1966 And Later Fuel Pumps

[Clarke]

Originally Posted by ilvracn

i still had my old pump laying on the bench from this summer when it went bad. i decided to take it apart, and see what went wrong. one of the check valves fell out. i was driving at hiway speed, i think the vibration from 3400 rpm probably vibrated the valve from it's seat in the pump. diaphram looked excellent. looks like manufacturer defect, should have been staked in.

Interesting... Did the engine die on the road or did it still run?

The first several that went bad on me, I took apart and they had a small tear in the rubber diaphragm towards the outer edge just inside the body flange. I’m assuming this due to having a hard point/edge that the diaphragm is repeatally flexing against? The pumps still worked with tear, but leaks oil from weep hole and I’m assuming you risk getting gas in the crankcase if you continue driving. Luckily, none have left me stranded.

Thanks for sharing.

[ilvracn]

lost power going up a long hill, pulled the hose off of carb, would still squirt a little gas out. i thought gas gauge was wrong and ran out of gas. a farmer stopped to help, and went and got 5 gal gas. put 4.5 gal in tank, same thing. filled holley carb float bowl from vent hole, got truck running i was able to drive it home. started to run out of fuel in carb again on next hill. backed off of throttle, ran about 1/4 to 1/3 throttle up hill, made it over top of hill. was able to nurse it back, just kept rpm's up and throttle light. i was expecting to have crankcase full of gas but it was ok. changed oil just to make sure and replaced fuel pump.

[zoulas]

Which fuel pumps are rebuildable? I have been getting the NAPA and they seem rebuildable but I am told there is no rebuild kit. The ethanol gas seems to destroy these pumps.

[Clarke]

Originally Posted by zoulas

Which fuel pumps are rebuildable? I have been getting the NAPA and they seem rebuildable but I am told there is no rebuild kit. The ethanol gas seems to destroy these pumps.

Not sure about the rebuild kit, but based on my experience, Ethanol gas has nothing to do (or very little) with premature failure of the rubber diaphragm. You can find detailed explanations to my therory in earlier posts.

In short, the pump arm stroke (travel) is too great for the (smaller) diameter of the pump. I have proven this by using and hauling non-Ethanol gas on many road trips, and by the type of driving. These pumps last a lot longer when cruising around town at low to mid RPM range.

Constant higher RPM is what kills these pumps quicker. I have proven this multiple times by replacing the pump twice within a few hundred miles, running constant highway speed at 3200-3400 RPM while burning non-Ethanol gas (versus driving the same miles back and forth to work through town over a longer period while burning Ethanol gas).

My experience is based putting approx 15,000 miles over the last ten years on my ‘62 305.

[FetchMeAPepsi]

Another big killer is drying out. If you use your truck then don't use it for extended periods you can get dry cracks in the diaphragm that keep your pump from being able to generate any pull - or push.

[Funky61]

GMC V6 Fuel Pumps comparison, rebuilding kit and swapping

Clarke:
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.

Reading this it got me thinking about rebuilding my old fuel pump from a 1962. It has the straight lever and not the bent lever and secured with 10 bolts. I believe Clarke is correct, that the bent levers were an update to reduce premature fuel pump failure. His thoughts on Ethanol fuel not necessarily being the culprit, but excessive, extended RPMs, made sense.
Trying to find a rebuild kit for the GMC unit that could be rebuilt, was a discovery. I found a kit from a company called, Then and Now, #FPA-225, which is for a GMC V6, but it doesn’t have the pressure spring. Cost is $52.50 plus about $12 for the spring.




Top Flight also has a kit that with the Pressure Spring that look’s more correct for my pump and runs $100. It’s listed for a 55-66 Corvette so maybe that’s the reason for the high price. Still cheaper to buy a new one but wait…




I researched and found the Corvette style pump, with the ten bolts, and it looks like a regular pump for a six cylinder Bel Air. It was cheap, about $30 and it weighed less, but it matched up, so I swapped the lower bowl and changed out the upper Diaphragm and reassembled. I could have just swapped the lever, but chose not to do that.




I also purchased the Carter M3955 for comparison. It has the straight lever and is much small and lighter with smaller inlet and outlet holes.



Also purchased a Carter crimped style, bent lever, fuel pump M4548 mentioned in the above thread.




Lots of tutorials online on how to rebuild a fuel pump and it’s a fun project. But as an alternative to the kit, I just decided to swap out parts from a newer unit.

The Carter Electric fuel pump (4070?) I used for many years, is now manufactured overseas. I have been reading about premature failures for these units, and it was suggested to buy NOS with “Made in the U.S.A.” stamped on them.