my 98 volvo s40 has a problem; crank but not start.
it 's been a long time ago. most of occasion car started in a few trial.
but 3 days ago, i never started.
please take this as a reference.

symptom)
battery ok.
as turning a start key to ignition position, crank but engine not running.
the fuel pump ran occasionally and fuel pressure is about 100kPa.
(normal reading is around 250kPa; volvo engineer told me.)

fix)
replaced a fuel pump. but the same symptom occurred.
there is a relay between fuel send unit and start key assembly and
this relay was malfunctioned. replaced it.
still same symptom.
check the polarity in the fuel pump(input wire); and switch the wire +,-.
this action made the engine run.

conclusion)
root cause was a bad relay.( even fuel pressure is low, engine had started )
wish you check wire (+,-) polarity.( i think previous owner already changed fuel pump once. and then he may switched input wire porality)

thanks for reading.