spacemanspif

iTrader: (5)

I see a lot of pictures of truck engines with return style fuel rails on the internet. Most of these pictures show a regulator that has the vacuum port pointing up. The FPR on my engine has the vacuum port sticking straight out (towards the side of the car).

Is this the difference between flex and non-flex fuel engines? If so what FPR do I have? Is there anything to be gained by switching to the other style?

Thanks,

74modified

I think it doesn’t matter as long as you get the correct pressure. Here is a little info on truck fuel systems. I like the return, most people don’t realize that returnless was installed to comply with emissions standards, not performance.
1999-2003 4.8, 5.3, 6.0 all had a fuel rail with 3/8" supply line and a 5/16" return line. The fuel pressure regulator is on the fuel rail, and has a vacuum line to the intake manifold. In 2004, All engines went to a single 3/8" supply line, with no return (returnless) except 5.3L L59 Flex Fuel vehicles. For 2004, Flex fuel engines KEPT the return line, BUT used a fuel pressure regulator with a lower pressure ratting. 2005 5.3L L59 Flex Fuel engines switched to a single 3/8" supply with NO RETURN line in 2005. Non flex fuel engines, fuel pressure spec is 55 to 62 psi with vacuum line disconnected, fuel pump running. 5.3L Flex Fuel engine 2004 and back, 48-54 psi with vacuum line disconnected, fuel pump running. 2005+ with returnless fuel rail all shared same fuel pressure spec, 55-62 psi. There is no vacuum reference on these, so fuel pressure should remain constant under all operating conditions. The computer is programmed differently for fuel flow rate depending on type of fuel rail. This is important to know when having a PCM programmed.

spacemanspif

iTrader: (5)

Thanks for the reply/info, 74. I have seen those numbers before and have wondered what they mean by unplugged, fuel pump running. Just to be clear, the values listed are for engine running at idle and vacuum line disconnected, right? Does it matter if the engine is in open or closed loop? I can't imagine it would but want to make sure that I take the reading properly. According to the guy I bought the engine off of its out of 2004 Tahoe. Electronic throttle body (using TAC module), return style fuel rail, gas pedal was adjustable before I chopped it up. Just in case any of that matters.

74modified

The rail mounted regulator uses the vacuum (high at idle) to cut back the pressure a little. If you disconnect the vacuum you get full pressure, no matter how the engine is running. The pump just pumps and the regulator bleeds off excess pressure, it doesnt even know the engine is running. They use the same regulator in the in-tank pump modules, so with no vacuum the pressure is constant.

spacemanspif

iTrader: (5)

Oh ok. Thanks for clearing that up. I just wanted to make sure the pressure while in "primed" mode wasn't different from "running" mode. I'll check it later when the weather gets nicer.

spacemanspif

iTrader: (5)

Checked it on a gauge today. At idle with vacuum disconnected: 54psi. So I'm guessing that my engine is a 2004 L59.

Is there anything to be gained by switching regulators? Or would it be more involved and lead to me needing new injectors to match the new regulator?