Fuel Pump won't draw fuel from tank

[74 Shortbed]

No mounting plate, pump bolts to the block and pump arm rides on the cam eccentric.

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[1987 GMC Jimmy]

Rats! It's a little tough to assume there were two bad pumps in a row, but with the QC they have in these foreign factories, anything's possible. I'd take the valve cover off and see if there's anything suspicious going on there. It should be easy to diagnose since there are only twelve valves all by each other. Just see if one rocker arm moves less than the others or not at all. Sorry for being stubborn back there. Pretty much all my experiences with Chevy motors are with SBC/BBC so I'm just basing my opinions on that. Hell, I've never done anything with a 4.3, a 250, or a 292. When you felt the lobe, were you able to rub the pointy side? The existence of a pointy side is a good sign, of course, and maybe if you can isolate it, you can post a picture if it's visually accessible. If the cam lobe is good, the lifter is probably exonerated from scrutiny so I'd check the pushrod for straightness and the rocker for relative tightness. The ones associated with the fuel pump that is. I think it's safe to assume that the pump is bad or the pump actuation mechanism is bad.

 

[Geektrucker]

Hey.. my ears are ringing!

Ok, after reading the thread, let me clarify a couple of things I've learned...

1. There is no push rod for an inline 6, the pump arm makes direct contact onto the cam lobe.

2. There should be NO shims or spacers between the block and the pump, the pump is bolted directly onto the block.

3. NEVER judge a pumps effectiveness by looking into a clear filter... I've seen them running fine without the filter being full

Okay, here's my $.02 worth.. take said same gas can, with a short chunk of fuel line (about 2' to 3' preferably on the ground below pump level) and connect directly to fuel pump inlet. Attempt to prime / start truck.

If truck starts / runs fine for more than a minute, you've narrowed it down to line back to tank, replace it all (there may be a pinched line that blows air fine, but not allow enough fuel through).

If truck does NOT run, back half is ok, reconnect and eliminate BOTH fuel filters and run line direct from pump to carb

If truck runs fine without filters, replace both (you really only need one good one) (get a good metal filter, those clear ones are infamous for leaking air!). Also, when installing in-carb filter, make sure it's the correct way... yeah learned that one the hard way and where I'm personally placing my bet...it goes butt end in first AFTER installing the spring...

If truck does NOT run, it has to be either pump itself, or cam lobe. Because you've replaced the pump (although technically still possible it's bad out of box.. how long has it been sitting?), it may be cam time.

Try those steps and let us know what you get...

 

[Geektrucker]

As a quick follow up.. if you get to the point of testing without filters, test direct to carb with new line if possible. Again, the line from pump to carb could be pinched somewhere and still blow air through it, but not allow enough fuel flow.

 

[Polyphemus287]

Hi all, apologies for the delayed response. I have been playing with this issue some more.

I rigged up an old gear oil container i cleaned out with some brake cleaner to make a gravity fuel feed to the carb. Truck ran fine this way. It ran thru approximately 1/2 a quart of fuel from the container before I shut it off or 5 minutes of run time.


At this point, I blew air thru the new fuel lines from that run the tank to the fuel pump. I held a red cloth rag at the fuel pump side. It blew so strong that the air pressure ripped a whole in the cloth rag.

I reinstalled original pump (6 months old) on the truck, and vacuum tested for a short run after i primed the carb floats again ( 30 seconds run time). It pulled above 16.5-17 hg consistently while running. While installing the old pump I rechecked the cam lobe for wear. I can't really lay eyeballs on the lobe while its in the truck, but I believe I felt the presence of a lobe on the cam. Definitely no scarring etc.

Next, I ran a rubber hose from where fuel tank meets line to a can, primed the carb again and let it run. No suction, clear filter filled 1/2 as usual, no gas at pump. Just because, i hooked the fuel line back to the tank, and of course nothing. Next, I switched the evap line with the fuel pick up line from the tank just to be double sure. No dice.

From this point I traced the fuel line for the third time. No kinks. The fuel line is more or less a straight shot down the frame rail to the pump. If anything it is straighter and has less bends than the original. I may attempt to remove the filter in between the tank and pump.

