Probably just quantity. Air can be pressurized too. The FPR can't tell the difference.

3 minutes ago, Zed Head said:

Probably just quantity. Air can be pressurized too. The FPR can't tell the difference.

Gotcha, that might be why we were getting standard fuel pressure when we had the gauge on, but still had to performance?

Maybe the crap in the tank is too large to pass through the filter screen in the tank. It just blocks it up instead

Edited March 23, 20251 yr by Patcon

S30.World. For a new tank and remove that variable. The only reason I am suggesting this is I had mine cleaned twice and the same results occurred until I put a new tank in.IMHO

2 hours ago, Yarb said:

S30.World. For a new tank and remove that variable. The only reason I am suggesting this is I had mine cleaned twice and the same results occurred until I put a new tank in.IMHO

Gotcha - that's odd and a bummer

Would throwing a bottle of that Lucas cleaner every time I fill up do anything to break this stuff down, or is the consensus just 'keep the tank topped off'?

Edited March 24, 20251 yr by chaseincats

I would "vacuum " that out with an electric fuel pump. Then analyze what you take out for a diagnosis...

On 3/23/2025 at 2:28 PM, Zed Head said:

I've heard/read that the pickup tube in the tank can get pinholes that suck air when the fuel level drops below them.

Here are some good pictures of the inside of an EFI tank.

https://www.atlanticz.ca/zclub/techtips//fuel/gastank/index.htm

@Zed Head Bringing this thread back from the dead since I am sure the above is the case.

The car was undrivable for a while due to the issues everyone has helped me through on various threads in this forum these past 1.5 years. Now that it's drivable again, the car will start when I manually trigger the cold start valve (using a switch I wired in the cabin), but if I don't, it's a lot of cranking - once it gets going, it will run rough for a few seconds before it smooths out - I am attributing this to the year+ old gas in there, (do you agree)?

There is fuel stabilizer in the tank (but put in after the problem started, so it wouldn't get worse) as well as a bottle of this Heet product recommended on this thread where we worked through why my fuel pump seized (fuel/water separating rusting the pump out internally requiring a periodic smack to turn on). The Heet product was supposed to quell the water issue.

I drove the car yesterday for its first proper highway shake-down after sitting for so long and the fuel pump was seized (second time ever) which was odd because I started it twice the day prior - albeit to move it in and out of the driveway for a well deserved wash.

Here's the question (finally): Since I have that pinhole air leak in the tank's pickup tube (not visually confirmed but the theory checks all of the boxes considering the symptoms), I can't drive the car past a half tank before it beings to run lean again (confirmed by my air/fuel ratio gauge). I'd of course like to have a fresh tank of gas in there, but I have no means to catch/store/dispose of 9+ gallons of old fuel. Since it has a half gallon of year(ish) old fuel, if I fill the rest of the tank with new fuel, will that be enough for the car to run properly, or will I now have a tank of better(ish) fuel and really do need to find a way to drain the tank in full?

Any ideas?

10 hours ago, chaseincats said:

I drove the car yesterday for its first proper highway shake-down after sitting for so long and the fuel pump was seized (second time ever) which was odd because I started it twice the day prior - albeit to move it in and out of the driveway for a well deserved wash.

Here's the question (finally): Since I have that pinhole air leak in the tank's pickup tube (not visually confirmed but the theory checks all of the boxes considering the symptoms), I can't drive the car past a half tank before it beings to run lean again (confirmed by my air/fuel ratio gauge). I'd of course like to have a fresh tank of gas in there, but I have no means to catch/store/dispose of 9+ gallons of old fuel. Since it has a half gallon of year(ish) old fuel, if I fill the rest of the tank with new fuel, will that be enough for the car to run properly, or will I now have a tank of better(ish) fuel and really do need to find a way to drain the tank in full?

Any ideas?

So... How did you drive it with a seized fuel pump? Replace it? And, if you were driving it with old fuel, why are you now nervous about driving it with old fuel - with some added new fuel...

L Series really don't like lean condition - don't mind rich at all. Are you really going to drive it half-a-tank at a time? The fuel tank will eventually have to be emptied and taken down and the intake tubing repaired or replaced if it really does have pinholes. Probably means rust inside the tank as well. Was the fuel pump seized due to rust or particulates? Rust in filter? The late 280 fuel tank is different than the early Z's and should be able to be inspected from the "top port" for the fuel gauge sender mechanism - once the fuel tank is emptied. I would suggest you have someone do this for you - that can also get a camera down in there.

48 minutes ago, cgsheen1 said:

So... How did you drive it with a seized fuel pump? Replace it? And, if you were driving it with old fuel, why are you now nervous about driving it with old fuel - with some added new fuel...

L Series really don't like lean condition - don't mind rich at all. Are you really going to drive it half-a-tank at a time? The fuel tank will eventually have to be emptied and taken down and the intake tubing repaired or replaced if it really does have pinholes. Probably means rust inside the tank as well. Was the fuel pump seized due to rust or particulates? Rust in filter? The late 280 fuel tank is different than the early Z's and should be able to be inspected from the "top port" for the fuel gauge sender mechanism - once the fuel tank is emptied. I would suggest you have someone do this for you - that can also get a camera down in there.

I was able to drive it after i gave it a few taps with a hammer. Nervous about it now because I can now drive it further than the driveway. Great news folks are actually able to repair the intake tube - that sounded really involved since you'd need to cut the top open and some how find the holes then weld it back shut. I've popped the top open and the tank itself is fine since it was boiled out/coated about 10 years ago and there isn't any rust on the walls. There's no rust particles in the tank since the Fram filter that sits between the tank and pump is clear - its just the water/gas separating and rusting the pump out internally.

Would I need to take the car to some sort of specialty shop to have them fix the intake pipe? I have no idea what kind of place I should be looking for.

I have no experience with the late 280 fuel tanks - only 1970-1976 - but on those the supply and return piping (tubing) is soldered to a plate which is soldered to the tank. I've repaired a few leaks on those (and the evap/vent hose fittings on those tanks which are also soldered on. I'm a Journeyman Plumber so I know torches, lead and soldering.) Although I've repaired leaks, I don't remember completely removing and/or reinstalling that tubing although those tanks are quite simple and I believe it can be done. Again, I have NO experience with the 1977-78. It, of course, requires removal of the tank and fuel and all the fuel vapors. I don't know what shop could get that done but Radiator Repair shops do a lot of soldering - though usually on brass...