Ok, I HAVE A 91 f-350 crew cab 4x4. Front pump was screaming at me, and the back pump was doing some talking of it's own. With over 200,000 on the truck I knew it was time for new pumps. Instead of dropping the tanks I pulled off the bed. It took about three hours, and I also wanted to straighten the front of the bed and re-inforce it. Anyway, I was told that both pumps always run regardless of the which tank the selector is on. I did my own testing and using a test light, found that only one tank had power running to it, depending on the tanks selector switch (front or back). So as far as I can tell only one pump will run at a time. I was also told that there was a valve located on the frame rail that worked off the selector switch, which would open or close for either front or rear. I traced the fuel line and saw no valve. Seems to me a valve is not needed. The delivery is dependant on the tanks pump. But every once in a while when I switch to the front tank, it will scream at me, and the back tank will also be talking at the same time. I tested the hot lead on each tank, and the front was hot, but not the back. I think there may have been some pressure equalization. What I want to know if I am on the right track. I think I just have weak pumps. But I want to be sure before I spend money on the pumps and put it back together.

 

ford pumps are somewhat notorious for getting noisy, but it doesn't necessarily mean they're bad. my family has owned a few Fords of this era with noisy pumps. the loudest was the 89 T-Bird that had a pump so loud it was all you ever heard. annoying? yes, but the car ran daily with that same noisy pump for the 2+ years we drove it.

Mike

 

Whoever told you that about both pumps running was completely full of it.

If you're worried about them, and 200k is a good time to be worried about them, get a fuel pressure gauge and test the fuel pressure. As long as they are in spec, don't do a thing to them unless you like replacing pumps that aren't bad. The noise doesn't matter. I have one that hums like a bumblebee - but works just fine.

 

Did anyone see this part? Maybe I'm dumb, but it sounds like hes saying that every once in a while when switched to his front pump, it will make noise ALONG with the rear pump. Actually I'm not dumb, thats exactly what he said. I think we might have a switch problem here....