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Old 02-15-2020, 06:26 PM   #1
subiguy
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Default Help! Fuel pressure drops 30 seconds after start up

Hi all, 2006 Spec B stock fuel system. Fuel pressure is 35 psi on start up but after 30 seconds drops to below 20 psi and engine leans out sputters, spits etc. This happened after replacing the fuel tank. I inspected all lines and hoses for blockage and even replaced the pump twice (yes I made sure the o-ring and clip was attached to pump) while assembling the pump housing.
There is a fuel pump control module located in the l/r quarter panel which I replaced but still happening. For some reason the ECM must be telling the module to cut psi but can't figure out why. Thanks all
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Old 02-16-2020, 02:18 AM   #2
some dude
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What happens if you pinch off return fuel line after fuel pressure regulator? Could regulator be bad?
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Old 02-16-2020, 08:26 AM   #3
subiguy
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No change when I pinch off return line. Today I'm going to test wiring from ecm to fuel pump control unit in case the harness got pinched during tank install. But visually it appeared clear.
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Old 02-16-2020, 09:55 PM   #4
BurninSTI
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Seems a little low across the board, tired fuel pump?
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Old 02-17-2020, 11:01 PM   #5
subiguy
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The fuel pump, when getting 12v is pushing 35psi, the problem is that after the engine runs for 30 seconds from start up, the ecm cuts voltage to pump down to 7.5V then the pressure drops to 18-20psi.
Havent been able to figure out why thats happening.
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Old 02-27-2020, 03:39 PM   #6
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Quote:
Originally Posted by subiguy View Post
The fuel pump, when getting 12v is pushing 35psi, the problem is that after the engine runs for 30 seconds from start up, the ecm cuts voltage to pump down to 7.5V then the pressure drops to 18-20psi.
Havent been able to figure out why thats happening.
That is kinda low is vacuum hooked up when getting that reading? What pump? Did you replace fuel filter? Could be check valve on the pump if that pump you have even has one.
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Old 02-27-2020, 05:41 PM   #7
subiguy
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I ended replacing the tank again and it fixed the problem. After doing some research it is normal for the ECM to cut voltage to pump after rinning for 30 seconds. That's when the fuel pump control module kicks in and brings it to 33% at idle. It is designed that way. Maybe I forgot to mention the tank I replaced originally was a used one even though I swap the pump still have the problem so put a whole nother tank in it with pump and sending unit and it is now fixed
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Old 02-28-2020, 09:01 AM   #8
spoolinsti05
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Quote:
Originally Posted by subiguy View Post
I ended replacing the tank again and it fixed the problem. After doing some research it is normal for the ECM to cut voltage to pump after rinning for 30 seconds. That's when the fuel pump control module kicks in and brings it to 33% at idle. It is designed that way. Maybe I forgot to mention the tank I replaced originally was a used one even though I swap the pump still have the problem so put a whole nother tank in it with pump and sending unit and it is now fixed
Yes that is normal. But swapping the tank wasn't the fix to your issue. Sounds like you had some issue with the vent system on the tank..

If the tank wasn't venting properly the pump wouldn't be able to suck the fuel out. Because it would build up a vacuum. You need an in and out doorway for fluid to flow.
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Old 02-28-2020, 09:07 AM   #9
subiguy
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I swapped fuel tank pressure sensor, drain valve, loosened the gas cap, and even left the sending units loose to make sure there was no vacuum being created all checked out ok. Still a mystery
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Old 03-02-2020, 08:27 AM   #10
spoolinsti05
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Originally Posted by subiguy View Post
I swapped fuel tank pressure sensor, drain valve, loosened the gas cap, and even left the sending units loose to make sure there was no vacuum being created all checked out ok. Still a mystery
Maybe your pump was bad. Did you change evap canister?
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