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Re: Fuel pump; when does it run?
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Posted by RayF (more from RayF) on Wed, 26 Jan 2005 10:19:19 Share Post by Email
In Reply to: Re: Multiple starts, Gary, Wed, 26 Jan 2005 08:10:54
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Don't think Gary's 10-seconds-on trick will do anything. Am I wrong, or doesn't the fuel pump only run on these cars when they are cranking or when the engine is firing? That's a safety feature so if there's a rupture the fuel pump doesn't keep feeding gas out after the motor quits. (You don't want to die in a flaming wreck after a crash that didn't go anywhere near the fuel tank.) But it's also why they crank for so long at startup when the fuel check valve at tank is bad - - pressure leaks down instantly after shutoff and there's no residual pressure there for fuel injectors on restarting. Sounds like the check valve might be Peter's problem to me, except that he says it won't start even after long cranking time.
(The usual check valve failure long crank time isn't because fuel pump would normally take that long to pump up pressure, but because, with the drain of running the starter, there's hardly enough current getting to the fuel pump to let it do its job. Peter's problem might be a combo of a bad check valve and a weak fuel pump or poor current getting to it.)
I have toyed with trying a work-around to the check valve problem that will be an intermittent switch - - a push-button - - that lets you pre-run the fuel pump just before cranking to put pressure in the fuel rail. Since this defeats safety features it needs to be a good push button that won't fail stuck on, and needs to be wired carefully so nothing shorts. I plan to run it off the fuel pump relay which I understand sits up next to the passenger's right knee. Will need to study Bentley first.

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