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1986 Ford F-150 with a brand new fuel tank and fuel lines has gone through 4 mechanical fuel pumps in a couple of months.

I put a sock on the pickup tube (was originally missing), an extra fuel filter on the line in front of the pump, etc. They keep going out.

I even put an electric pump on as a stop-gap backup and that burned out after only around 100 miles (it's possible I wired it wrong or something but it seems suspicious).

At this point I am thinking about dropping the tank and completely emptying the gas and checking for sediment or something (but the tank is only a couple of months old!).


UPDATE: The mode of failure is acceleration is increasingly sluggish and the engine bucks, then sputters and dies. Sometimes it will restart briefly, like long enough to get where you're going. Eventually it won't start at all.

I cut one of the bad pumps in half and observed that the diaphragm had become brittle and cracked through.

I decided to pump out all the old gas through the fuel line so I could remove the tank and inspect the inside of it. During that process the diaphragm in my hand pump suffered the same fate. Some of the fuel (that which I pumped out first) was very discolored (dark amber) and smelled like varnish. When I removed the tank and opened it up, the whole thing had the varnish smell.

Absent a better answer, I'm chalking it up to filling up at some pump with contaminated fuel. The truck has been driven regularly, so I don't think the varnish-y gas is due to it getting old while in my gas tank. Maybe it was already old when I pumped it in there.

I refitted the tank and filled it with clean gas, put on one more mechanical pump (these have all been Spectra pumps I believe), and it has been running correctly for about an hour's worth of driving.

We'll see what happens.

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    When you say gone through what is the actual failure, and how was it diagnosed?
    – GdD
    Commented Jun 6, 2022 at 19:12
  • There is nothing to be miswired in an electric pump. It has + and - and it has to stop with the ignition off.
    – fraxinus
    Commented Jun 9, 2022 at 19:13

2 Answers 2

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What has been the mode of failure? Given the many failed mechanical pumps in a short time frame, incorrect installation, the wrong fuel pump, or some sort of fuel restriction are your likely culprits. Assuming you're blowing out diaphrams: do your new lines include any rubber lines involved? Are the lines sized correctly? Not sucking in air anywhere? Have you checked for any pinches or restrictions? Is your tank building excessive pressure? Is the fuel filter correct, and orientated correctly? Clogged?

No comment on the electric fuel pump, I would ignore that and focus on checking every item mentioned above. If you've been using the same mechanical fuel pumps, I'd recommend trying a different brand in case there's a bad batch.

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  1. Hmmm… I'd say ethanol, or perhaps even methanol, is the culprit. A lot of gasoline, especially in the US Midwest, is "contaminated" with ethanol these days.

Ethanol degrades natural rubber. Vehicles newer than 1996 have Viton™ or other synthetics in place of natural rubber. Older vehicles do not.

Older vehicles can generally tolerate 5% ethanol with some grace, but it will eventually degrade natural rubber. If you've put 15% ethanol in it, it will degrade three times faster. And if you've added any fuel-line de-icer or other gasoline "treatment," you may have added methanol, which is even more destructive of natural rubber than ethanol is.

My experience and research is primarily from Diesel engines, because I make my own biodiesel, and methanol is used in its production. The rule-of-thumb among biodiesel brewers is that you get a few tankfuls to a few tens of tankfuls before your natural rubber begins to seriously degrade. But the principle will be the same with gasoline engines.

https://www.treebuzz.com/forum/threads/does-ethanol-speed-up-rubber-breakdown.10370/

It is odd that the new fuel pumps you bought would not have Viton™ components. You might see if you can get a fuel pump that is certified for "E85" fuel, which has 85% ethanol. But you'll still eventually have trouble with flexible fuel lines and other vintage components that inevitably have natural rubber in contact with the fuel.

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