I've developed another issue on my '87 F-150 4.9L 4 speed that I suspect is the in tank pump going bad, but I'm going to throw it out here for other points of view.

I have developed a stumble above 40-45 mph that is more noticeable when the fuel level in the tank gets below 1/2. I'm running off the front tank only as I have not dug into why the POs didn't use the rear tank so I can't compare one to the other.

Over the last several months I have replaced the plugs, wires, cap & rotor, EGR, vacuum hoses, coil, distributor & TFI module, injectors, fuel pressure regulator, throttle position sensor, lower intake manifold, upper & lower intake manifold gaskets, O2 sensor, engine computer, and the main engine wiring harness. I know - that's a lot of parts, but you have no idea of the mess the POs of this truck made trying to correct the infamous high idle issue.

I haven't re-tested the fuel pressure since the stumble developed. It is a struggle to find a rental pressure gauge around here that will work on it as other people have jacked them up one way or the other. It was in spec before this started, but I can't swear to it now.

I suspect that the in-tank pump is on its way out since things improve with a full tank, but am trying to avoid jumping to conclusions without considering other options. I need to drop the rear tank to see what kind of nightmare is waiting for me there, but if the front tank needs some love, i might just pull the bed to make life a little easier...

 

Kinda sounds like it doesn't it? Sure would want to eliminate anything else first though, maybe deteriorated rubber hose sucking air or bad venting fuel cap? Thing is, if it's got some serious miles on it, + the age (1987) it stands to reason fuel pump is on the way out. Definitely want to have a replacement on hand at the minimum.

 

Quick update...

I decided to start with the rear tank. It was pretty rusty on the outside so I chose to replace it rather than have it rust out on me later. The fuel gauge sender was bad so I also replaced that along with the pump. Once that was back in, it on the rear tank a bit until the fuel reservoir thing got stuck between tanks. A quick run to a self-serve junk yard for another one (only 2 in the whole lot) and it's now running fine on the rear tank. It still has an occasional hiccup, but not like before. The front tank is going to get a new pump in the near future - too hot out to mess with it any more today.