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Lower than Normal Fuel Pressure - Subaru STi
Lower than Normal Fuel Pressure - Subaru STi
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Old 08-01-19, 02:20 PM   #1
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Default Lower than Normal Fuel Pressure - Subaru STi

I recently had my car tuned on Flex Fuel. I noticed after going WOT for the first time after the tune, my Fuel Pressure was lower than my last tune. My last tune was at roughly 60-65psi of Fuel Pressure when WOT. I took some datalogs and will post them separately. I know my injectors compensated for the lower fuel pressure because my car still felt like it was pulling strong. Any ideas? my mods are as follows
Turbo XS TMIC
Tomei ELH
Grimmspeed EWG Uppipe
Tial 38mm Wastegate with .9bar spring
Cobb SF
Cobb FPR
Cobb Flex Fuel Kit with FPS
AeM Fuel Pump
1050x injectors
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Old 08-01-19, 02:27 PM   #2
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https://drive.google.com/open?id=1wb...geJzmxShj6L2zh
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Old 08-01-19, 02:30 PM   #3
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https://drive.google.com/open?id=1Av...0ba3UyWUtKhVrS
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Old 08-01-19, 03:53 PM   #4
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Default Re: Lower than Normal Fuel Pressure - Subaru STi

I just requested access. Feel free to email me the logs at [email protected] too.

Happy to look them over.

Josh
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Old 08-01-19, 05:17 PM   #5
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Originally Posted by [email protected] View Post
I just requested access. Feel free to email me the logs at [email protected] too.

Happy to look them over.

Josh
just added you to those datalogs, thanks
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Old 08-01-19, 06:59 PM   #6
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Default Re: Lower than Normal Fuel Pressure - Subaru STi

Do you have a datalog from when you were making 65psi of fuel pressure?

Post tune:


Pre tune:
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Old 08-02-19, 04:12 AM   #7
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Default Re: Lower than Normal Fuel Pressure - Subaru STi

* Injectors don't compensate for for anything. They open and close in response to how they are driven. That's all they can do. Perhaps you meant injector duty cycle? I see it rising while boost is falling. This in itself is not normal in our cars. Normal pressure 43.5+ "boost" - so it looks to me as if your fuel system isn't keeping up. Fuel pressure is a mechanical adjustment with a setting only for base pressure. I see high enough RPM and IDC to indicate the car is in open loop i don't think so there should would be compensation applied to correct. I'd be concerned about AFR.

Again: 43.5+19 (ish)=62.5 Fuel pressure should track boost - I believe you are on the right track thinking something is wrong. All I/(we?) can tell is it has to do with fuel pressure which you already knew.
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Last edited by mheyman; 08-02-19 at 09:20 AM.
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Old 08-02-19, 11:33 AM   #8
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Quote:
Originally Posted by mheyman View Post
* Injectors don't compensate for for anything. They open and close in response to how they are driven. That's all they can do. Perhaps you meant injector duty cycle? I see it rising while boost is falling. This in itself is not normal in our cars. Normal pressure 43.5+ "boost" - so it looks to me as if your fuel system isn't keeping up. Fuel pressure is a mechanical adjustment with a setting only for base pressure. I see high enough RPM and IDC to indicate the car is in open loop i don't think so there should would be compensation applied to correct. I'd be concerned about AFR.

Again: 43.5+19 (ish)=62.5 Fuel pressure should track boost - I believe you are on the right track thinking something is wrong. All I/(we?) can tell is it has to do with fuel pressure which you already knew.
Thanks for the response. What’s interesting is that my AFR is spot on
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Old 08-02-19, 05:03 PM   #9
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Default Re: Lower than Normal Fuel Pressure - Subaru STi

I didn't see the pre-tune before. It looks like what we'd expect. Your injector duty cycle is higher at the right with resulting higher pressure . . . you mentioned injectors? You modded with larger injectors? Then you could run less fuel pressure, - it could be set up that way. Most cars use IDC, but changing fuel pressure is sometimes done - -

Wait -
* do you have an adjustable regulator?
* was it tuned previously with higher pressure?
Maybe now a stock regulator and bigger pump?
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Last edited by mheyman; 08-02-19 at 05:15 PM.
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Old 08-03-19, 01:43 AM   #10
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Quote:
Originally Posted by mheyman View Post
I didn't see the pre-tune before. It looks like what we'd expect. Your injector duty cycle is higher at the right with resulting higher pressure . . . you mentioned injectors? You modded with larger injectors? Then you could run less fuel pressure, - it could be set up that way. Most cars use IDC, but changing fuel pressure is sometimes done - -

Wait -
* do you have an adjustable regulator?
* was it tuned previously with higher pressure?
Maybe now a stock regulator and bigger pump?
Hi,
I currently have the Cobb Fuel Pressure Regulator, 1050x injectors. My regulator is non adjustable. Still waiting to hear back from the tuner. But the new tune is where I’m having my issues with fuel pressure. It was fine with the previous etune which was a couple months before the protune
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