s5 TII fuel pressure question

[FerociousP]

S5 TII with a Sprint RE
stock s5 turbo
550cc/750cc injectors
FD fuel pump

Car originally had a Rtek 2.0 and ran great. Fuel pressure was 32psi at idle and 39-40 at 0psi or with engine off. Switched to the Sprint RE this week and when we started tuning I noticed that the fuel pressure was around 50 psi at idle. The fuel pressure doesn't change with vacuum and when revving the gauge sways a 1-2 psi either way. I am getting voltage for the pump from haltech fuse block so it is getting full voltage when running as it is no longer going through the fuel resistor box. No other changes to the fuel components were made and I didn't make any changes to the vac/boost reference for the FPR.

Is the FPR damaged?
Is the full voltage causing the FD pump to overwhelm the regulator?
What else could cause this?

*** Subquestion: Is the stock TII fuel pressure regulator rising rate?

[RETed]

Vacuum line was moved or fell off.

It technically not a "rising rate".
"Rising rate" implies a higher ratio than 1:1.
It's technically a "boost dependent" FPR, which is a true 1:1 ratio.
+1psi of boost = +1 psi of fuel pressure


-Ted

[FerociousP]

Ok, good to know about the boost dependent.


The line to the FPR is not off and was the first thing checked. The MAP source for the ecu is being taken from the same source and the map reading is correct. Also, as a test I changed the FPR reference location to the nipple on the outside of the UIM (instead of the inside middle) and found the same results. Whether the line is connected or not, the pressure reads basically the same.

[FerociousP]

Got home to the fuel rail in my mailbox. Installed it and now I have 41-42 psi priming the system. Much better than 50+ psi! Looks like somewhere between removing the old harness and installing the Haltech, the regulator failed. Weird. Oh well. I Pulled a little vac with my mouth and the pressure went down. Everything looks good. I'll be reinstalling the mani and firing it up tomorrow as today is the girlfriend's birthday. This weekend I'll be reloading the base map and retuning as well!

[My5ABaby]

[FerociousP]

ok. Everything is NOT OK. I bolted everything back down and started the car. Fuel pressure stayed at 40psi until revved and went to 50psi. Seems to still coorelate with the battery voltage as I was getting 12.5V at startup and 13.5V volts AFTER i revved. Engine vacuum is at 17inHg and is definitely what the fpr is seeing. After the car warmed up I revved a little more aggresively. It even got a little boost, about 3-4 psi, and the pressure actually started falling!! It fell to just under 40psi as the rpms rose and entered boost. It seems to be doing the OPPOSITE of what it should be! I'm so confused. Ideas anyone?

** With the gauge on the return, I saw NO pressure at all, so I have no restriction in my return line.

With the new (used) secondary rail:

Car off 12V: 40-42psi
13.5V and 17in/hg vac at idle: 50psi
pressure falls with revs

[RETed]

Originally Posted by FerociousP

** With the gauge on the return, I saw NO pressure at all, so I have no restriction in my return line.

This is normal.

I would try and dead-head pressure the fuel pump?
It sounds like the fuel pump might be failing?


-Ted

[FerociousP]

Yeah I was I know its normal, just confirming that it wasn't one possibility. Tested the "dead-head" pressure this morning. 95-97 psi when I clamp off the hose right after the gauge.

Took a few pics to show my setup and to give context to this whole issue.

[FerociousP]

Took a few vids to show some of the symptoms... Pardon the person (Jacob Cartmill) talking in this vid... He's "special".

First is with the fuel pump running. It starts at 41-42psi and when vacuum is put on it, about 20inHg, the pressure barely moves to 40psi or so.


In the second vid, I have turned off the pump. It is still holding about 38-39psi, and when I put vac on the FPR, about 15inHg, pressure drops to 30psi!

[FRED]

An idling engine should read about 28-32 psi, sorta what you saw in your LAST picture above. So that seems to indicate the FPR is doing its job to me anyway.

So the question is...............did you ever check the fuel pump pressure by dead heading the line from the fuel pump/filte to the gauge????????????? I guess not or you'd have said so. So maybe you could do that for us? Just remove the line off the filter and shove it into the fuel pressure gauge and turn the pump on and the pressure you see is??????????????????

Maybe the fuel pressure of the pump is overwhelming the FPR???? Only way to tell is to look at the fuel pump pressure.

About your FPR source of vacuum. It tees into three lines. Where is that third line going?????????

[FerociousP]

Tested the "dead-head" pressure this morning. 95-97 psi when I clamp off the hose right after the gauge (which is after the filter, but before the rails).

The 3rd line goes to the MAP sensor on the Haltech SprintRE. Also goes to the boost gauge. Both the SprintRE and manual gauge read 15-17 inHg with this configuration. When I changed the reference line to go from the nipple on the outside of the UIM directly to the FPR, there was no change in pressure (from the other reference location).

[FRED]

Stk fuel pump pressure is 71-92 and the way I see it you say you have 95-97 which is just a little higher. So my GUESS is that the FPR might need replacement.

Although when you did that bit where the rail read 38-39psi and when you put 15hg vac on the FPR the pressure read close to 30 psi...............that show at least me that the FPR is working right in that respect. Mostly I say that because a stk car idling will read 28-32 psi at idle and that would be having approx 15hg or so inhg on the FPR.

A stk FPR will regulate to approx 37-39psi with the pump running and engine off and your a bit over that with your 41-42 psi. Almost normal.

IF you put Pressure on the FPR with the pump running what kind of pressure does the fuel rail read??? I'd estimated that a stk rail would read approx 50psi doing that. I remember doing that to a stk car, stk fuel pump sometime back. I don't remember it going over that 50 psi.

Anyway maybe someone else can figure out what is going on.

[FerociousP]

Took some vids today.

[NoDOHC]

The FPR regulates fuel pressure by dumping fuel to the return line. If the pump is flowing more fuel than the FPR can handle, it will not be able to regulate pressure. The fuel pressure falling when under boost is an indication that this is the issue, as the injectors are using some of the fuel from the fuel pump, allowing the FPR to dump enough.

I would look into replacing the FPR with either a bigger aftermarket one or a newer (FD) stock one. I have not had this problem, but I have never required a bigger fuel pump than stock either.

[RETed]

Originally Posted by NoDOHC

The FPR regulates fuel pressure by dumping fuel to the return line. If the pump is flowing more fuel than the FPR can handle, it will not be able to regulate pressure. The fuel pressure falling when under boost is an indication that this is the issue, as the injectors are using some of the fuel from the fuel pump, allowing the FPR to dump enough.

I would look into replacing the FPR with either a bigger aftermarket one or a newer (FD) stock one. I have not had this problem, but I have never required a bigger fuel pump than stock either.

That doesn't make sense?
The FPR is a mechanical bleed-off device - nothing fancy.
With the fuel injectors firing, the fuel flow through the whole system is less.
If the FPR is working correctly, this should not be a problem.
The problem arises when there is too much flow, and this "overpowers" the FPR - you end up with too much fuel pressure at the rails.

If the FPR is okay, then dropping fuel pressure is indicative of a fuel leak or a bad / failing fuel pump.

It's funny you started this thread...
I have a customer whose S5 FC turbo FPR just failed just like yours, except his was stuck at 60psi at all times.
He swapped it out and running parallel rails with an Aeromotive FPR now.
:P

As another guess at the problem...
Can you confirm the fuel pump is getting "full voltage" at all times?
Run a DMM / voltmeter to the fuel pump leads as close to the fuel pump as possible and monitor...


-Ted