Sideways7

iTrader: (7)

Ok, long story short, I am only getting about 25-30 psi of fuel pressure.

I have a new fuel filter, new in-tank fuel filter (fuel sock), and I just broke down and bought a brand new Bosch fuel filter from Advance Auto. The pump is getting battery voltage. And I still have 30 PSI of @#^@#$ fuel pressure.
What else is there that can cause this?
Could a bad fuel regulator cause this? I have an s4 with an s5 intake on it, meaning I have the s5 fuel rails, if that has anything to do with it.

I really need some help, because I am at a loss.

Any help is appreciated.

fcdrifter13

Is this running pressure, or just kicking the pump on with the jumper wire

Sideways7

iTrader: (7)

Running pressure is 30-35 with the new pump. Just jumpering it is a little below 30.

fcdrifter13

isnt the pressure only 34-39psi?

KhanArtisT

iTrader: (2)

A kinked fuel line can cause low fuel pressure, something to check.

Sideways7

iTrader: (7)

Well, when I originally put in the pressure gauge, it was directly at the output of the fuel pump, and it read the same pressure. Also operating pressure is about 28 PSI at low throttle, and about 39 psi at high throttle. The pump is supposed to put out 65-80 psi. It will actually drop into the low 20's at high rpms.

Sideways7

iTrader: (7)

Well, I still have my old fuel rail, so I think I will dig it out tomorrow and test to see if I can get decent pressure with it.

HAILERS

I'm guessing your doing the pump output check all wrong. You get a D- for that.

But I could be wrong.

The output pressue of the PUMP ONLY, is by installing a fuel gauge in the outlet of the fuel filter. Only the gauge is in the system. The fuel rail is NOT connected in any way when you do this.

So the pressure with the jumper in should be the ??? 70-80 psi (see FSM for good figures).

If you have a gauge tee'd into the line coming out of the fuel filter, then the pressure should read approx 37psi with the yellow plug jumpered with Key ON. That Tee'd pressure is the fuel RAIL pressure.

Or idle the engine and the figure should be approx 28psi...but I usually see 31-32. Fuel RAIL pressure.

And if I read your thread wrong.......I get a F minus and no supper tonight.

Sideways7

iTrader: (7)

Well, that makes perfect sense, and I feel like an idiot.
I still think the fuel pressure is low, but I think I'm going about it wrong.
Hailers, I think you already know this from my previous thread, but the a/f ratio is good until I get to a certain throttle position. In fact, it seems like a certain vacuum, since the throttle position depends on the RPM. Once it reaches that place, it IMMEDIATELY goes to super lean, almost to the point of non-combustion (I am getting lean hesitation at high RPMs)

Now my understanding of the fuel pressure based on reading the FSM is this: it is roughly 28 PSI in low throttle/high vacuum situations, then bumps up to 39 psi at high throttle/low vacuum situations.

So my theory is that for whatever reason, the fuel pressure isn't ramping up like the ECU is expecting it to, creating the sudden lean condition. Is this sound reasoning, or way off base?

So this comes back to the fuel pressure regulator: maybe it isn't switching to the high pressure mode?


EDIT: One odd thing I have noticed recently is that when I first start the car, I can floor it and the a/f ration will dip to stoich, then go right back to rich. Once the car reaches close to operating temperature it goes back to what it normally does, going super lean.


Edit #2: I just went and pulled the vac line off of the fuel rail and it made no difference in fuel pressure. It reads a pretty steady 31-32 psi at idle.

Sideways7

iTrader: (7)

I just had a genius moment (maybe).

- When I first installed the new pump, it read up to about 40 PSI. This is what it should be with the test connector jumpered.

- When I tested it later after driving it, it was only reading in the 25 PSI range under the same conditions.

- When I start the car, it runs ok a/f wise.

- When it warms up, it runs lean when the fuel pressure should be bumped up.


This tells me that something is definitely happening when it warms up. The first thing that comes to mind is the fuel pressure regulator. Or maybe the fuel pump voltage is dropping (fuel pump relay problem). The most obvious is the fuel pressure regulator.

So does that make sense to anyone else?

HAILERS

Well, you could put a vaccum hose on the FPR.....turn the key to ON........jumper the yellow connector.........then watch the fuel gauge as you put a vacuum on the FPR and then put presssure on the FPR. The gauge should respond.

I'd use my MITTYVAC if I were doing that. Been there, done that some yrs ago. Even on a non turbo, if you put 10psi on that FPR line, the fuel pressure will come up close to a even 50psi. I don't remember the FPR pressure when I put a vacuum on it.

Anyway, that would verify if the FPR was any good. That's IF the fuel pump deadhead pressure is in the 70-80 psi range.

Sideways7

iTrader: (7)

Well, I went out last night and did a bit more testing. I started up the car and let it idle. It was sitting at around 32 PSI. When I pulled the vac line off the FPR, the pressure stayed the same.

arghx

iTrader: (3)

^ measured after the rail/FPR correct?

Sideways7

iTrader: (7)

All of these measurements are taken at the outlet of the fuel filter. The sender is tapped into a fitting which is put inline with the fuel line.

Sideways7

iTrader: (7)

Ok, so I've done some more testing.
Yesterday, I ran the car and to see if the fuel pressure was effected by temperature. I started off and almost immediately ran full-throttle to redline. The afr's stayed the same as I have observed when the car is cool: they dipped to stoich for a split second, then went back to slightly rich. What I noticed for the first time was that the fuel pressure was the same as it always is regardless of temperature: it stayed around 32, and lowered at high rpm, high throttle situations.
What this tells me is that the ECU is trying to run the car VERY rich when cold (which I am pretty sure is what it is supposed to do), which is somewhat compensating for the lack of fuel pressure.

Second, I tested the static pressure of the fuel pump. I did this by jumpering the fuel pump connector and turning the key to 'ON', then I used pliers to clamp the hose shut after the pressure sensor. It ran up to around 60-62 PSI, so I know the pump connection is good.

So what this leaves me with is that either the fuel pressure regulator is bad, or something isn't flowing well. The latter is unlikey, since all of the hoses and filters are brand new with about 200 miles (hoses) and 10 miles (filters) on them.

So what do you think, is that a reasonable assesment?


Also, I made a video of the fuel pressure and afr's when reving in neutral (I have a flat tire so I can't drive it). I will post it hopefully tonight.

eage8

iTrader: (9)

when it warms up it idles lower so the fuel pump is getting less Volts? (because the alt is spinning slower)

what kind of FPR do you have? a stock one? Is the vacuum hose going to it leaking?

Sideways7

iTrader: (7)

No, the fuel pressure change when warm I am talking about is while I am driving the car. At idle it is around 32 psi. It holds that pressure until I get to 5k+ at full throttle, and will get down to the low 20's at 7k and full throttle.

Sideways7

iTrader: (7)

Well, I went ahead and replaced the secondary fuel rail (which includes the FPR) and it fixed the problem! So after all of my problems, it was the damn fuel rail, so that means that its probably had the problem since I started using it. Anyway, its fixed.

So if this happens to you, remember to check your fuel pressure.