History:
Leaking front tank. Pinholes in top, underneath rear frame contact point, from wear and rust. Insulator pad wore through on top, turned into goo on bottom. Bottom front pad also turned to goo.

Replaced original holed tank with new plastic tank. Replaced pickup screen, because it broke off in handling. Did not replace the fuel line connectors to the sending unit.

Fired right up, no sign of air in lines, ran perfect romping on the interstate. But one of the connectors leaked... more than enough to be a problem.

With some help here, found what seemed to be the right connector... maybe. The original connector is a garter spring unit with internal O-rings. The Dorman replacement unit uses a hairpin clip instead of a garter spring to keep the connector on the sending unit tube, but seems to work.

Something that may be an issue... Bronco Graveyard lists 2 different sizes of connectors, and doesn't give anything to say which is correct. The size I got is for 3/8" OD tube on the sending unit, connecting to 5/16" ID hose on the truck side. The other size is for 5/16 OD tube on the sending unit, and 3/8" ID hose on the truck side.

So I installed the 3/8" OD tube connector. It seems to fit well; it does have a bit of play sliding along the tube, but I did lube the outside of the tube pretty well with sticky grease. It looks like the hose on the truck is 3/8" ID, and the barb on the replacement fitting is for 5/16" ID. I double-clamped the hose.

As for fuel level... I did not completely drain the tank. I siphoned it down from nearly full, and got 12 to 14 gallons out with some still left in the tank. I put back about 4 gallons before testing, to make sure the pickup was covered. The gage read 1/4, which should have been good.

Today's symptoms: Truck started and initially ran well on the front (worked on) tank. After about a mile easing through the neighborhood, it seemed maybe a little rough. But the engine also still was cold. Accelerated onto the highway, and the roughness kept increasing. Seemed like it was missing, but not in a repeatable pattern... generally rough and fewer cylinder firing noises than there should have been, and started shaking. Roughness and shaking kept increasing and power decreasing. Switched to the rear tank, after 10 seconds or so of continued shaking and very low power, the problems started decreasing and were gone in maybe 30 to 45 seconds. Truck ran fine on the rear tank, romping on the interstate and going up steep little hills.

I switched over to the front tank several times, with the same results. After 1 to 3 minutes... shorter with more throttle, longer with less... the shaking returned, got worse, stayed bad for a bit after switching to the rear tank, then progressively went away.

Back in the driveway, I crawled under the truck and looked and felt around the tank and connections... no leaks.

So what gives?

I suspected that the leaking connection that I replaced was on the fuel return line, thinking that the fuel feed line, being under suction, shouldn't leak at a connection on top of the tank. If correct, then whatever is going on is not related to that line. But the truck ran fine before I replaced that leaking connection.

While working on the leaking connection to the sending unit might I have bumped the other enough that it is sucking air? Is the connection I replaced actually the fuel feed, and it's now sucking air... but not leaking... at the 5/16" hose barb to 3/8" hose connection? Or is something else going on?

Thanks in advance for any help you can offer.

 

could have crud in the line to the fuel tank transfer solenoid


can use section of clear hose to see if it has air in the line when using front tank