Re: fuel pump won't run unless cranking or running - Saab 900 Bulletin Board - Saabnet.com
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Re: fuel pump won't run unless cranking or running
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Posted by RayF (more from RayF) on Thu, 24 Feb 2005 16:54:19 Share Post by Email
In Reply to: troubleshooting a fuel delivery problem, Balzun, Thu, 24 Feb 2005 16:02:24
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You are wrong if you think you remember that the fuel pump should run for 5 seconds on turning on the ignition. It DOESN'T come on then, only when cranking or running. That's why the notorious "long crank time" problem with 900s, when the check valve at fuel tank begins failing and leaks down residual pressure in the fuel rail. (No pressure there means no squirt of gas from the injectors; but cranking starter draws so much current there's barely any left to let the fuel pump work to build pressure so that the injectors CAN squirt gas.)
So though you might in fact have a fuel problem, not hearing the pump with ignition on before cranking doesn't tell you anything - - you'll NEVER hear it even with a healthy car.
To see if your fuel pump runs (section 234-4 of the Bentley manual) pull fuses 27 and 30, and install a jumper wire with a switch in it between the outer-wall-of-fusebox terminal of Fuse 27 to the interior-of-fusebox terminal of Fuse 30, and, with ignition OFF, switch your jumper on. Fuel pump should run.
Expect it to, but if it doesn't run, the fuel pump or the wiring to it would be bad.
If it does run, you could still have a fuel problem if you have a bad fuel pump relay, located covered by the carpet up on passenger side inner wheel well.
Bentley 234-6 gives two tests of pump efficiency (volume/time, and (with a special gauge) max. pressure), to be done with the test lead jumper described above, if you doubt its efficacy.
Other tests also in Bentley, same area.
Note (Bentley 234-8) that your car also has a "turbo over-pressure switch" meant to kill the fuel pump temporarily if the turbo somehow permits vastly excessive boost, but I'd doubt it is your problem, it should drop right back to letting the fuel pump run again as soon as the intake boost pressure drops down to normal range.
Happy hunting.

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