Interesting but I'll have to try it to believe it. My disbelief arises from several factors. First is that I don't think the stock FPR has a damper built into it. The only thing I see is the spring and diaphragm assembly for setting the pressure. Next is that it's hard to believe that there is insufficient fuel flow to be allowing this to happen. Since its rpm-specific, it would seem to be a resonance-driven issue. Anyhow, I'll log fuel pressure as soon as I get a chance. A 18% fuel reduction will require about a 30% fuel pressure drop. Should be easy to see.

 

The theory may carry water though, I know at idle, my car suffers from light pulsation which can be seen on the fuel pressure gauge which i've got mounted to the fuel rail.

This stops once I bring up the rpm from idle.

 

Another issue I have with the concept of a lack of a pulsation damper is that I have yet to see one that is properly designed to work with a variable pressure system, i.e., a typical port injection fuel system like what's on our Evos. Dampers need to be tuned to the operating pressure and are typically only effective within about +/- 20% of that pressure. A proper damper for a our fuel system would need to have a manifold reference just like an FPR. The dampers I have seen have no such reference.

 

Here is some info on troubleshooting from the Evo manuals:

4. Connect scan tool MB991502 or MB991958 to the data link
connector.
5. Use the Actuator test 07 to drive the fuel pump. Check that
there is no fuel leaking from any section when the fuel pump
is operating.
6. Stop the fuel pump.
7. Start the engine and run at idle.
8. Measure fuel pressure while the engine is running at idle.
Standard value: Approximately 230 kPa (33 psi) at
curb idle
9. Disconnect the vacuum hose from the fuel pressure
regulator and measure fuel pressure with the hose end
closed with your finger.
Standard value: 289 − 309 kPa (42 − 45 psi) at curb idle
10.Check to see that fuel pressure at idle does not drop even
after the engine has been revved several times.
11.Revving the engine repeatedly, hold the fuel return hose
lightly with your fingers to feel that fuel pressure is present in
the return hose.
NOTE: If the fuel flow rate is low, there will be no fuel pres-
sure in the return hose.
12.If any of fuel pressure measured in steps 8 to 11 is out of
specification, troubleshoot and repair according to the table
below.

SYMPTOM PROBABLE CAUSE REMEDY
• Fuel pressure too low
• Fuel pressure drops after racing
• No fuel pressure in fuel return
hose
Clogged fuel filter Replace fuel filter
Fuel leaking to return side due to
poor fuel regulator valve seating or
settled spring
Replace fuel pressure regulator
Low fuel pump delivery pressure Replace fuel pump
Fuel pressure too high Binding valve in fuel pressure
regulator
Replace fuel pressure regulator
Clogged fuel return hose or pipe Clean or replace hose or pipe
Same fuel pressure when vacuum
hose is connected and when
disconnected
Damaged vacuum hose or
clogged nipple
Replace vacuum hose or clean
nipple
Defective fuel pressure regulator Replace fuel pressure regulator
AK204070AB
MB991502
16-PIN
AK204071AB
MB991911
16-PIN
MB991827
MB991824
AK202411
ON-VEHICLE SERVICE
TSB Revision
13A-764 MULTIPORT FUEL INJECTION (MFI)
13.Stop the engine and observe fuel pressure gauge reading. It
is normal if the reading does not drop within two minutes. If it
does, observe the rate of drop and troubleshoot and repair
according to the table below. Start, then stop the engine.
(1) Squeeze the fuel return line closed to confirm leak-down
occurs from defective fuel pressure regulator.
(2) Squeeze the fuel supply line closed to confirm leak-down
occurs from defective fuel pump check valve.
(3) If pressure continues to drop with both fuel lines
squeezed closed, injector(s) are leaking.
14.Release residual pressure from the fuel pipe line.
SYMPTOM PROBABLE CAUSE REMEDY
Fuel pressure drops gradually
after engine is stopped
Leaky injector Replace injector
Leaky fuel regulator valve seat Replace fuel pressure regulator
Fuel pressure drops sharply
immediately after engine is
stopped
Check valve in fuel pump is held
open
Replace fuel pump


I hope this helps with the troubleshooting for the slight lean at 2250-2750

 

My thought is the aftermarket fpr regulator valve is not seating well at low rpms.

