m880 fuel pump

I just picked up a 75 M800 with a 77 parts truck. There is fuel to the pump. I tested the pump on the bench and it seems fine. Mounted in the engine it does not pump fuel. I assume that the rocker arm on the pump rides against a cam of some sort. When I install it, there is a slight tension downward on top of the arm as should be. Acts like the cam or what not might not be turning...? Doesn't make sense to me, unless it has a key or something that could be sheared. I've yet to pick up a manual.

If there is an internal problem, I've considered just going to an in line electric pump. I'm not particular about the truck being origninal. If it is not a major tear down to fix, than I prefer the mechanical...

Any input will be appreciated.

There is an eccentric mounted to the cam gear in all small black Chryslers that the fuel pump arm rides on.

How did you bench test the pump? I have had mechanical pumps that seemed like everything was fine externally, but actually had a rotten baffle on the inside or a busted check valve. It should be pretty cheap at any local parts place. You might just go buy you a new one and put it on there. Advance Auto lists pumps for $14-35.

I had a problem with my SnoCommander (78 dodge like the M880). I was like the pump was going out from time to time. I would blow air across the inlet line to the pump and fuel would come out. After I primed it like this it would work for a while then die like it was not getting fuel. It ended up being a few pin hole leaks in the metal line near the tank.

I tested the pump by clamping it in a bench vise, running a length of fuel hose from a small gas can to the inlet on the pump, and rapidly pumped the arm by hand. While obviously insufficient for a pressure and flow test, it did readily move fuel through the pump. Pretty good spray at that. On the vehicle: not a drop of fuel. This lead me to believe that the problem lies within the engine and that the arm on the pump is not getting rocked.

That eccentric on the cam, shown in the above image, looks like it has an indent for a key or something. What secures it in its place on the cam? Does the cam shaft need removed to replace it or is it at the end of the shaft and removable without lifting the shaft out?

Before you go to the trouble of a tear down, crank the motor with the pump in place, running a piece of rubber fuel hose from the pump suction side to a container of fuel and a hose from the output fitting to another can. If it pumps fuel while cranking, the engine eccentric and likely the pump is OK. If this works, take an air hose and blow back toward the tank by inserting the air nozzle into the suction hose from the truck frame that connects to the pump intake side. An assistant should hear a strong bubbling in the tank. I have had tank problems with two trucks I have bought. The old fuel had been in them so long it turned to gum and plugged the intake. I have had to drop the tank out of one truck and clean it. There is a plastic screen on the end of the tank pickup tube that can get clogged with gum. The advice about looking for holes in fuel lines and cracked rubber connector lines is also good. If you have holes in a line or connector hose, you will see wetness there from fuel seeping out with the motor off given there is fuel getting to the bad spot. You can also check by putting the output hose form the pump into a glass jar and watch for large bubbles in the fuel that is pumped. Since you have no flow at all, do the pump on the motor check and blow through the line as your first checks.
Regards Marti

That eccentric on the cam, shown in the above image, looks like it has an indent for a key or something. What secures it in its place on the cam? Does the cam shaft need removed to replace it or is it at the end of the shaft and removable without lifting the shaft out?

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There is a woodruff key (like what goes in crank snout to keep a harmonic balancer from spinnin on the crankshaft) on the end of the camshaft. The end of the camshaft has a threaded boss, a large washer goes on to hold the eccentric into place and a bolt in the end of the cam holds the washer in place. You should not need to remove the cam itself, but you will need to pull all the engine accessories off the front, the harmonic balancer and the front timing cover. You will also need to remove the timing chain.


I would highly recommend following all the diagnostic steps that Marti outlined, as the odds of the eccentric being a problem are slim. i have been playing with Mopars for almost 20 years and never had an issue with a fuel pump eccentric.

Clinto says:

Quote:

That eccentric on the cam, shown in the above image, looks like it has an indent for a key or something. What secures it in its place on the cam? Does the cam shaft need removed to replace it or is it at the end of the shaft and removable without lifting the shaft out?


