E36 fuel system questions/issues with my LS1 swap

[squashman702]

I have a 1997 M3, LS1/T56, that I'm trying to diagnose a fuel system issue with. The car is set up with a Walbro 255 in tank to a C5 Corvette fuel filter/regulator in the BMW filter location (set to 58 psi), then a single line to the engine. I have excess fuel pressure at the rail (70-78 psi) which I believe is causing running issues, after going from an Aeromotive Stealth 340 to the Walbro 255, and swapping out the regulator/filter with another new unit. I have the BMW evap purge hooked to the LS1 set up on the intake manifold, and this system is coded to function like it should with the LS1 computer. The car seems to idle fine but will stumble and die after a few minutes of driving, and smells like it's running rich. The plugs show signs of running rich as well.

I did some research and some people stated that high fuel pressure could be due to a blockage or restriction in the return. I pulled the return line off the regulator/filter, and blew a quick blast of compressed air through it. When I pulled it off, I had a trickle of fuel coming from the tank. After blowing the air through it to the tank, I had a steady flow of fuel coming from the tank to where the return line meets the regulator, which does not seem correct. My understanding is that that the OBD2 fuel tanks have an internal siphon, and the return line sits at the top of the tank on the passenger side. My testing seems to show that there would be fuel flowing backwards from the return, which may be a cause of excess pressure.

I had the tank out to do some chassis welding- is it possible to hook up the return line incorrectly? Is the pressure flowing backwards out of the return normal? Nothing seemed to indicate a problem when I put this all back together.

[bmwdirtracer]

I am not familiar with how a Corvette's pressure is regulated, but certainly, 12 to 20 psi too much pressure will cause terribly rich conditions. I'd think that it is possible to hook up the feed and return incorrectly, but in my experience, on pure BMW systems, this results in inadequate pressure, not excessive pressure.

Although you've been able to blow through the return line, perhaps the rubber hose has become kinked somewhere, (enough to increase pressure) during all your work?

Return lines usually don't dump the fuel in at the top, they usually have some mechanism for returning the fuel to the bottom of the tank, without splashing. Therefore, you might certainly get a siphon effect after blowing through the line.

For testing purposes, you might consider disconnecting the return line, and allowing the return fuel to dump into a bucket, for long enough to check whether that addresses your excessive pressure issue?

[squashman702]

I am not familiar with how a Corvette's pressure is regulated, but certainly, 12 to 20 psi too much pressure will cause terribly rich conditions. I'd think that it is possible to hook up the feed and return incorrectly, but in my experience, on pure BMW systems, this results in inadequate pressure, not excessive pressure.

Although you've been able to blow through the return line, perhaps the rubber hose has become kinked somewhere, (enough to increase pressure) during all your work?

Return lines usually don't dump the fuel in at the top, they usually have some mechanism for returning the fuel to the bottom of the tank, without splashing. Therefore, you might certainly get a siphon effect after blowing through the line.

For testing purposes, you might consider disconnecting the return line, and allowing the return fuel to dump into a bucket, for long enough to check whether that addresses your excessive pressure issue?

The C5 Corvette uses a filter/regulator combo- apparently it's just a spring in the regulator controlling pressure. I did check to make sure that I had the return and feed hooked up correctly by turning the pump on with both lines disconnected, so that is not the issue.

I also figured/was hoping that I was getting a siphon effect by blowing air through the return line.

I also was planning to disconnect the return at the regulator return port for the exact reason you said, and see if there's some sort of restriction between it and the tank. I'm thinking I should have enough time to check pressure at the rail without making too much of a mess.