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I am getting desperate to fix this, any help is appreciated beyond words.

A month ago, my car (2008 Impala SS) wouldn’t start. I could fix it by disconnecting the battery momentarily, but eventually not even that would fix it. I got a new fuel pump and it ran again for a few hours... the next day, it didn’t start again. I am positive it is a lack of fuel pressure as I tested with starting fluid, and I’ve checked pressure at the fuel rail.

Took it in, got a new BCM, and it was fine for a few days. Took it in yet again, and they tightened some electrical components, then it started fine for two whole weeks. On occasion though, my lights and other electronics would dim momentarily, which I believe to be because of unexpected decreases in alternator output due to fuel pressure drops. Additionally, my car will idle rather weirdly and will go down the highway at unusually low RPM (1250-1500) and just feels unresponsive.

A couple days ago, it was doing this particularly badly, and I was getting small acceleration surges. The next day, my car wouldn’t start again. I listened for the pump to prime, and it doesn’t. I have a new Fuel Pump Relay as well.

I feel as though the problem is in the electrical circuit to the fuel pump, but I don’t know what the crucial points would be to check. I am going to try a slight bend in my relay pins to make sure they are making contact, but what other crucial points should I check? Also where is the ground for the Pump located?
 

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Check your grounds. Bad grounds often cause unusual symptoms. There are a lot of grounds both engine and frame. Check engine grounds, and your fuel pump ground. If you are in the rust belt, it is common to have rusty ground connections.
 

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This forum focuses on the 1993-'96 B-body and D-body platforms, and based on LT- motor. I don't know how many aspects of electrical, mechanical and assembly those share in common with a W-body LS- motored car manufactured a decade later. But, your details and well-ordered background will probably net you better probability of results by posting the same info on a forum more suited to your car. Here's one example: Impala Forums

"I got a new fuel pump" is confusing. Did you install it, or did a shop? You said you're convinced it's a FP problem, and then you said you did a pressure test. So, what's the pressure? And if it was low, then what has been done to correct it?

With just a first reading it strikes me you may want to consider fresh eyes on your symptoms and problems. I read what sounds like too many visits to the shop (or shops) for the same issue (or issues) without a sustainable repair. For instance, if a new FP has been installed and it does not prime, then the installation is the first suspect. Unless there is faulty power feed or bad ground. [EDIT]: just caught Fred's post too. Also, what made you think to disconnect the battery for reset in the first place? Finding it keeps dying to me shows there is likely one or more components adding defective parameters/inputs that gradually corrupt the program.

If the work performed so far can be trusted (which I don't, based on the above), then be careful more trips to the same shops that may attempt throwing other parts at it (that you pay for) in place of sound diagnostics. I'll volunteer that some words I did not see above which commonly cause problems in our cars include maint. items related to emissions, coolant reading and electrical service. The collection of your symptoms COULD BE related to poor charging or dying battery. [EDIT]: just caught Fred's post too.

A real big question would be what other seeming unrelated work has been done in the near past, and investigate any parts that were disconnected or broken nearby. Good luck.
 
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