There is a woodruff key (like what goes in crank snout to keep a harmonic balancer from spinnin on the crankshaft) on the end of the camshaft. The end of the camshaft has a threaded boss, a large washer goes on to hold the eccentric into place and a bolt in the end of the cam holds the washer in place. You should not need to remove the cam itself, but you will need to pull all the engine accessories off the front, the harmonic balancer and the front timing cover. You will also need to remove the timing chain.


I would highly recommend following all the diagnostic steps that Marti outlined, as the odds of the eccentric being a problem are slim. i have been playing with Mopars for almost 20 years and never had an issue with a fuel pump eccentric.

Click to expand...

Clinto says:

Quote:

There is a woodruff key (like what goes in crank snout to keep a harmonic balancer from spinnin on the crankshaft) on the end of the camshaft. The end of the camshaft has a threaded boss, a large washer goes on to hold the eccentric into place and a bolt in the end of the cam holds the washer in place. You should not need to remove the cam itself, but you will need to pull all the engine accessories off the front, the harmonic balancer and the front timing cover. You will also need to remove the timing chain.


I would highly recommend following all the diagnostic steps that Marti outlined, as the odds of the eccentric being a problem are slim. i have been playing with Mopars for almost 20 years and never had an issue with a fuel pump eccentric.

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I appreciate the input guys. I did run a line from a gas can to the input on the pump while cranking the engine, so I know that it is not the fuel lines or tank. I've never heard of an eccentric being the problem either, but the fact that the pump would move fuel when tested outside of the vehicle but won't when mounted and the engine cranked left me stumped for what else it could be.

This may be impossible, but I'd try seeing inside the timing cover with a mirror. That may "shine a light" on things.

In my experiences with these trucks.
The fuel pickup in the tank and the fuel lines get quite rusty from just sitting.
This rust restricts the flow of fuel, making the pump work harder.
The rust particles get sucked into the fuel pump damaging it. While it can suck fuel through a shor clean line to a gas can, it won't pull through the clogged line from the tank.
I now have the see thru filters before the fuel pumps on my trucks.
and spare fuel pumps on hand.

for a simpler test of the lobe on the cam, just place the rod in its spot and hold it up against the cam with your finger, then have a buddy rotate the engine with a bar on the crank bolt, if the rod moves your finger, its good.

I have never seen an eccentric come loose. If it did the bolt on the cam gear of the timing set would be loose also and the truck would jump time. I really think you need to check the lines from the tank or use a boat tank hooked directly to the fuel pump inlet before you start tearing the engine apart. A messed up eccentric is very unlikely and a long shot.

CCATLETT1984 said:

for a simpler test of the lobe on the cam, just place the rod in its spot and hold it up against the cam with your finger, then have a buddy rotate the engine with a bar on the crank bolt, if the rod moves your finger, its good.

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I think you might be thinking of something like a small block Chevy that uses a pushrod between the eccentric and the arm on the pump. The small black Chrysler is not like that, the arm on the pump is what the eccentric operates.

oh, okies

yeah i was thinking like my cucv and the chevies that i have worked on, although couldnt you do the same thing by placing something to ride on the lobe to see if its the problem?

You could but due to the large open area, I would be worried about it coming off the eccentric and hitting something or getting hung up in the chain and damaging the engine, or more importantly, your finger.

Is it possible that the arm on the pump is either A: worn till it is too short to be pushed by the cam, or B: Not seated properly to be moving?

It's got to be something stupid as to why he's not getting fuel.

I really think you need to check the lines from the tank or use a boat tank hooked directly to the fuel pump inlet before you start tearing the engine apart.

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The problem does not lie with the tank or lines in this case. As stated above, I've run a short flexible line directly from a container of fuel to the inlet on the pump. It has to be the pump or what runs the pump. The pump tests on the bench. Pumps are cheap and I will probably replace it with a new one anyway, but fail to see why if it works on the bench, it wouldn't on the block. Eccentrics aren't too expensive either, but obviously more labor intensive. I've little spare time to work on this unit, but will let you all know that I came up with when the problem is solved. Everyone's input has been appreciated.

Loosen the pump ,and crank the engine. IF the pump jumps up and down the arm is moving. your bench test may have moved the arm further, or in a differant spot than when on the engine. the pump can be shot in the area that it always works. i see a lot of pump falures. never the cam.