84Red

Hi guys, I have an 86 F250 equipped with the 460. Dropped rear tank to check crappy rear pump. My symptoms for it are: pump only supplies as idle, any more and it starts to cut out, also, the gauge either reads empty or up to half full, even when the tank is full. So I tested the pump and it puts out 6 psi, and the sending unit and it is putting out the correct 73 ohms when empty and 8 when full. Gauge still only reads up to half. Front tank and gauge work perfectly. Any ideas? I'd rather not pay $80 for a new pump.

84Red

Also, I was looking at new fuel pumps, and the pump that is apparently "correct" for my truck, has an entirely different electrical connector. Here is what my pump looks like:

Steve83

The truck behind that pump looks like an '87-91: pinholes in the wiper valance, 2 wiper motor connectors, blackout ring on the windshield, the hood bracing, and the pump itself is the pre-'90 style. Is the truck a flatnose, or does it look like the '84 F150? Your profile lists an '88 - does it look like that one? Click these & read the captions:


(phone app link)


(phone app link)

84Red

Sorry for the confusion. The pump shown is for my 86 f250 with a carbed 460. The red truck is an 88 F350

Steve83

Well, that clears up some things...

You said it's making 6 psi, but you didn't say how much it flows at its nominal (working) pressure. I'm not familiar with the 460 (other than that it has lots of tricky problems and sprays gas out the tailpipe), or pre-'87 2-tank plumbing. But any pump (fuel, water, etc.) has to be able to make its target pressure AND supply a specific flow rate (volume in a given time). Testing only pressure isn't enough.

As to the gauge - it sounds like there's high resistance somewhere in the wiring between the tank switch (possibly INSIDE the switch) and the rear tank. Try connecting the sender from the rear tank to the front tank's wiring, and see if the gauge travels through its range as you work the float by hand. If it does, the fault is in the rear tank wiring or the switch.

84Red

I'll see if I can do that. Also, I believe the in tank pumps for 460 powered trucks were the same from carb to efi until '89 when they switched to high volume in tank pumps and ditched the frame rail pump

Steve83

AFAIK, the change to hi-PRESSURE in-tank pumps (FDMs) was '90 for all F-series/Bronco.

84Red

Plugged the rear tank into the front tank connector and it reads past full, indicating an open circuit. Not sure why the readings are different, but at any rate I will buy a new fuel pump. Also got off the rear tank strap that I couldn't fully get off. One of the bolts rust welded itself to the nut. A little persuasion from my dremel helped. Nice for spaces the grinder can't fit into.

Steve83

Why would you buy a pump to fix a gauge problem??? They're separate. And on pre-'87 trucks, the needle going past full indicates a SHORT circuit (to ground) - not an open circuit.

84Red

The fuel pump assembly is rusty first of all, so I would rather replace it entirely. Even if this pump is working correctly, it is 32 years old already, and I would rather have 1 new pump and 1 old pump that work good than 1 good pump and 1 pump that I don't still know works 100%. I've always heard that past full indicated an open circuit...