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Discussion Starter #1 (Edited)
What is the functionality and operational mode of the 2 separate fuel pumps on the V8 ?

I assume that the primary pump always runs continuously and that the secondary pump cuts in & out only under certain demand conditions ? Or, are they both supposed to run simultaneously (e.g., as a low pressure & high pressure paired system?)---- or, is the secondary pump just a "back-up"?

Currently, when I look at the OBD data while my car is running, only the primary pump is "on" --- secondary pump is "off". Is that normal, or do I have a problem with my secondary pump / relay --- should it also be "on" ?

Car seems to be running a bit rough & stumbling at idle, so I'm looking at possible fuel delivery issues as the cause.....although, strangely enough, I'm getting no error codes at this time. Fuel trim values do look a bit unbalanced though...

Thanks
 

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The secondary fuel pump is "exercised" during start-up and is turned on by the ECU during periods of maximum demand, such as reaching 130 MPH and above. If the motor is not running well at idle it may be because you haven't driven the car enough for it to load the tables. The ECU "learns" and adjusts itself to get the idle smooth. Before that it relies on default values that are not optimized for each motor. If you disconnect the battery or reset the ECU it has to do this all over again. Other possible causes of a bad quality idle include worn spark plugs, dirty air filter, bad injectors, a weak battery (low voltage) a leaky EGR valve and a vacuum leak in the plumbing for the IAC and air induction system.
David Teitelbaum
 

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Discussion Starter #3
Thanks David ----

Yes, the car has been "unused" for a few months, and the battery was disconnected recently prior to re-start-up. So, I expect the ECU will take a while to re-learn. Probably, it will need to go for an actual, several mile drive to get everything set right --- rather than just letting it idle as I have only done so far to ensure that all the repairs I have done are working well.

Also, is there any advantage to electronically re-setting (using an OBD tool) any of the systems / ECU --- or, is it best just to let things set themselves up by running?
 

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If it keeps stalling you can tweak the idle motor by inputting a higher value till it can learn a better one. It takes driving cycles to fill the tables, just letting it idle won't do it. IIRC the only way to set values is with a Tech 1. All of the others can just scan, not actually change anything.
David Teitelbaum
 
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