mateospeed

EDIT: Crap, I posted this in the wrong forum. If a moderator sees this, would you be kind enough to move it over to the fueling forum, pretty please?

Folks, I have an interesting problem that's stumping me.
First, background: Car is an LS1 retrofit from a 2001 Trans-Am. Fuel comes from an in-tank Walbro 255lph pump, ~2ft line to a corvette filter/ regulator, with ~2ft return back to the tank. -6 Hardline to the engine bay, into the stock fuel rails, into a stock engine.

Engine has run maybe a total of an hour since the install. Initially, it ran wonderfully. Engine wasn't started for a few months, and now we have a new problem: fuel pressure isn't stable, and the car will BARELY fire to life, one cylinder at a time. Any attempt to apply power kills it immediately. We have so much fuel that it throws fireballs out of the (currently) open headers when it finally does fire. More recently, it won't fire up at all.

With the key on, or the pump hardwired to the battery, fuel pressure sits at about 44-60psi. Good enough, considering it's a relatively cheap, un-damped gauge.

When you crank the starter, or when the car is running, the pressure skyrockets up to the 90/100psi range, and the needle fluctuates rapidly, say ~30Hz.

I've tried the following to diagnose what's up:
-Disconnected accessory belt to rule out alternator/regulator failure. No effect.
-Tested voltage at the pump. ~12.5V ign on, ~10.5V cranking. Seems fine.
-Changed gauges to another, similar, undamped gauge. Same response.

I've never seen this kind of thing before. I would suspect a restriction in the return line if I was seeing high pressure with the pump running normally, with (relatively) high voltage input. But with the starter cranking, and LESS voltage supplied to the pump, the fuel pressure spikes! So strange.

Brainstorm, go! All ideas considered.
Thanks in advance!
-Matt

kingtal0n

iTrader: (1)

I would turn the fuel pump on manually- run direct current to it with the engine and key off, everything off. The vette filter/regulator is a return style regulator, it sends fuel back to the tank, and it should pressure the system to exactly 60psi while you run it like this, while returning fuel to the tank. Run it for a while, maybe 30 seconds or so to determine if the same terrible symptoms occur, or if it truly is only when the key is on, or engine is turning.

This diagnostic procedure will help you see if the fuel system is at fault as a solo entity, or if the problem is perhaps electrical (ignition power/voltage related) or even compression related.

If the fuel system appears to be working normally while running the pump by itself, the next thing I would do is turn the engine manually (with a socket) a few revolutions while the fuel pump is running. Watch the fuel pressure gauge while you do this.

Then, disconnect the engine's fuel related items somehow (disable the injectors, unplug them if necessary or unplug the main harness if there is one) and crank the motor with the starter while still running the fuel pump manually, while watching the fuel pressure gauge. If the injectors are disabled and you still get a combustion event, it points to a leaking injector.

To me it sounds like you have a return line blockage or something that is acting like one, perhaps in the filter/regulator, or in the tank, but that is just my guess.