Fuel pressure gauge won't read over 40 psi

[bergs]

I have an auto meter fuel pressure gauge that won't go above 40 psi when in boost. It's a 2002 gt with a p1sc. The air fuel ratio is great and the tune is spot on. It's been supercharged for 6 years and I never had a problem. Could the gauge be bad?

 

[01yellercobra]

Has the gauge been on the whole time?

 

[bergs]

I put the gauge in years ago and was worried that it read low so I went back to my tuner and he dyno'd it and logged the fuel pressure. He said it looked good, see the graph below. back then it would read 40 psi at WOT and jump up when I shifted. now it reads 35 psi at WOT and only jumps up when I shift sometimes. back then at idle it would be at 28-30 psi, now it reads 20 at idle. My system is stock with GT supercar pump in the stock basket and 42 lb/hr injectors. My tuner said it was maxing out the pump at 6000 rpms but below that it was ok. I took off the sensor to see if there was any junk in there and it was clean. I'm hoping it is a bad sensor and I can just replace it. I have an SCT X3 can I watch fuel pressure on it as I drive? I really never used it. I don't use the car a lot so that's why I never looked deep into this but now that the gauge is reading really low I am worried.

Here is a vid and some pics:

https://www.youtube.com/watch?v=UY0PlcP72zY

 

[bergs]

heres another vid, sorry for the bad quality. its hard to film and drive.

https://www.youtube.com/watch?v=2osM_ICb8AI&list=UU2twJngDGgcMVnGzRHRnQ6w

 

[MaximumVelocity]

You did take the valve core out of the fitting on the rail, right? You can rig up a fitting and hose and apply compressed air (with the sensor off the car of course) and watch the gauge to see of it still tops out at 40psi.

 

[bergs]

yes i took the schrader valve out. I am trying to data log with sct live link but I think my x3 might not work with it. It is an X3 power flash part number 3000. The serial number starts with X not XP. Does this mean I can't use the live link program? It wont connect to my car when I try to start.

 

[bergs]

ok so I figured out that I need the older program and was able to get a good data log. It looks like the gauge is reading correct at WOT but it reads 20 psi when cruising even though the data log says it is at 40 psi. I fear there is a problem with my fuel pump. Can I send the datalog file to someone to interpret? It would be awesome if someone who knows what they are talking about could take a look at it. is .500 on the fuel pump duty cycle 100%?

 

[bergs]

I also wonder if on the GT's returnless set-up it is supposed to be at 40 psi all the time. It seems like it is trying to keep at 40 psi and it spikes when I let out of WOT. I could be wrong but I'm just trying to make sense of it.

 

[01yellercobra]

Are you watching fuel pressure or pressure drop across the injectors? The ECU tries to keep 40psi on the intake valve side of the injector. So if you're boosted the fuel pressure has to rise to keep that 40psi. If your blower makes 10psi then the fuel pressure has to be raised to 50psi to keep the 40psi on the intake valve side. If you're watching pressure drop across injectors then it'll always show 40psi. Well, ideally it will.

 

[bergs]

I'm reading fuel rail pressure. I think I need a boost a pump

 

[bergs]

I really think this has to do with the way i installed the supercar fuel pump. I didn't use submersible hose, i used the stock connector that the stock pump uses and I think it is a bad seal that allows leakage. And thats why i'm not getting good fuel pressure and i'm maxing out the pump at 5500+ rpms. I'm going to drop the tank and re-install it using the submersible gates hose and see if that solves the problem. If that does not work i'm thinking of using this pump:

http://www.lethalperformance.com/sh...el-pump-module-assembly-excl-03-04-cobra.html

Anyone have any experience with this one? i think either way i'm going to add a BAP when its all said and done

 

[bergs]

so I installed that new walbro 290 lph pump. I took it out for an easy shakedown to make sure the install went ok. No leaks so I data logged a little low boost 1/2 to 3/4 hits in 2nd gear just up to like 4000 rpms. The fuel pressure gauge still reads like 36-40 psi. The datalog shows that the fuel pressure is trying to stay at 40 but the duty cycle goes up in a steady up hill line. the fuel pressure will flutter sometimes up to 50, sometimes below 40 but it kinda averages out to about 40 while the duty cycle is a constant increase.

I'm starting to wonder if the thing is supposed to keep 40 psi at the rail at all times even under boost. Funny thing is when at idle the fp gauge reads 20 psi and the handheld reads 40 psi like it should with 20 in/lb of vacuum. Anyone want to look at the data log? I'm going to do a more aggressive log soon and probably take it back to my tuner for a re-tune because of the new pump and to verify the a/f ratio.

I'm also thinking of doing the wire upgrade and BAP at this point

 

[TXEnginerd]

I can't really offer any explanation as to why your gauge seems to be reading lower than you expect it to, but for what it's worth, my electric fuel pressure gauge attached at the schrader valve location always reads about 10psi higher than the mechanical gauge on my regulator of my return fuel system. And it definitely never reads the same as the fuel pressure drop across injectors PID because that is not measuring outright fuel pressure. It's the difference between fuel pressure and manifold pressure. Bottom line is I don't use the electric gauge as a tuning tool. I just have it for peace of mind since I can't read the mechanical gauge under the hood while driving. I think having your tuner double check the A/F is a good idea, but beyond that I wouldn't worry too much about what the actual value is on the gauge unless it goes to 0.