does anyone know what kind of problem it can cause the line going from the intake to the fuel pressure regulator being collapsed? i've tried looking around and couldn't really find anything. i dunno if i was looking right or not. could it cause bad gas mileage, or not wanting to idle under a load?

 

A check of the fuel pressure will tell you if there is any effect. Never really heard of one of those hard plastic vacuum lines collapsing.

 

Realized you sent me a PM on this one I never answered.

Have you tried replacing the line? As Shoebox already said, its odd for the rigid plastic line to "collapse". If it did, it would prevent the intake manifold vacuum from adjusting the rail pressure, to control the differential pressure between the rails and the tips of the injectors inside the intake manifold runners. The pressure at idle would be excessive, and that would cause the injectors to flow about 10% more fuel than the engine needs. But the O2 sensors should pick up on the rich condition, and reduce the flow through the injectors by shortening the pulse widths.

So.... yes, anything that prevents the fuel pressure regulator from adjusting to intake manifold vacuum could cause poor fuel mileage or rough idle. Not likely, but possible.

But its so easy to replace the vacuum line, why not check it. When you replace it, verify that there is no wet fuel in the vacuum line. That would would indicate a faulty fuel pressure regulator, and cause the same problems you describe.