Are these the rising rate type? Is there a way to modify the stock fpr to hold better pressure having the best of both worlds?

Another question IIRC MrFred is using higher fuel pressure and voltage to the pumps. Can the onset of the increase fuel flow and pressure be overriding the fpr up to a point allowing fuel to bypass to the tank again?

Still be a good idea to log the fuel pressure regulator solenoid to make sure it is cutting off properly at the correct time. is it possible that the solenoid cycles slowly during that rpm range thus allowing lower fuel pressure and the lean condition? Is there a 2d table that controls this versus rpm ?

 

Ive had this occur on three different regulators.
With and without the FPR solenoid connected.

 

From what I've seen in the ECU code, the FPR vacuum cut off solenoid is only utilized to deal with vaporlock during startup. It is not used during normal operation. And besides that, I removed it from my car.

I did some reading on the internet about the fuel pressure instability issue due to resonance, and there are claimed examples out there. What I found more interesting is that one guy found his problem with using an aftermarket FPR was elminated by moving the regulator from the engine block to the firewall. My FPR is attached to the intake manifold, so I'm wondering if a vibration resonance is causing it. Balance shafts are installed, and I don't feel any additional vibration at that rpm, but perhaps it could be this. At any rate, logging rail pressure should provide insight, but its going to be a few weeks before I can do this again.

 

I suspected the same as mine was also on the inlet manifold.

Relocated to the firewall and it didn't make a change.
My car is setup for logging fuel pressure off the rail, it's in storage atm but I'll try and grab it as get some logs.

 

+1^ My Aeromotive FPR is mounted to the firewall.
Still been having a similar lean issue. Driving me nuts

I am also logging off of the rail with an AEM sending unit.

 

Are you seeing a differential fuel pressure (rail minus boost) drop in the lean area?

 

I'm back in town and was able to take my Evo out this evening to log fuel pressure across the injectors. I turned off unneccessary data acquistion which increased the data logging rate to one point ever 0.05 sec, so 20 points per second. If it was a fuel pressure issue such as a resonance centered in time right on injector opening, pressure across the injector would have to drop by 33% to produce the 18% lean AFR I am seeing. For the 44 psi base pressure I'm using, a 33% lower fuel pressure would be 30 psi. It could be argued that a resonance would produce a very momentary drop in fuel pressure which is the reason I increased the data acquisition rate to 20 Hz. In this scenario, I wouldn't ever expect to see 30 psi because of limitations in the speed of the sensor and the low chance of a sensor reading taking place right at the moment of the pressure resonance, but I should be able to see a drop down to the high 30s if there was a resonance. As I expected, fuel pressure was rock-solid at 44 psi. No signs of any momentary drop in fuel pressure, i.e., not one data point was below 42 psi.

I'm back to looking for other reasons. I think I've mentioned before that I've noticed that the motor is not always entirely smooth in this rpm range, and that this makes me wonder if it could be a load calculation error. It occurred to me this evening that perhaps its a misfire. No idea what might be causing it, but it would cause a lean condition and make the motor feel a bit more rough.

 

Try those Misfire maps mrfred.

 

Running a built head with S3s cruising around 2400rpms certainly feels more like a misfire to me. My AFR swings but it never goes LEAN. This became much more pronounced when I swapped to the S3s. I noticed changing the TPS % Delta helped out a little bit, and it generally happens when the TPS is around 24%. I figured this was an issue when the ECU wasn't sure whether or not it should fuel cut or not.

 

quick update: i looked at the misfire tables that merlin mentioned and found that they will cause fuel cut when a certain level of misfire is detected. however, they are only active when the p0300 misfire diagnostic is running, and i have it turned off on my evo, so the the tables are not active. i also drove my evo a bit more, and if its misfire, its very subtle. for the most situations, the motor is perfectly smooth when traversing the rpm range where AFRs always go lean.

as far as the 2150s go, they are awesome. best driving injector i've had in my evo.

 

I agree, mine is very smooth when traversing this area.. It's still just the very very light throttle almost completely off the throtte. It's the 15%-25% area that starts to stutter for me... but perhaps it's my cams being grumpy.