At this point the only thing I can fathom is I have two faulty brand new pumps, or the pumps still read good vacuum even though my cam lobe is finished. However, The truck idles well, and has no hiccups at higher rpms. I wonder if the cam has a separate lobe for the pump, and it could be wiped but the lobes for the valve train be fine? Is there a way to verify cam wear via the fuel pump hole?

If this is the case the engine isn't new enough to justify a new camshaft etc. If the fuel pump cam lobe is wiped out I will probably just install an electric pump.

Edit: I will try to post some pics up ASAP.

 

[Georgeb]

Looking at pics online it appears that the pump lobe is separate than the calve train lobes. It is a round lobe that is eccentric to the centerline. Did I read right that you have a fuel filter between the tank and pump? Thats not typical. What happens if you put a little pressure in the tank? You could hit the evap vent line with the air hose. Can you get fuel to come out the line at the pump?

 

[Geektrucker]

Hi all, apologies for the delayed response. I have been playing with this issue some more.

I rigged up an old gear oil container i cleaned out with some brake cleaner to make a gravity fuel feed to the carb. Truck ran fine this way. It ran thru approximately 1/2 a quart of fuel from the container before I shut it off or 5 minutes of run time.

Hmm.. with the pump putting out the Hg you're saying, that tells me the diaphram is good in the pump.

What I don't see, have you tried removing both filters?

Have you tried turning engine over while output on pump is setup to pour into a can, physically see what you're getting from the pump?

I'm still inclined to think it's between the pump and the bowl and I don't see where you've ruled that out yet

EDIT: Nevermind...re-read your original diagnosis

 

[Camar068]

Hey.. my ears are ringing!

Ok, after reading the thread, let me clarify a couple of things I've learned...

1. There is no push rod for an inline 6, the pump arm makes direct contact onto the cam lobe.

2. There should be NO shims or spacers between the block and the pump, the pump is bolted directly onto the block.

3. NEVER judge a pumps effectiveness by looking into a clear filter... I've seen them running fine without the filter being full

Okay, here's my $.02 worth.. take said same gas can, with a short chunk of fuel line (about 2' to 3' preferably on the ground below pump level) and connect directly to fuel pump inlet. Attempt to prime / start truck.

If truck starts / runs fine for more than a minute, you've narrowed it down to line back to tank, replace it all (there may be a pinched line that blows air fine, but not allow enough fuel through).

If truck does NOT run, back half is ok, reconnect and eliminate BOTH fuel filters and run line direct from pump to carb

If truck runs fine without filters, replace both (you really only need one good one) (get a good metal filter, those clear ones are infamous for leaking air!). Also, when installing in-carb filter, make sure it's the correct way... yeah learned that one the hard way and where I'm personally placing my bet...it goes butt end in first AFTER installing the spring...

If truck does NOT run, it has to be either pump itself, or cam lobe. Because you've replaced the pump (although technically still possible it's bad out of box.. how long has it been sitting?), it may be cam time.

Try those steps and let us know what you get...

perfect example of the "Half Split Method" of troubleshooting.

 

[Polyphemus287]

Hi All,

I removed the filter in between the tank, and the fuel pump with no change. Still no fuel getting to the pump, or coming out of the pump outlet. Its pretty unreal.

The thing that gets me about this whole problem is that when the problem initially started, the truck was starving for fuel under throttle, but would still idle all day no problem. Somehow with new line, filters, and pumps I have arrived at less flow than I originally had. However, the problem remains the same as still not enough fuel is being pulled from the tank to the pump.

With all things considered I have to assume at this point that some camshaft problem exists, thus causing the fuel pump to not perform well.

Any thoughts on what electric fuel pump to use a stock carb'd vehicle? I was thinking about the Mr. Gasket P#12s pump. It states it produces 4 to 7 psi. Anyone know if I'd have to run a return line?

 

[1987 GMC Jimmy]

I've got a silly question, but have you tried mounting it the other way? Like inverting it 180 degrees from how you have it now? I've seen pictures, and they're upside down to how SBC/BBC pumps mount, and I've read that they can mount both ways. If that's a dead end, wouldn't you think that if it is the cam lobe, you've got a valve not opening all the way, and your engine's AFR or exhaust is being restricted? Hell, there could be other wear potentially. If your confident that an electric pump will fix the issue, I'd get the low pressure one so you don't have to use the regulator. I've seen the regular O'Reilly shelf pumps mounted on the fan shroud so it'd be easy to wire, but you can pretty much put it anywhere. I'd think it'd be safer away from electronics and exhaust.

