So I finally figured out why my car was periodically running lean/down on power:

I was driving the car yesterday and it started to drive poorly in the same way as it had in the past - so I went home and popped open the top-loading fuel sending unit (since it's a '78) and looked into the tank. There is some what looks like brown mud that was sitting on the bottom of the tank that I suppose gets diluted when the tank is full (because I saw it disappears with a full tank when I looked in after filling it)?

I filled the tank and took it out for a spin, and sure enough, it ran perfectly again. So my question is what is that brown stuff, is there a way to get it out, and why is a very low fuel level (gauge reading near empty) an issue when the clear paper filter between the gas tank and fuel pump looks brand new (it's a few years old)?

NOTE: The tank was boiled out/treated in 2017

Edited Sunday at 03:16 PM1 day by chaseincats
added additional information

That’s rust in the tank. Are you running a pre-filter before the pump. Fram 33003 is an in line 3/8 filter that’s clear.

These cars didn't have coated tanks when new so they get "gas varnish" whatever that is. I coated both of mine with Red-Kote after a good cleaning but that's a lot of work. Until your ready for that just keep the fuel level at half a tank and enjoy the ride.

Here's a couple of things you could read over...

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

https://www.google.com/search?q=gas+varnish&rlz=1C1VDKB_enUS1121US1121&sourceid=chrome&ie=UTF-8

EDIT: Some gas stations have ground water leaking into their old tanks so sometimes they will get moisture causing rust but if you keep fuel in it the rust won't happen, it takes oxygen and moisture to rust.

Edited Sunday at 04:32 PM1 day by siteunseen

32 minutes ago, Yarb said:

That’s rust in the tank. Are you running a pre-filter before the pump. Fram 33003 is an in line 3/8 filter that’s clear.

It sits on the bottom though and when the tank is filled it floats away. Are you sure it's rust since it doesn't stick to the metal?

Edited Sunday at 04:44 PM1 day by chaseincats

18 minutes ago, siteunseen said:

These cars didn't have coated tanks when new so they get "gas varnish" whatever that is. I coated both of mine with Red-Kote after a good cleaning but that's a lot of work. Until your ready for that just keep the fuel level at half a tank and enjoy the ride.

Here's a couple of things you could read over...

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

https://www.google.com/search?q=gas+varnish&rlz=1C1VDKB_enUS1121US1121&sourceid=chrome&ie=UTF-8

EDIT: Some gas stations have ground water leaking into their old tanks so sometimes they will get moisture causing rust but if you keep fuel in it the rust won't happen, it takes oxygen and moisture to rust.

I had it boiled out/coated about 8 years ago, so it shouldn't rust I'd have thought. Since it looks kind of goopy and doesn't stick to the metal I didn't think it was rust at the time.

Interesting articles/google post - thanks! The google post talks about using a "fuel system cleaner." I added one of those small Lucas bottles (this stuff) last fill-up - should I add another bottle?

Edited Sunday at 05:04 PM1 day by chaseincats

4 minutes ago, chaseincats said:

I had it boiled out/coated about 8 years ago, so it shouldn't rust I'd have thought. Since it looks kind of goopy and doesn't stick to the metal I didn't think it was rust at the time.

Any young kids around? My next door's 5 year old put gravel in my Nissan D-21 p/u fill hole. Cost his Dad some money too. Maybe a Snickers bar in there?

2 minutes ago, siteunseen said:

Any young kids around? My next door's 5 year old put gravel in my Nissan D-21 p/u fill hole. Cost his Dad some money too. Maybe a Snickers bar in there?

haha, nope - I believe the shop that was working on it sent it to a radiator shop but I'm not sure. It was around $400.

I would use a small electric fuel pump and some hose sections and maybe a small tube section and pump it out into another container. Diesels can get algae in the tank but I've never heard of that in a gasoline car...

Never heard back whether you’re using the pre-filter that’s highly recommended here. Being clear you will see exactly what’s going to the input of the pump itself. I personally installed a shut off valve right before the pre-filter to make it easier and simpler to service without spilling fuel. If you’re interested I can provide pics.

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

13 minutes ago, Yarb said:

Never heard back whether you’re using the pre-filter that’s highly recommended here. Being clear you will see exactly what’s going to the input of the pump itself. I personally installed a shut off valve right before the pre-filter to make it easier and simpler to service without spilling fuel. If you’re interested I can provide pics.

Sorry I missed that. Yep I have one of those clear fram filters between the tank and pump. The weird thing is the brown floating junk visible in the tank isn’t in the filter.

8 minutes ago, 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

Would air in the line show up on a fuel pressure gauge?

Edited Sunday at 07:38 PM1 day by chaseincats