Fuel Pressure

[mcstang]

I've been messing with my predator and been looking at fuel pressure.
05 GT Auto.
Car bogs down when you accelerate up to 2000revs although she's fine at highway speeds.Been like this for the past 20000miles.
Engine note is also no where near as good as it used to be.
Only mods are C&L and SLP's.
Changed plugs,fuel filter, transmission fluid,cleaned maf,replaced throttle body,checked cats.. etc bloody etc.
So now looking at fuel pressure.
From a warm engine she idles at 39psi which I understand is normal but when I pump the pedal (stationary) she drops to 32psi.
Also sometimes I can hear a little squeaky noise coming from under the rear seats. Is the pump under there?
No check engine lights and no codes.
MPG is fine I guess.
Is this drop to 32psi normal?

Thanks

 

[Paps]

Fuel pump is located under the seats on driver side. Maybe your pump is diying, that's why you can hear it crying!

 

[kojack]

Fuel filter, fuel pump in gas tank!!!

 

[skwerl]

When was the last time you changed your fuel filter? It is located along the frame rail behind the left front tire. Consult your owner's manual for specifics.

 

[mcstang]

Thanks folks.
Fuel filter was changed about 2000 miles ago and again, 10000 miles before that.
What about this drop in pressure?
Surely it would rise if you pump the pedal?
Car bogs worse when going up hills but is generally crap unless your doing 70mph+.
Cheers

 

[skwerl]

Sounds like a problem. Fuel pressure should remain at 39psi regardless. If it drops then that indicates your engine is using fuel faster than the pump can deliver. It could also be a programming issue. Do you have a tune in the car or is it on the factory tune?

 

[petersonb]

If you are on factory tune, I would guess fuel pump. I had the same pressure drop thing happening, and it was the pump for me.

 

[mcstang]

Cheers Gents,
I have the bogging down regardless of what tune is in the car.
She did have the stock tune when she initially went bad.
I then bought a predator and regardless of what tune is in she still bogs down.
My car has been doing it for nearly two years, she still drives and is still awesome on the highway but just hates it when I accelerate up to 2000 revs.
Did your engine note change as well?
Mine sounds like it's shitting itself whereas she used to sound awesome.
Thanks

 

[petersonb]

Cheers Gents,
I have the bogging down regardless of what tune is in the car.
She did have the stock tune when she initially went bad.
I then bought a predator and regardless of what tune is in she still bogs down.
My car has been doing it for nearly two years, she still drives and is still awesome on the highway but just hates it when I accelerate up to 2000 revs.
Did your engine note change as well?
Mine sounds like it's shitting itself whereas she used to sound awesome.
Thanks

Have you datalogged? That's where I would start. My fuel pump duty cycle (fpdc) was okay at idle, but any time I asked more of it than that, it would peg out at 50.0 (corresponds to 100%). Also, my rail pressure dropped to about 20psi (measured off the rail, not through the datalogger) and the AFRs went lean.

You can also check the vacuum hose on the rail pressure sensor; if you have gas in that vac line, the diaphragm is likely busted and that would likely be your issue. A dealership would have one in stock for around $150, and it takes about 5 mins to swap.

That being said, i think it's more likely to be your pump than it is your frps (fuel rail pressure sensor, but the dealership would call it a fuel injection pressure sensor).

 

[petersonb]

Ummm just saw you're in Scotland. I can't say if the dealerships there have a sensor in stock, haha

If you need something like that shipped, PM me.

 

[mcstang]

Thanks for your continued help.
I need to go out this evening, but here's some stuff I logged earlier.
I have no idea if any of this makes sense to anyone but I thought it may be of interest;
47% FLI
22.5% FP
39PSI FRP
2.71-2.76 FRP V
2.56-2.58 FTPV
127.39H20 FTP
112F IAT
1.98V IAT V
-3% LONGFT1
-3% LONGFT2
4.96 GMSMAF
0.74V MAF V
0.74AMP HTR12CM
1.25AMP HTR11CM
0.01AMP HTR22CM
0.02AMP HTR21CM
0.86V 02S12
0.11V-29-63-73% O2S11 (this one is all over the place)
0.84v 02S22
0.83V 02S22SV
13.5V VPWR
0V MFFTP

All taken at idle with a warm engine using a Diablo Predator U7140.
I can only compare driving my car to driving a manual in 5th gear all the time.
It stops and starts and is fine at highway speeds but at lower speeds,as soon as you press the pedal she bogs,especially bad on hills.
Oh and she was perfect until I put a new battery on her 2 years ago-got a couple of jump starts using leads.
Relevant?


