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My '93 940 has started the same problem at start-up after sitting overnight. The rest of the day it starts right up. Sometimes, if I let turn the key to the first position and let the pump run until it shuts off, then turn to the start position, the car starts right up. This makes me think the residual pressure in the fuel rail is too low. I don't yet have a fuel pressure guage but I plan to get one. There is a valve on the fuel rail for measuring the pressure.
Last summer when it was very hot, the car had a momentary power loss as you describe. It only seemed to happen after a hard, hot run. Then when I'd be sitting idling at a light, I'd step on it and there was very little power for maybe 10-15 seconds then off we'd go. Art Benstein has noted the same thing on his mother-in-law's 940. We discussed the possibility of vapor lock but neither has an answer yet. This problem has never happened this winter. If it happens again this summer, I plan to try using foam pipe insulation around the fuel line and rail. My old pickup truck had a vapor lock problem and this insulation on the fuel lines cured it.
The only other possible cause I can see for the momentary loss of power would be the crank position sensor. When I first got the car I had a few no-start mornings. Eventually I got around to replacing the crank sensor since it was original and the forum recommends replacing at 100K miles. When I got it off, I saw that the outer rubber insulation has split right at the sensor. There were three wires inside and the insulation on these wires had also flaked off. This meant these three wires were hanging down vertically only a tiny distance apart. Any vibration or engine movement under torque could have shorted these out and give either a no start or momentary engine cut out. Anyway, since replacing that sensor (easy) there has never been a no-start. We'll have to see how the car acts during the hot days this summer.
Hope this helps and I'll follow this thread to see what others think.
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