 

[Polyphemus287]

So I think I've figured out the issue.

Today I wired & plumbed up a Mr. Gasket electronic fuel pump. Truck got fuel all the way to the carb, and the truck ran like a champ, very misleadingly . I decided to change the oil as I've been priming the carb so much that I was worried about gas in the oil.

The truck has one of those magnetic oil drain plugs. When I dropped the oil i found about a 1/4 inch of what I can only describe as sludge and tiny metal particles on the drain plug magnet. So that explains why the mechanical pump isn't working! I assume the engine's cam and bearings are toast.

I dissected the old oil filter and it had massive sludge deposits in it. This is the first oil change that I have done to truck since its initial oil change back when I first got it. Approximately 2500 miles ago. I knew the engine was tired when i bought it, but just not that tired.

I reckon that the mechanical pump pulled vacuum because the cam still had enough meat on it to pull vacuum but not actuate it fully.

However, the truck still runs fine at this point. It makes no noise or lacks power. I guess I will run her till she dies, and then rebuild the engine or swap in a V8.

Thanks to all for the help and advice!

 

[chengny]

Under what conditions was the following vacuum check conducted? Was it done at the pump end of the steel line or at the tank end? Secondly, was it done with or without supplying an artificial suction head pressure?

I reinstalled original pump (6 months old) on the truck, and vacuum tested for a short run after i primed the carb floats again ( 30 seconds run time). It pulled above 16.5-17 hg consistently while running.


If I am reading this correctly, it appears that the pump is consistently able to develop suction/pressure/flow, but only under the following conditions:

1. The rubber dutchman hose has been disco'd from the forward end of the run of steel
tubing, and

2. The hose is then submerged in a temporary container of gas, and

3. The engine is started,

4. The pump is allowed to pull directly from the container

This is true - regardless of whether a positive suction head is applied to the
pump or it is forced to pull the gas up and out of the temp container.

I think it would be safe to say; the pump works as designed as long as everything aft of the rubber hose has been eliminated from the system. Correct?


But, on the other hand, when the rubber hose (that connects the pump's suction port to the forward end of the steel tubing) is reconnected - as would be expected in normal operation - things start to get more complicated.

Now, when an attempt is made to produce flow in the system (by running the engine), the pump appears to only perform as designed under the following conditions:

1. The rubber hose at the aft end of the steel tubing is disconnected from the sender
outlet, and

2. The end of the hose is submerged in a temporary container of gas, and

3. The container is raised to some point above the level of the pump suction, and

4. The engine is fired

When set up like this, the pump appears to be able to supply the carburetor with sufficient gas to allow for continuous operation.

If either of the following are attempted, proper pump operation quickly ceases - flow/suction/discharge are lost:

1. The temporary container is relocated to a point below the pump suction, or

2. The rubber hose is removed from the container and reconnected to the sender outlet

IOW, with the fuel pump connected to it's normal supply (the gas tank) - but now via the steel section of gas line - it stops pumping.

Check the steel line with compressed air and soap suds. I can almost guarantee you are sucking air. Where did you mount the new fuel pump? If it is piped into the suction line back near the tank, it will keep the section of steel tubing pressurized. Now the leak (if it exists) will let gas out rather than letting air in.

 

[chengny]

The truck has one of those magnetic oil drain plugs. When I dropped the oil i found about a 1/4 inch of what I can only describe as sludge and tiny metal particles on the drain plug magnet. So that explains why the mechanical pump isn't working! I assume the engine's cam and bearings are toast.

Aaah, that's nothing. The motor isn't toast. You would know it if the bearings were shot. A 250 is bullet proof. Keep the faith!

 

[Polyphemus287]

Chengy I hope not! Honestly, the truck hasn't burnt any noticeable oil since ive owned it. It does have a small oil leak at the pan, but no blue smoke etc. With the new electric pump it runs as good as it ever did. No noises from the engine.
I kind of astonished this old 250 still runs this good, with the amount of metal particles I drained. When I worked at peterbilt I saw trucks that had less metal in them be completely destroyed. Maybe the death of the motor is still coming, or maybe that magnetic plug, and the napa gold filter saved it from too much damage. Fingers crossed. I just want her to keep on purring.