Many 900 fuel pumps have been replaced with no need... - Saab 900 Bulletin Board - Saabnet.com
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Many 900 fuel pumps have been replaced with no need...
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Posted by RayF (more from RayF) on Sun, 30 Mar 2008 19:38:16 Share Post by Email
In Reply to: Re: How would fuel pump fail simultaneous with..., scott, Sat, 29 Mar 2008 12:06:36
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It's a curiosity of classic 900s, at least the later 16 valve versions such as yours, that the fuel pump won't run unless the engine is cranking or running. Many fuel pumps get replaced because a mechanic notices that the fuel pump isn't running when the car won't start, which may be true, but when the reason the car won't start is NOT that the fuel pump isn't running, but some other cause.

For instance, the electronic computer determines if the car is cranking or running by whether it is receiving a spark signal. If there's some failure in the ignition, such as the "Hall sensor" connector, a tiny set of wires plugged into the distributor via a brittle plug on your year, then the entire car fails to run including the fuel pump. Shops not familiar with the system detect no fuel pressure or no sound of a fuel pump and presume it needs a new one. There are simple ways to trigger the fuel pump and see if it's working, mentioned in the Bentley manual for these cars or by doing a search on here and a little reading. Unless that test has been done and shows no fuel pressure I wouldn't okay him going ahead and replacing the pump.

A crude quick test of whether the engine is getting spark but not fuel is, to spray something combustible like starting fluid or carburetor cleaner into the intake manifold thru a vacuum port, then crank. If it fires up briefly but dies, then yeah, there's been no fuel getting to the engine but it has a spark.

If that's the case it still might not be the fuel pump but instead the fuel pump relay or the "system" relay, which sits right next to it, both above the carpet beside the passenger's right foot, reached by taking off a black strip of metal trim, peeling back the carpet flap and stretching, diagnosed by either using the Bentley manual or searching how on here.

Why not the bad compression and the fuel pump failing at same time, you ask? Why I'd suspect that diagnosis is, two bad things very rarely happen simultaneously. And if they do they're usually linked things, the first obviously causing the second.

Bad compression is not linked in any way to the fuel pump or any of the systems that control it. And as I think I said earlier it would have to be pretty bad compression indeed to come on suddenly and stop the car from starting.

Good luck and post back here.

posted by 71.241.209...

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