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Old January 28th, 2019, 06:02 PM
Clarke Clarke is offline
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Join Date: Jan 2013
Location: Eureka Springs, AR
Truck: 1962 1500 Wideside 4 speed (305D / SM-420)
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Default Re: 1966 And Later Fuel Pumps (My experience with 1962 Straight Arm Pump)

Right or wrong, on this particular subject, I don’t believe ethanol gas and/or NOS is the issue, for the following reasons:

I’ve had my truck for about 12 years now and have driven it about 12K miles with 9 or 10 mechanical fuel pump changes. My truck originally had 4.56 rear end and for the first few years I only drove it during the summer months, because I spent the winter months restoring the truck. During that time, I had to replace the fuel pump (O’Reilly brand) two times and maybe put 1K total miles on the truck during those first few years, running the cheaper “up to 10% ethanol gas”. In addition, those several winters sitting idle with the same ethanol gas without following any long-term storage procedures/any fuel additives/etc.

The third year I swapped out the 4.56 rear end for 3.54, so I could keep up with highway traffic and not beat the motor up so bad, yet I could still do some moderate towing with my original transmission and oversized tires. With the 3.54 rear end, I put about 5K miles on the truck over a five year period, running the same cheaper “up to 10% ethanol gas” and had to replace the fuel pump (O’Reilly brand) one time.

Then, we got a 4K lb. camper and couldn’t keep up with highway traffic, so several years ago I swapped out the 3.54 rear end for 4.10, and this has been working great for our set up/driving habits. However, I’m back to swapping the fuel pump every 300-700 miles. I have tried running non-ethanol gas, adding fuel conditioners, added 1/16” thick shim between pump housing and block to possibly reduce amount of travel on the straight arm, and tried multiple fuel pumps from three different auto stores (FYI, only got 500 miles from the NAPA pump in 2018). I am confident none of these pumps were NOS, as the packaging was new and the finish on all the aluminum and steel subcomponent parts were fresh looking. Additionally, all three brands had the exact same part number casted into the housings.

Now, there has to be a reason why GMC changed the fuel pump design, making the fuel pump body/diaphragm larger diameter and bending the arm… My theory is, the bent arm reduces the amount of travel on the diaphragm; therefore, reducing the amount of stress/stretch on the rubber diaphragm. Furthermore, due to the shorter (arm) travel, GMC increased the diameter of the fuel pump body to increase the surface area for sufficient pump/vacuum, then increased diameter of fuel line to increase fuel volume. Does this sound logical? Do the later model pumps with bent arm last longer? Can the bent arm pump be used on the earlier 305s, without making any adjustments to the cam/etc?

I know ethanol gas is hard on certain fuel components, as I have used two stroke tools for decades without issues until I started running ethanol gas and soon had to replace all fuel lines because they became brittle. I no longer use ethanol gas in my two stroke tools, but I have proven via multiple “straight arm” pumps running non-ethanol gas/etc within the last few years with my many 600 mile round trips between KS and AR; this is not the case. I truly believe the (straight) arm travel is too great for the (smaller) diaphragm, and when running higher RPMs (up to 3.4K), the result is premature failure of the diaphragm).
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