85 318i failing to accelerate at low rpm

[redhotchilipepper]

My '85 318i has a problem accelerating when under partial (normal) throttle. It is almost like a there is a plugged up line that i have to use an extra amount of gas to overcome before the car responds. When i accelerate from a stop i get the rpms to about 1300 at which point they stop rising and I have to just about double the throttle to get the rpms to rise again. When it does kick in i get surge in rpms and the car finally responds, lunging forward. This happens in each gear but is more subtle as I rise in gear. On the highway I generally won't notice because the rpms stay about 1900 but if I drop them down a bit the same "stopper in the bottle" effect occurs.

I bought this car a year ago and have done quite a bit of general maintenance since then, including:

Fuel:
Both pumps replaced
New fuel filter
Relay replaced
Pressure reg replaced
Pressure tested: 37psi no load, 35 under load

Spark:
Wires replaced
Plugs replaced
Ignition coil replaced
Rotor and cap replaced

Air filter, afm, oxygen sensor, and vacuum lines all replaced. Grounds checked out okay.

What else could cause this??

[TobyB]

uhh, I never let an M10 drop below 2000.

The M42 is worse- nothing much happens below 3500!

But seriously, how's your throttle position sensor?

t

[Jean]

possibly a nice dead spot in afm.

[pandaboo911]

I'm pretty sure that's just a lack of torque you're getting. Shift later.

Is it worse when the engine is cold?

[thebutterson]

Your 85 should have a 3 bar fuel pressure regulator, so the pressure should be around 45 psi. I don't know how that would explain your problem though, it seems like having low fuel pressure would cause the engine to run poorly everywhere.

[redhotchilipepper]

Still haven't found a solution to my low fuel pressure. :(

To reiterate, I have done the following:
Both internal (in-tank) and external pumps replaced
New fuel filter
Fuel relay replaced
Pressure regulator replaced (3 bar)

I get a static pressure reading of 43 psi, which is dead-on, but a dynamic reading of 32 psi, which is incorrect. Bentley says this should be 40.6 to 46.4 psi. I get 32 psi while idling and also while driving around with my hood propped open which makes the neighbors give me funny looks. If I bump the throttle, this goes up to about 37 psi briefly. If I pinch the fuel pressure regulator's return line, the pressure goes up. If I remove the vacuum line from the fuel pressure regulator, the pressure goes up. Everything I've tested seems to indicate that all is well, except the fuel pressure is low and the car misses/stops accelerating all together under heavy load.

What is left in the fuel system that could be giving me this low pressure reading? Possibly a failing fuel accumulator?

[Smelser]

could be a weak spring in the regulator, even if it IS new, it WASNT built yesterday.
try using an aftermarket inline one

[thebutterson]

Were you getting the same pressure readings with your old FPR? Have you double checked the location you're pulling vacuum from? Is it possible you have the pressure and return lines switched? I'm not sure what the result of that last one is, it's just a random idea.

[redhotchilipepper]

I had low pressure readings with the old FPR and low pressure readings with this one as well. In fact, they have gotten even lower over time (35 psi 6 months ago, 32 psi now).

Vacuum line is properly run to the manifold. FPR is properly mounted to the fuel rail as is the return line to the tank.