1966 And Later Fuel Pumps

[BobBray]

While the old style 'rebuildable' screw-together fuel pumps still seem to be available, the non-serviceable 1966 and new pumps often show as discontinued on auto part supplier websites. This is not altogether true, the later style pumps are still available though they are hard to find. The problem is the fuel pumps on 305E's and 351E's used in light trucks came with a 5/16" hose nipple inlet, and those are discontinued. However, the same pump with a 3/8" inlet is still manufactured. You can find the 3/8" inlet pump listed for V-6's in medium and heavy duty trucks with the larger 'Magnum' V-6's. They have the same 1/2" flare fitting outlet, so all you have to do to use these pumps on light duty trucks is get a 3/8" to 5/16" reducer hose nipple. Part numbers for the 3/8" pumps are as follows:

NAPA NNP B0143P
Airtex 40050
Carter M4548
Precision M20022 (O'Reilly's)

[lizziemeister'sV6]

Great information! Would you give us the insight on the curved arm and the straight arm versions of pumps depending on the year/model of the engine. Was the reason of this the difference in the camshaft? OK to use straight arm version instead of curved arm and vice-versa? Good knowledge to share - THANKS!

[BobBray]

That's a good question. The non-serviceable '66 and newer pumps all have the straight arm. I believe at some point the mounting bolt holes were changed to 5/16". I think the curved arm pumps were very early, maybe '60-'62. Couldn't say if the difference was the fuel pump eccentric on the camshaft or the timing cover.

[LEWISMATKIN]

Bob, the only difference between the '65 pump (AC #4578) and the '66 pump (AC #40050) is the inlet bib. GM increased the inlet size of all vehicle lines to 3/8" from the earlier combination of either 5/16" or 3/8" depending upon division. Whether or not the arm is bent or straight is dependent on the manufacture of the specific pump (AC, Airtex, or Carter), either will work on the V6 engine.

[BobBray]

Funny thing, my '67 has 5/16" fuel lines to the pump. This is where I ran into a problem, the 5/16" inlet pumps seem to be all obsolete. Only pumps for my truck listed at places like Rockauto now are universal electric pumps.

[LEWISMATKIN]

Bob, I know it is possible that GM continued the 5/16"-3/8" during production in the light duty trucks. The I-6 still carried a 5/16" line into the early 70's. However, the '67 my dad had (which he had sold when he was a dealer) used the 40050 pump. I replaced it at least once while we had it.

[Clarke]

My ‘62 has the 5/16” line with the straight arm pump, which I have replaced seven times within the last two years. I have driven my truck several thousand miles the last couple of years and after several hundred miles the pump starts spewing oil out of the weep hole near the body flange that is screwed together.

I’m assuming this is mostly due to the rubber diaphragm being too small diameter and constantly running the engine at 3,400 rpm to maintain 65 mph.

A couple years ago I almost swapped the pump to a later model, which has a larger diameter diaphragm with angled arm, but I wasn’t sure if it would work and for that reason was hesitant to change my fuel lines from 5/16” to 3/8” dia. I’m assuming with the bent arm the stroke travel would be less, but with the larger diameter diaphragm (increased surface area) maybe there would still be ample fuel volume and the pump would last longer? I ended up taking the later model pump back to Napa and still changing out the original. I will most likely upgrade to electric fuel pump.

[BobBray]

Is there a lot of alcohol in the gasoline you are using? I have seen alcohol do funny things to older rubber, maybe the pumps you are getting are old stock. The new NAPA fuel pumps are supposed to be alcohol resistant.

[Clarke]

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).

[BobBray]

Makes sense. I would agree with your theory about pump diaphragm size.