mkd

iTrader: (8)

I recently put a rebuild in and went though the break-in period. The car ran great for a couple days then stopped boosting (it never ends...). In the process of troubleshooting the boost issue I noticed something concerning -- the fuel pressure never gets above about 40psi. It sticks at either about 37 when the FPR solenoid is off, and 40 when it's on (see the graph below -- the orange line is fuel pressure). I looked back at Power FC logs I took during the break-in process and the pressure has not been right ever since the motor went in.

• I had the RP pump out and sitting on a shelf for a few weeks (with OEM pump installed) while dealing with emissions testing. The RP pump is now re-installed in the car.
• I pulled the RP pump out and found that the o-ring at the pump outlet was installed incorrectly. That must be it? No. Re-installing correctly did not fix the problem.
• I am pretty sure the soft lines off the firewall are installed correctly into the hardlines under the UIM.
• When logging "Sensors" on the PFC, I see that the uptick in pressure (from about 37 to about 40/41) corresponds exactly to when the FPR solenoid turns on.
• I have not pulled the UIM or rats nest yet but that is likely next.

Help appreciated.

apexhittinbull

iTrader: (21)

Fuel pressure will rise with a boost, it is natural to see atmospheric pressure like NA engine.
Pressure won't rise without the compressed air from LIM pushing the springs inside of a FPR.
Solenoid only exists to prevent flooding in hot start by cutting vacuum to FPR, so yours are working well showing 5psi difference when on.

mkd

iTrader: (8)

The problem is that when the car was boosting to 20psi, the fuel pressure still only increased to 40psi.

DaleClark

iTrader: (55)

Yep something is wrong.

You want a 1:1 relationship with fuel pressure and boost. You have your base pressure, which is manifold air pressure at atmospheric, and it should go lower than that under vacuum and increase above that with boost. So, for example, if your base fuel pressure was 40psi you should see 60psi fuel pressure under boost.

This is easy to test without the car running, just use a Mityvac, shop air, what have you to pump pressure to the nipple on the FPR with the fuel pump running (jumper F/P and GND and turn key on).

The fuel pressure solenoid, as stated, just switches the vacuum source to the FPR from vacuum to atmospheric pressure (it has a little filter) when hot starting the car. That bumps fuel pressure up to compensate for any fuel boiling in the lines on top of the hot engine.

It's possible that solenoid is plumbed in wrong or is stuck.

You definitely don't have fuel lines backwards, if you did the car wouldn't start.

So, you need to check that solenoid and make sure it's good and plumbed in right and check the FPR to make sure its doing the job right. Shouldn't be too bad to troubleshoot. Could also be a logging or sensor problem, may want to back it up with a manual fuel pressure gauge. I think you can even rent a fuel pressure test kit from Autozone or something and tee it into the fuel line.

Dale

Pete_89T2

iTrader: (4)

Which fuel pressure are we talking about here? Fuel pressure in the rails or Differential Fuel Pressure?

Fuel pressure in the rail should increase/decrease in proportion to boost/vacuum or MAP; Differential Fuel Pressure should stay relatively constant at whatever value your fuel pressure regulator is set to.

mkd

iTrader: (8)

I think that only works with stock ECU right? Might have to measure the sensor voltage with a multimeter since it's normally plugged into the datalogit.

That's interesting. I thought it was doing something somewhat different. The reason it's interesting is that, for some time after the rebuild the car wouldn't start when warm. Hot, sure. Cold, sure. Warm? no. Maybe the fuel in the lines started boiling after sitting still for a few minutes.

But then, why is the FPR solenoid being told to turn on while driving? That is what I am seeing in the logs, which when on corresponds to the higher pressure sections seen in the plot above.

This has occurred to me, too. Will need to figure out how to test the sensor. I guess I could make some sort of test rig for it as it just screws into 1/4 NPT threads.

Both. See plot above. "PIM" is "pressure in manifold."

DaleClark

iTrader: (55)

The F/P and GND jumper works with the PFC and most any aftermarket ECU's. That just hot wires the fuel pump to run, ECU has no involvement.

As for the solenoid, there are 2 things in the sensor check screen that are fuel related - FPD, Fuel Pump Operation, and FPR, Fuel Pump Control. One is the 12v/9v switch while the car is driving, 9v for idle and low load and 12v for high load. I think it also bumps to 12v on starting. The other should be for the solenoid that switches the vacuum source of the FPR to atmospheric. I'm not sure which is which, unfortunately, but looking at the Sensor Check screen on the Commander for a few minutes will probably answer that question.

Anyhow, not sure which of the 2 you are logging.

Do you have a fuel pressure sensor hooked up? There is no fuel pressure sensor on a stock engine and the PFC doesn't see it at all. You could install your own and wire it to an AUX port on the Datalogit. Are you looking at something else thinking it's fuel pressure?

Dale

mkd

iTrader: (8)

Cool. For some reason I thought I had tried that in the past and it didn't work with the PFC.

I checked my logs --

FPD: Regulator vacuum disconnect? Active while cranking and running.
FPR: 9v/12v switch. This is what is going on/off while driving in my logs. Turns on around 0psi. This is also where I see ~4psi increase in pressure, so that makes sense.

Yes I have a sensor on the fuel inlet side between the firewall and rats nest hardlines, connected to an aux input on the Datalogit.

Here is what it looked like when working properly before the rebuild:




The scaling is off, but fuel pressure overlays PIM nicely, as you would expect.1


I have a theory. I have been having mice problems recently. I occasionally find gnaw marks on vacuum lines. I bet one of those little fuckfaces gnawed a hole in the one of the FPR vacuum lines. Either the one between the regulator and solenoid, or between the solenoid and manifold.

mkd

iTrader: (8)

Well I guess the fuckface is me.

DaleClark

iTrader: (55)

Yeah that's not helping

Renamed the thread as that was driving me nuts.

Dale

mkd

iTrader: (8)

Ha! Thanks. That was bugging me too.