Cheers

 

[petersonb]

Thanks for your continued help.
I need to go out this evening, but here's some stuff I logged earlier.
I have no idea if any of this makes sense to anyone but I thought it may be of interest;
47% FLI
22.5% FP
39PSI FRP
2.71-2.76 FRP V
2.56-2.58 FTPV
127.39H20 FTP
112F IAT
1.98V IAT V
-3% LONGFT1
-3% LONGFT2
4.96 GMSMAF
0.74V MAF V
0.74AMP HTR12CM
1.25AMP HTR11CM
0.01AMP HTR22CM
0.02AMP HTR21CM
0.86V 02S12
0.11V-29-63-73% O2S11 (this one is over the place)
0.84v 02S22
0.83V 02S22SV
13.5V VPWR
0V MFFTP

All taken at idle with a warm engine using a Diablo Predator U7140.
I can only compare driving my car to driving a manual in 5th gear all the time.
It stops and starts and is fine at highway speeds but at lower speeds,as soon as you press the pedal she bogs,especially bad on hills.


Cheers

EDIT: I'd say you need to check out the O2 sensors. At idle, should be moving pretty good, I think.

Can you upload the datalog file, like as an attachment?

Fwiw, my fpdc was around 17-18 at idle with stock cams, and now is around 19-20 (less vacuum means fuel pump has to push harder.... Higher duty cycle). 22.5 fpdc means your pump is running 45% of its capacity just at idle. Seems high to me, but I bought this car with aftermarket pump, so that might be on track for a stock pump. I'm not sure.

Also, when I had bad O2 sensors, it was nearly impossible to accelerate without the motor really cutting out and sputtering.... But got better and went away once the motor had warmed up. Maybe yours are worse than mine were? Someone more experienced can make that call.

Again, I'd like to see the datalog, if possible.

-Brad

 

[petersonb]

Oh and she was perfect until I put a new battery on her 2 years ago-got a couple of jump starts using leads.
Relevant?


Cheers

I've never personally experienced it, but people say crazy things happen with these cars when the alternators are bad.

I don't know much on the matter, but something to keep in mind if you run out of other possibilities.

 

[petersonb]

/nevermind

 

[mcstang]

Hey Brad,
I'm learning daily how to use the Predator so I'm fairly hopeful I can data log and post a file.
Watch this space.
Thanks again

 

[RocketcarX]

For what it's worth, according to my Autometer fuel pressure gauge reads (sensor is on the fuel rail) about 10 or so PSI below what I'm told my car should be at. THe most pressure it ever shows is occasionally a spike to 40 PSI and then it settles back down to somewhere around 30 PSI. My car runs great all the time, doesn't seem to have any issues and doesn't detonate or anything. I'm not sure if the car had a tune on it when I bought it, although I think it does because it isn't happy with any fuel less then 93 octane.

 

[petersonb]

For what it's worth, according to my Autometer fuel pressure gauge reads (sensor is on the fuel rail) about 10 or so PSI below what I'm told my car should be at. THe most pressure it ever shows is occasionally a spike to 40 PSI and then it settles back down to somewhere around 30 PSI. My car runs great all the time, doesn't seem to have any issues and doesn't detonate or anything. I'm not sure if the car had a tune on it when I bought it, although I think it does because it isn't happy with any fuel less then 93 octane.

The 39.15 psi is "delta pressure" used for returnless fuel systems.

