Fuel Pressure and Vacuum Sadness

[recall]

Alright guys, I've been reluctant to seek help but I'm stumped. I have an 88 325is and it doesn't run like it's got any grunt at all until 3.5-4k. I know this is around where the torque peaks on the M20B25, but I can't help but feel like in normal driving the car just lugs around anytime I'm below 3k. Having driven a few other B25s, mine feels slow. So, here's the symptoms I've got:

-Poor off-idle acceleration
-Lumpy idle (no bounce in tach, but has an audible "duh-da-duh-da-da")
-Hard start when cold or warm

New parts:
Fuel Pressure Regulator
Fuel & Air Filters
Rotor/Cap/Wires/Plugs

The injectors were just professionally cleaned and rebuilt and I've also checked the throttle position sensor, coolant temp. sensor, crankshaft position sensor, and made sure the AFM doesn't bind anywhere through its travel.

Thinking the fuel pump might be on the fritz, I picked up a fuel pressure gauge from Harbor Freight and did some tests earlier today. I get an idle pressure of ~38psi with the vacuum hose attached to the pressure regulator and around 43-45psi with no vacuum. The weird thing is the fuel pressure drops when I rev the car up (and the vacuum line is attached), bottoming out around 34-35psi. I think this is the opposite of what I should expect, and I wonder if a vacuum leak might be to blame. To test the pump, I pinched the return line and was able to get pressure up to around 90-95psi. I've driven the car around without the vacuum on the regulator and honestly it drives about the same, should this be a drastic difference?

The car holds pressure on the line for at least 30 minutes, but regardless of this it takes a couple of turns to fire up.

I've done a carb. spray test everywhere I can think of and haven't found anything, so my next step is to go for a smoke test. But having thrown a good deal of money at this thing, I'm hoping to get some ideas on whether I'm on the right track. Why's my car lazy?

[Adrian_Visser]

Check the fuel pump's output volume as well. Just because it can put out sufficient pressure doesn't mean it can deliver enough fuel.

[browntown]

check to see if prev owner chipped your board. If it has been tinkered with consider a known good chip or just swap ecu's with a buddy and drive around for a while and see if things get better.

Of course there is the universal auto thing to blame, your o2 sensor. That however is semi pricey. You can swap out your pump for a walbro unit for pretty cheap if you're sure you need to. Frankly I would expect the pressure to drop a little when you demand more fuel but I can't comment on your readings.

[recall]

Check the fuel pump's output volume as well. Just because it can put out sufficient pressure doesn't mean it can deliver enough fuel.

Good point, I forgot to mention that I did check this before getting the pressure gauge and it was spot on by the Bentley.

check to see if prev owner chipped your board. If it has been tinkered with consider a known good chip or just swap ecu's with a buddy and drive around for a while and see if things get better.

Of course there is the universal auto thing to blame, your o2 sensor. That however is semi pricey. You can swap out your pump for a walbro unit for pretty cheap if you're sure you need to. Frankly I would expect the pressure to drop a little when you demand more fuel but I can't comment on your readings.

That's a good idea, I actually have a chip to put in this car but I've been holding off as I want to figure out what's up with it first, so I'll dig in there to see if anything has been tinkered with. Given the rest of the car, it seems unlikely, but anything is possible. I've considered replacing the O2 sensor, but doesn't the ECU ignore that until near operating temperature is reached? The car isn't much if any different when cold or warm (there's a bit more choppiness in the throttle), and going WOT doesn't change things.

From what I've read, the pressure regulator should respond to a drop in vacuum (under acceleration) by closing the valve inside to increase fuel pressure while the demand is higher.

These probably explain it better:
Fuel pump pressure & volume testing
Making sense of sensors

Then again, I have no idea how my car could do the opposite, unless the vacuum pressure is too high and doesn't drop when I get on it.

I also forgot to mention I've done a compression check with high results consistent across the cylinders (~170psi) and get no codes from a stomp test.

[jlevie]

Intake vacuum should drop when you rev the engine and fuel pressure should rise. Seeing it drop suggests a bad FPR or a weak fuel pump.

[recall]

Intake vacuum should drop when you rev the engine and fuel pressure should rise. Seeing it drop suggests a bad FPR or a weak fuel pump.

The FPR is brand new (and there's no fuel in the vacuum hose), so that's probably out. Do you have any suggestions for other tests to check if the pump is weak? It delivers the right amount of fuel and pushes over double in a deadhead pinch test. I'm going to check the current draw to make sure that's in spec, but if it is I'm stumped on where to go next.

[jlevie]

Even a new part could be bad. The first thing I'd do would be to try a different FPR. If that makes no difference, replace the pump and filter.