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Yikes! Doesn't sound like the fuel pressure regulator, though I'm gonna stick by what I said, and advise replacement if it looks like it's the original part, as that was suggested to me recently by a reputable independant Volvo mechanic.
My first impression is that it sounds like it could be the throttle position switch, though I wouldn't want to have to defend that hypothesis. Lemme look up a test procedure...
Hmmm. I could be wrong on this. The throttle position switch on a 240 is quite different from the one on my 1800. The 1800 D-Jet has a throttle position sensor with something like 19 seperate positions to feed info to the ECU on just how open the throttle is. On a 240, it's really a switch--off at idle, on if the throttle is open. So, it shouldn't cause the car to run different at high as opposed to low revs. To test, though, with the car off, disconnect the yellow and black leads to the switch and hook up an ohmmeter to the contacts on the switch. With the throttle off, you should get infinite resistance. With the throttle open, you should get no resistance.
Okay, I'm going to rationalize a little bit, here. Let's say your TPS is badly out of adjustment, so that it doesn't engage until a few degrees after the throttle opens. Let's say it's also faulty, so that it disengages at the top of its travel. So, although you are at high revs when it disengages, your ECU thinks you're idling, and cuts your fuel supply. Yeah, that's the ticket.
Again, drawing on my 1800 repertoire, there's a TPS trick. You can judge whether or not this is apropriate or even possible on a 240. If you take the black cover off of the TPS, you should, and I believe will, see a copper brush which is connected to the throttle shaft. It rides on a track made of a copper trace on the back panel. The trace should have a zig-zag at the beginning so that the brush will disengage at that point. You may also see a bunch of black corosion and crap on the brush and the trace, similar to the nastiness you see on the contacts of an old distributor cap. Take a pencil eraser and clean up the trace and. If such crap is causing your TPS to lose continuity at the top of its travel, that might take care of it.
Good luck,
-EdM.
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'90 240DL Wagon 'Lola' -- '72 1800ES 'Galadriel'
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