GT40 Low Pressure Fuel Pump Should Sound Like?

[h2oski]

Howdy,

I guess after 20+ trouble free years it's time to start having problems with my '99 Air :-) To make a long story short, she let me blow in the wind over Thanksgiving week. I made it back to the ramp (a passing pontoon towed me) and figured out how to get her on the trailer, tail between my legs and went home.

My year has the two totally separate fuel pumps. The low pressure pump draws fuel from the tank and keeps the FCC full of fuel. I knew something was up because the FCC was empty, which narrows the scope to something before the FCC. I did a bunch of searching, reading, learning, debugging and realized I needed/should've spent more time maintaining my fuel system over the years. Mind you, I do all of my own maintenance religiously, but took for granted that parts never fail and CC & PCM thought of everything. My issue is the low pressure fuel pump is just tired.

To the question... after I replaced the low pressure fuel pump (check the rubber grommets on yours, mine needed replacing) it draws fuel from the tank, fills the FCC and I see fuel returning to the tank now. However, I notice the low pressure fuel pump making a sound like it's getting a little air, speeding up slightly or slight cavitation sound. Again, not terrible, just an obvious, inconsistent difference in pitch/tone/sound.

Thoughts about the pump noise?

-Scott

[NautiqueJeff]

I haven't noticed a sound like that before. How full is your gas tank?

[h2oski]

The EEC test port connection to force the pumps on without starting the engine is a very handy trick.

I was only familiar with the pump sounds from the 2 or 3 sec. key on moment, but the "trick" lets you hear them run without the engine running.

Wondering the same and how the fuel level might effect the pump, I tested with full, 1/2 and 1/4 on the fuel gauge, no change in sound.

-Scott

[h2oski]

Here is a sound file that I recorded of the new LPFP pumping. The sound is consistent but every so often is a hiccup or slight change in the pump sound. Maybe it's fine, but I don't want to find out it's not a mile from the ramp....

New LPFP pumping sound 1.m4a.zip (download and expand)

Concerns are:
-The low pressure fuel pump pushing air bubbles into the FCC. I imagine the bubbles collect in the top of the FCC and return to the fuel tank or do they? When the engine is running, it seems to idle fine. I don't sense any miss fires or vibration from air reaching the high pressure fuel lines.
-The fuel return line dumps about 10 inches from the fuel pickup tube. I wonder if the fuel return is creating bubbles (spray is strong, I can see the fuel churning in the tank) and the fuel pickup is picking up some bubbles. If this is the case, that has not changed and has always been this way.

-Scott

[charlesml3]

Seems to me like everything here is OK. The FCC is staying full of fuel. You have good pressure on the rail.

Start with some short runs out from the ramp and back. If it runs strong you should be good to go.

[h2oski]

Thanks. Good idea on the ramp and back. There aren't many people boating this time of year to get a tow in again :-)

Speaking of fuel pressures, key on only/jump EEC port, 40psi, engine idle, 35psi, engine idle/remove fuel regulator vacuum hose, 40psi.

-Scott

[charlesml3]

Anything above 30PSI is good. You should also check it on plane and under load to make sure it's maintaining.

-Charles