. So there is NO WAY to depressurize by killing the pump, as so many believe. There is a way to interrupt the main pump, but not by simply removing a fuse; a wire must be disconnected.
3- "The bigger issue now is that once we took out the fuse, the fuel pump is "whining", something it has not done before."
Can't explain that, because Fuse 6 isn't even wired to anything in your car. Did you finally open any lines and maybe induce some air? Possibly it was fuse 4 that was removed, so depriving the main pump of feed from the tank.
4- "So do we need to look for a fuel pump problem?"
Possibly, but it's too early to say. Let's see if we get any other suggestions. I would think the change from quiet operation to whining is a positive and hopeful sign of a healthy main pump, if removing fuse 4 was the cause. It cannot hurt to closely examine the fuel injection fuse at the fender for corrosion and intermittent poor connection, and its wiring exposed to weather.
Depressurizing fuel system is not as critical as the Haynes and other books make it sound. There is not much volume released compared to older systems with fuel accumulators - maybe a tablespoon, easily contained by a shop rag around the fittings. If the procedure is yet sought despite this, a plug can be found above the main pump itself.
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