Is my fuel pump on it's way out or something else?

[amoebamoebaaa]

Hi R3v community, I'm new to E30 knowledge, looked around but couldn't find a problem like mine, so here goes.

Is it possible for the low pressure in-tank "transfer" fuel pump to fail/malfunction and the car still be able to run solely from the external high pressure in-line fuel pump [I replaced this a few months ago when the car crapped out on me a few times, which resolved the issues of poor mpg and no start], it just ends up working harder? I own a '87 325is, an early model as I have discovered 06/87 (Sealed-headlight beams, dual fuel pump system, etc...)

I just feel as though my car isn't pulling as much as it can, lack of power. Although it's been a little more then a week since any occurrence. Story from the beginning after going on a dyno test, car was running fine. On the free way cruising roughly 2000-25000 RPM going 65mph car hesitated, noticed that the MPG needle dropped but regained itself again same as the Tachometer. I thought something was up but was able to make it home. Later in the day it got worse, I was driving as usual then suddenly when I would go in Neutral or let my foot off the pedal the car was surging. I pulled over and shut off the car, when I started it again it stopped, but ended up happening again later in the day. After that the car started hesitating as if not enough fuel was being pumped, began rocking back and forth but the car wouldn't shut off as long as I held the RPM above 1000, below that the car would shut off. I pulled over and shut off the car, when I started it again it started up fine. But the same symptoms would occur again the longer the car was on, occasionally slight hesitation when down revving from 2500 RPM (which through on the CEL on for less then a second then off), idle surging would start.

It's been a week and the car is driving okay now without the problems I described, no CEL, hesitation, surging, stalling from insufficient fuel, plus since then I changed the O2 sensor since the dyno-tech guy said that it wasn't reading correctly. I can't figure out if it's my fuel pump going out, or something else, I did not unplug my battery when installing the O2 sensor so I don't know if anything needs to be reset/calibrating to the new sensor as well. Pretty low on cash as well, so looking forward to see if it's just a simple cleaning on a part such as ICV or AMF. Car Idles a lot better after tune-up I did. I'm hoping it's not something with TPS/CPS because looking around seem pretty expensive.Thanks in advance for any help and reading this long description o_O.

Edit: Fuel filter was replaced along with the new in-line fuel pump I replaced a few months ago. As of right now I don't know what to be listening for as early signs on fuel pump failure in the cabin. With the radio off I don't hear anything out of the ordinary, just the slight engine tick and humming* idling

[jlevie]

The high pressure pump, if in good condition, will work even if the transfer pump is dead. However once the tank gets below 3/4 full the high pressure pump may start to starve out.

A change in the econometer and tachometer while driving is a sign that the DME is loosing timing data. The usual cause is a bad CPS.

[amoebamoebaaa]

The high pressure pump, if in good condition, will work even if the transfer pump is dead. However once the tank gets below 3/4 full the high pressure pump may start to starve out.

A change in the econometer and tachometer while driving is a sign that the DME is loosing timing data. The usual cause is a bad CPS.

Thanks jlevie!

I've driven the car below 1/4 tank and haven't noticed any starvation from the high pressure pump. If the CPS is bad, would that also cause any mpg loss or power loss? Would it throw a CEL? Where would I be able to get a good deal on a CPS if I need it? I've bought some parts already from Pelican, just checking to see if there is a better offer somewhere else too.

Yesterday I unplugged the battery in attempt to reset everything. I notice now that when I'm sitting in Neutral and rev the engine once I let the RPM fall and it goes below 500RPM, the car will sound as if it's going to shut off but will regain itself and go back to around ~700RPM. Is that normal? Plus, also feel as though my car is not pulling as hard as it once was, would it help if I tried cleaning the MAF and ICV? I was thinking about buying a can of CRC Mass Air Flow cleaner and spray them both out.