After closing the book on the EGR thread, I had hoped not to start another one for awhile, but here we go...

A little background - my front tank does not work. Switch works, front sender works, fuel pump is shot. The rear tank will cross-fill into the front one until the front tank is full to the filler cap. If the truck sits outside in the sun during the day, the front tank will pressurize enough to push some fuel past the filler cap (if you unscrew the cap, fuel will actually spray out).

First question - shouldn't that overflow into the vapor canister?

More important question - on these days when the temperature causes the fuel to gain all of this pressure, after starting the truck it runs for about a minute before it starts chugging like it did when the rear fuel pump went out. It only lasts for 10 seconds or so, then the chugging goes away and it runs smooth for the rest of the trip.

Is this due to the pressure in the tank? I'm wondering if the vapor line is plugged up and not allowing pressure out. I had thought that a new front fuel pump would cure the cross-filling, but I'm not sure if it will cure the pressure build up. Could there be a problem with the filler caps? It seems odd that the front tank builds up crazy pressure, but the truck runs off the rear one, so I'm not sure if the two issues are even related.

Any thoughts?

 

The vent runs from both tanks to the canister, so if it's not making it to the canister it's being pushed back to the rear tank through the vent line. With the fuel being pushed back to the rear tank through the vent line there is no venting anymore. The vent line is filled with gas. Although I'm not sure if there are 2 separate valves for supply and return, if the supply valve is also malfunctioning then the fuel being pumped to the engine will be pushed into the front tank starving the engine for fuel. If the front tank is really that full then the few seconds it takes to cover the bad valve with gas might stop the fuel from entering the front tank and then force it to the engine.

 

I should probably try to track those vent lines from the tanks to the canister. It might be hard to find the blockage without taking a bath in gas, I assume... I'll try to investigate as soon as I get a chance and see if anything looks suspicious.

I'd think the vent tube would need to be really blocked up to overcome the gas filler cap.

Just to be sure - the vent lines run from the tanks to a Y (more than likely) and then on up to the canister in the engine bay, right?

 

Sounds good to me. I haven't actually looked at them but that should be how it is.

 

I haven't actually tracked the vent lines yet...

But - a few days ago, I was filling the rear tank at the gas station, and left the filler cap laying on the edge of the bed. (the little plastic leash is long gone)

So, I had to buy a new gas cap. Decided to buy one for both tanks while I was at it. In the past few days since putting new filler caps on, I haven't had any issues with the truck doing wierd things after start-up no matter what the weather is. It also doesn't build up the crazy pressure in the front tank.

What the hell?!?!?!?! Am I dreaming, or could this be as simple as a plugged up/worn out filler cap?

 

Hey, what was the end of the story on this one? Was it really just an old fuel cap issue? Swap them out first to see if that solves all your problems? I've got some of the same issues on my '89 right now, and I'm trying to trouble shoot the easy stuff first.

 

Both tanks join into a Y to the evap but in both tanks is a check
valve so raw fuel doesn't make it into the charcoal canister.

And if its like my truck the pumps have check valves to keep
one pump filling the second tank I had the same problem front tank
pump died had chugging and everything switched to rear.
All was good then the rear died chugging and all.

Replaced rear ran it front was empty until check valve went
long story short I now have one gas tank (rear) capped the 3 lines to front
removed front to make room for true dual exhaust.. Truck is a 95 f-150

 

OK - now you've got my attention!

This issue (for me) has woven it's way through a few other threads, and continues to be a real pain. When the sun comes out and warms up, the front tank builds pressure and leaks through the filler cap. The check valve in that tank is shot, I'm sure. I don't get pressure issues unless the front tank is overflowing.

I would love to cap the lines and forget the front tank, but wasn't sure if there was a way to do it that would hold pressure.

Sooo.... how'd you do that?

 

I capped the lines to my front tank today. Aside from a gasoline bath, it wasn't a bad deal. Like Dominick did, I cut the rubber lines that connect the plastic lines to the fuel pump. Inserted a hose barb fitting in the lines, and capped the hose barb. It's holding pressure, and the front tank is isolated from the system.

If I decide to put a new tank up front, I can just replace the rubber lines and hook 'er up.

At this point, I've drained the front tank, then ran out of daylight. Next decent day we have, I'll pull the front tank out completely.

 

There is a pesky part called a fuel reservoir that you may want to replace. It acts as a switch between the two tanks. When you flip the switch in the cab all you are doing is selecting which low pressure pump (in the tank) gets power. The fuel pressure from the tank that has been switched to is supposed to kick over the fuel reservoir so the supply and return lines are for the same tank.

Sometimes the reservoir goes bad and you can have the situation that you describe, it gets fuel from one tank but returns excess to the other. In my case (I've had it happen twice) the reservoir gets stuck in one position. When the tank switch has the right tank selected everything works. When switched to the other tank the engine will be starved of fuel since the reservoir is stuck and wont allow the newly pressurized line to go through, even if evything else is working properly.

I'm guessing that the excess pressure is because the front tank is filling up enough to close the check valve in the vent. It sits right in the middle on top of the tank. Once the check valve is closed there is no where else for the pressure to go. Also speculation: once this pressure builds up it may kick the reservoir over and everything returns to normal. That's a big conjecture but hey, that's what the internet is for.

Hope this helps.

Oh and the reservoir is on the rail behind the high pressure fuel pump.