I have an 88 F150, 351W, that idles fine with reasonably full tanks, but consistently develops a lumpy idle when the tanks get down below about 1/4 full.
If it was just on one tank, I would suspect an air leak in that intake line, but the symptoms are the same on both front & rear tanks.
Good power & performance otherwise & no codes.

Has anyone had this problem, or got any ideas? Thanks

 

Anyone have any insight into this?

 

Maybe there is trash in the tank and when its low it gets circulated to the front.

 

I'm curious about this one. I don't have anything to add, but I want to know what's causing it.

 

Try changing the fuel filter and see if the problem is still there or how quickly it comes back. Also when changing the filter, pour it out and look and see what the fuel in the filter looks like.

 

Would the fuel filter un-clog when he filles up with fuel?

 

The filter is nearly new, but as soon as the tank is topped up, above about 1/4, it's all fine; & there is no problem at high rpm or heavy load, at any tank level.

Have stripped & cleaned the IAC valve but it made no difference. Perfect idle except with low tanks.

I can't see it being water or dirt in the tank causing the problem because, full or low, the intake is still drawing from the same level in the tank.

Could it relate to the vapor collection system?

 

After the truck sits for a while , i would take a fuel sample from the bottom of the tank with siphon hose into a clear jar , let it sit for a while & see what it looks like...Lew