Fuel Pump

Maybe this might save a bit of grief working on these old mechanical pump systems, maybe not.

I am on pump #4 since I got my truck. It was sold with a "bad" pump. I am not driving this on the street yet but I have already bought 3 pumps, make that 2 one was defective out of the box, but maybe it wasn't defective after all.

I finally have one that is working as is should since saturday.

With the most recent pump I replaced the entire fuel line from the tank with 5/16" fuel line, this goes into a 5/16" clear housing filter (we are trying to learn here) then the pump, then a second fuel filter and finally all new lines up to the carb.

Works like it should.

Observations.
1. Primary filter only has a SMALL amount of fuel in it while running.
2. Secondary filter is mostly filled with gas, small air bubble in top.
3. You can feel the pump "ticking" when you touch the line or filter at idle.
4. Previous pump, cut open and found likley cause of check valve failure to be old rubber fuel line debris under check valve diaphragm, unconfirmed.
5. The pump has an inlet and outlet check valve, looks like low pressure is created at diaphragm during pump cycle, check valves allow fuel in, then pushed out on return stroke, but not to flow back into tank. My old one always seemed to loose prime, needed a splash of gas to get it woke up.
6. New pump hand cycled on the bench made a definet WHOOSH WHOOSH.

Didn't find much help in any manual. Other than hooking up a pressure gage which I do not own. And just R&R a ned pump with old line debris would have just ruined a new pump.

This old stuff needed refreshing in my diagnostic control system, me.

Sounds like you have diagnosed the problem correctly regarding the debris compromising pump valve function. The whoosh sound should happen with a good pump, you should feel suction while cycling the pump when holding your finger over the intake, pressure when holding your finger over the outlet. The pressure at the outlet should hold for a while after you quit cycling the pump. On the truck, you can run the truck for a bit, shut it off, then loosen the fuel line at the carb and you should have spray of fuel under pressure as the pump holds pressure. Be careful of doing this with any ignition source present, or with a hot engine!!! The pump diaphragm has a spring under it that is set to give the correct fuel pressure. When this pressure is achieved, the pump diaphragm actuating mechanism is held off the cam until fuel demand again allows it to cycle. Because of this feature, there is no need to divert unneeded fuel back to the tank. It is not unusual, but frustrating that fuel filters do not fill completely. There is enough flow through the filter media to keep it supplied without all of the air having to be removed. If the outlet of the filter can be mounted or held in a vertical position with the motor running, the air will come out. All of our trucks, after sitting for a week or so, need a prime. The carbs seem to vapor dry easily.
Hope this helps! Regards Marti

Marti,

You filled in several gaps. Thanks. I found the spring under the diaphragm, I didn't know how it functioned as a pressure control system but after you see it, makes sense.

It started up tonight without much fuss at all, sits there and idles nicely. I think up north, we would be reworking the fast idle cam system, but ain't a problem yet.

I think the biggest mechanical work I did to this engine so far that transformed how it runs was the timing chain project earlier this year, major difference. Previously posted.

Pretty soon, the 727 will be coming out and the NP435 handshaker will take its place. I have as far as I know, all the parts I need. Bell housing does need some thread repair, but it can be dealt with. Even found a new flywheel, price is right also.

My M38 that I owned had a primer handle that allowed you to pump fuel w/o the engine, saved cranking and batteries. My 5.9L 12V Cummins has a hand primer for use after filter changes.

My 880 went though the same issues until I added the filter before the pump. Then for years the filter would gradually plug up resulting in lower and lower top speed. Had several filters in the glove box at all times.

you guys gave me some new stuff to look at. I have an M886 with the 318. I probably got it about 6 years ago. I cleaned the fuel tank, new fuel pump, and new carb at the time. There is probably around a 1000 miles since then. For the last year, if it sits for a week or more, it needs to be primed to start.

I have a clear filter after the pump and it runs with very little in the filter. If it is started every two of three days, there is no problem. What it seems that you are saying is that there is a check valve that may be clogged in the pump? I do not have a pressure gauge either.

I will add a filter before the pump and replace the pump to see if that works. It used to run fine with no problem leaving it for a few weeks, it would start right up. Once I get it primed it runs just fine.

Any thoughts or updates on this would be appreciated.

Filters and pumps are cheep enough, But with only 1000 miles on the pump I think I would try cleaning it and putting the filter before the pump first, to see if that works.

Good point on the pump, will try that. Unfortunately too many green vehicles, so the M886 is in line after the M54 brake issues.