Delta pressure is "Rail Pressure" (what your gauge displays) - "boost/vacuum"

At idle on a stock car, the vacuum is 19-20 in. Hg, or about 9-10psi.

At WOT on a na car, it's closer to zero vacuum.

Idle: 30psi (displayed on gauge) - (-9psi) = delta pressure = 39
WOT: 40psi (displayed on gauge) - (0psi) = delta pressure = 39

On my car with 14psi boost, mine is 60psi on gauge minus 14psi boost = delta pressure = 45psi

Delta p is controlled by your tune, and that's what the data logger will display. The pressure at the rail is sometimes called "actual pressure" and is different from delta p.

Hope that made sense......

 

[RocketcarX]

The 39.15 psi is "delta pressure" used for returnless fuel systems.

Delta pressure is "Rail Pressure" (what your gauge displays) - "boost/vacuum"

At idle on a stock car, the vacuum is 19-20 in. Hg, or about 9-10psi.

At WOT on a na car, it's closer to zero vacuum.

Idle: 30psi (displayed on gauge) - (-9psi) = delta pressure = 39
WOT: 40psi (displayed on gauge) - (0psi) = delta pressure = 39

On my car with 14psi boost, mine is 60psi on gauge minus 14psi boost = delta pressure = 45psi

Delta p is controlled by your tune, and that's what the data logger will display. The pressure at the rail is sometimes called "actual pressure" and is different from delta p.

Hope that made sense......

I think that makes sense, not to thread jack here, but I think you're saying that my reading are normal because of the difference between "delta" pressure and "actual" pressure being the fuel overcoming the engine vacuum or boost, whichever mode it is in for your application. Or are you saying since an NA motor never sees boost that my readings are actually close to 10 PSI off? I'm not sure why the gauge at the rail would be affected by "delta" pressure if the measurement is taken before the injector.

Does the gauge not read 60 with the actual fuel pressure being 60 even though 45 is what the motor actually sees at boost? How does the ECM know what the pressure is (delta) after the injector?

 

[RocketcarX]

The 39.15 psi is "delta pressure" used for returnless fuel systems.

Delta pressure is "Rail Pressure" (what your gauge displays) - "boost/vacuum"

At idle on a stock car, the vacuum is 19-20 in. Hg, or about 9-10psi.

At WOT on a na car, it's closer to zero vacuum.

Idle: 30psi (displayed on gauge) - (-9psi) = delta pressure = 39
WOT: 40psi (displayed on gauge) - (0psi) = delta pressure = 39

On my car with 14psi boost, mine is 60psi on gauge minus 14psi boost = delta pressure = 45psi

Delta p is controlled by your tune, and that's what the data logger will display. The pressure at the rail is sometimes called "actual pressure" and is different from delta p.

Hope that made sense......

Wait...was that supposed to say "Actual" pressure?

 

[petersonb]

I think that makes sense, not to thread jack here, but I think you're saying that my reading are normal because of the difference between "delta" pressure and "actual" pressure being the fuel overcoming the engine vacuum or boost, whichever mode it is in for your application. Or are you saying since an NA motor never sees boost that my readings are actually close to 10 PSI off? I'm not sure why the gauge at the rail would be affected by "delta" pressure if the measurement is taken before the injector.

Does the gauge not read 60 with the actual fuel pressure being 60 even though 45 is what the motor actually sees at boost? How does the ECM know what the pressure is (delta) after the injector?

I was saying your car is acting perfectly normal. You would see 30psi on your gauge because you have about 9-10psi vacuum if stock/na.

30psi minus a negative 9, which means addition of 9psi, equals the 39.15 which stock tunes are set to for delta P.

I am in no way an expert on this stuff, but I think the computer knows delta pressure because the pressure sensor on the rail connects to the vacuum line, and also measures fuel pressure at the rail on the underside of the sensor.

Somehow, though, the computer determines a value for "pressure drop across injector" or something like that. I see it when I datalog. I'm not really sure how it comes up with that number.

Not too bad a thread jack since it kind of relates to the issue at hand, because diagnosing fuel systems requires some background knowledge! My 2c ...



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