I have a 1990 F150 with a 4.9l and dual tanks. I bought the truck for $650 knowing I would have things to fix, and there's one thing I just can not seem to solve.

I can't get the rear tank to run the engine. The truck runs great on the front tank, but barely idles for a few seconds on the rear tank. I have tried everything I know of to find the problem and can't get a definitive answer, so I'm hoping someone here can give me something else to try.

Each pump will run to prime the system when the key is turned on. The front tank will start just fine and run well. When switched to the rear tank it will idle slightly lower rpm, and after just a few seconds will sputter and die. The rear usually won't start the truck, and will sputter and die if it does. While the truck is sputtering, the rear pump is still running, up until the engine dies.

I had the rear tank out of the truck, ran the pump with a 12v power supply and it builds pressure. I had a clear vinyl hose on it, and when I kinked the line it bulged very quickly.

With both tanks hooked up I tried running with the rear tank, and I put Vice Grips on both lines from the front tank, just in case pressure was bleeding off to there instead of the engine, no change.

I tried blowing air through the fuel line by disconnecting it at the rear tank and just before the fuel filter, no obstruction there either.

I'm hoping someone has some ideas for me to try, and thanks in advance for any help!

 

There are only 2 possibilities here, corroded connections on the rear tank electrical connection causing a low voltage to the pump, or the pump is weak. If your 12v power supply was a battery then that isn't really a valid test of the pump since it will supply a lot move current than the trucks electrical system. I went through the same thing on my truck and it was the pump.

 

The pump has been replaced, but I have not checked the voltage at the pump. I will try that tomorrow and post the results. Thanks

 

My guess is that you have a bad fuel pump selector switch at the dash.

 

It's definitely not the switch, I have been under the truck and had someone else toggle the switch and the pumps turn on just like they should. My schedule this week is busier than I thought so I probably won't get to testing the voltage until Thursday night.

 

Well, I found the issue, and it wasn't the voltage. It was the brand new pump I installed 2 weeks ago. It pumps like crazy but it all gushes out inside the plastic canister the pump is in, none of it gets to the fuel lines. New pump from the local parts store and at least 10 times dropping the tank for troubleshooting, I'm running on both tanks again.