Fluctuating TBI fuel pressure???

Fluctuating TBI fuel pressure???

I am curious if anyone else has ever noticed if the fuel pressure changes from cold start to operating temp.

I am running an adjustable TBI regulator with a 15 lbs spring from TBI parts, an inline pressure gauge with an AC ep381 in-take fuel-pump (96 Vortec 350 pickup fuel-pump). The fuel pump, spring and regulator diaphragm are all brand new.

The problem I am having is if I start the motor and adjust the fuel pressure to 15 psi then go on a drive after then engine and engine compartment comes up to operating temp the fuel pressure drops to 12 to 12.5 psi. I can readjust the pressure back up to 15 psi but after it cools off and I restart it the pressure will be up to 17.5 to 18 psi. Again after it gets warmed up it comes back down to 15 psi.

Does anyone know if this is normal? Because I am working with Dave W of Old School trying to get a final tune done. I have been trying to record some consistent data for him and I feel like I am chasing my tail trying to keep the fuel pressure stable:mad1:. I wonder if I would be better to just adjust it and leave it alone, if I didn’t have the gauge I wouldn’t even know I was having an issue.:confused:

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Originally Posted by devind

Fluctuating TBI fuel pressure???

I am curious if anyone else has ever noticed if the fuel pressure changes from cold start to operating temp.

I am running an adjustable TBI regulator with a 15 lbs spring from TBI parts, an inline pressure gauge with an AC ep381 in-take fuel-pump (96 Vortec 350 pickup fuel-pump). The fuel pump, spring and regulator diaphragm are all brand new.

The problem I am having is if I start the motor and adjust the fuel pressure to 15 psi then go on a drive after then engine and engine compartment comes up to operating temp the fuel pressure drops to 12 to 12.5 psi. I can readjust the pressure back up to 15 psi but after it cools off and I restart it the pressure will be up to 17.5 to 18 psi. Again after it gets warmed up it comes back down to 15 psi.

Does anyone know if this is normal? Because I am working with Dave W of Old School trying to get a final tune done. I have been trying to record some consistent data for him and I feel like I am chasing my tail trying to keep the fuel pressure stable:mad1:. I wonder if I would be better to just adjust it and leave it alone, if I didn’t have the gauge I wouldn’t even know I was having an issue.:confused:

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If the gauge is liquid filled and in the engine compartment, only trust its reading when the engine is cold. Set the cold pressure and forget about it. The pressure is not varying at hot idle, the gauge is not accurate when hot.

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Originally Posted by Fast355

If the gauge is liquid filled and in the engine compartment, only trust its reading when the engine is cold. Set the cold pressure and forget about it. The pressure is not varying at hot idle, the gauge is not accurate when hot.

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Yea it's liquid filled. Setting it cold and leaving it sounds good to me.:cool:
Thanks

Quote:

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Originally Posted by Fast355

If the gauge is liquid filled and in the engine compartment, only trust its reading when the engine is cold. Set the cold pressure and forget about it. The pressure is not varying at hot idle, the gauge is not accurate when hot.

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^ X2 Happens all the time...

Thanks guy,

I probably should have said inconsistent fuel pressure instead of fluctuating.

In addition to starting this thread I did some google searches on this issue and it backs up exactly what you told me. So thank you; now finally I can get back to recorded some good data for Dave so that he can get this thing fine-tuned for me.:happy:

Now that I know I plan to set the fuel pressure at 15 psi cold and leave it alone.