Fuel Pump won't draw fuel from tank

[Polyphemus287]

Hi All,

The issue that I am having trouble solving is that the mechanical fuel pump on 1983 C10 won't pull fuel (or enough fuel) from the tank.

Some info on the truck:
1983 c10
250 inline six
2 barrel 2ese carb (factory)
Stock mech. fuel pump


The problem first rose it's ugly head when I was driving the other day, at cruising speed, and the truck began to cut out like it was not getting enough fuel. It would idle fine, but when the throttle was applied for any duration of time (1/2 throttle that is) it would stall. As if the carb's float bowl was draining and not refilling quickly enough. I verified this by popping the hood and looking at the clear fuel filter between the fuel tank line and the pump. It had fuel but was approximately an 1/8 of the way full. Prior to this issue It would always be completely full.

From this point I ran a line from pump line to gallon gas can and the fuel filter filled itself to the brim and the truck ran fine with no issues.

I figured there was an issue with the sending unit I installed 6 months ago or a pin hole leak in the fuel lines from the tank. Also, at the same time I installed the new sending unit I installed a new fuel pump. As a result, I dropped the tank again, and removed the sending unit. It looked as clean as the day I installed it, and the tank has no visible rust or debris inside it. I blew compressed air threw both inlets, and no blockage was apparent. From this point, I ran all new steel lines from the tank to the pump, and new rubber hose sections. I also installed a new clear fuel filter.

I reinstalled everything and the pump only pulled a splash of fuel into the clear filter. I primed the float bowl in the carb 3 times and the engine ran about a minute each float bowl prime. I installed a new mechanical pump I had lying around still had the same result. I verified that this pump was working with a vacuum gauge. It pulled 16hg (spec 15 hg). I also put my finger on the inlet and outlet of the pump while cranking and it is defiantly pulling vacuum, and pushing air at the outlet. Suspecting some vapor issue in the tank I removed the gas tank cap, and primed the float bowl. The result was that the filter filled approximately halfway full. I primed the bowl two more times and the filter remains 1/2 full, and no fuel at the outlet of the pump.

At this point I am bit lost, because I am unsure what would prevent the pump priming. I have rechecked my new lines for kinks, and for leaks. I also re-tightened all hose clamps on the rubber fuel lines. All would appear to be kosher. Anyone have any ideas regarding this? Any help is appreciated!

 

[1987 GMC Jimmy]

Okay, so was the strainer sock okay when you pulled the sending unit, or did you replace it? Do you still have the Rochester Dualjet with the filter in it? If so, how's that look? Does it have the evaporative emissions stuff on it still? If so, you should check for vacuum leaks and make sure that line is clear of obstruction. Maybe try running it with the fuel cap off and see if that helps. It sounds like everything's covered here short of the fuel pump pushrod.

 

[Polyphemus287]

87 GMC jimmy thanks for the reply!

When I bought the truck around six months ago I replaced the sending unit, the new sending unit came with brand new sock on the pickup tube. When i pulled the new sending unit out it looked basically new, like the day I installed it (I've only put around 3,000 miles on the new unit) Also, yes, it still does have the Rochester 2 barrel on it with the small filter that goes into the float bowl housing. This filter was also replaced when I rebuilt the carb (around the same time that the sending unit was replaced). The truck ran excellent after the carb rebuild up until this new problem arose. Also, at this point, the pump won't even push fuel up to the carb inlet.

It does have some evap emissions equipment still on it. Everything other than the air pump and the charcoal canister appears to be removed. However, it is missing an emission equipment under the hood so I cannot verify what is missing, if anything.

When redoing the fuel lines from the tank I did run a new evap. line to the charcoal canister. However, I realized after the fact that charcoal canister outlet lines where disconnected. I must have been driving the truck since I have owned with them unplugged with no ill effect (unless they have caused a problem that now has come full circle).

I did remove the gas cap in an attempt to trouble shoot. I primed the float bowl three times with the cap off it ran for almost a minute each time. This resulted in the clear inline filter filling halfway, but still not fuel at the fuel pump outlet.

So at this point I am guessing pushrod, but would the pump still be showing good vacuum if the pushrod was off?

Anyhow thanks agian for the response! I need all the ideas i can get!

 

[Georgeb]

What gets me is you placed a hose in a gas can to feed the pump and it ran fine if I read the original post right so it may still be an issue with the sender. I used to use an outboard motor tank as a temporary tant on my project trucks. Maybe you could rule out the the pump and carb by running on a temp tank of some sort. Or you could look at the sender again.

 

[1987 GMC Jimmy]

No problem, buddy. Well, I have a few issues here. You already resolved one by replacing the evap line and sending unit. If that line is hooked to the sending unit, but the system's open by being disconnected at the canister, it'll suck whatever is floating around in the engine bay down that line. I understand if you don't wanna replace the canister, but you should place a small filter on the end of that line so it can suck freely and not swallow something detrimental. I'd double check that area. Also, I'd double check the pump functionality using a fuel pressure gauge (you can borrow one from Autozone, I believe) with fuel in the system instead of just air. I feel that you may get a more accurate reading using PSI instead of inches of Mercury. A lot of people would say just run an electric pump and say goodnight to the mechanical one, and I just wanted to present that option. However, I'm not a believer in adding electric components. Just me. Well, the push rod can bend sometimes, it can get jammed, and it can get really gunky up in there. For some unlucky folks, if the fuel pump cam lobe wipes, it'll cause the fuel pump to underperform or not actuate at all. I'm not even considering that for you, though. I'd like to see your pressure numbers in PSI instead of your vacuum in inches of Hg. You teeing that in by the pump would be easiest, but I don't want you to run out and get a bunch of supplies if you can find a simpler resolution to the problem.

 

[Polyphemus287]

GeorgeB-Thanks for the response! I will hook the pump back up to a gas can again when I get home today and verify it will draw from a can. I'd just hate to drop the tank again and not resolve the issue.

87 Jimmy- I do have a fuel pressure gauge in my box, but the issue is that it won't even draw up enough fuel to the pump inlet to provide a reading. I do know that it is pulling a marginal amount of fuel from the tank since the clear inline filter is halfway full.

Also, I primed the float bowl one more time and watched the clear filter. You can see that the pump is pulling vacuum but it is almost as if the vacuum from the pump is fighting another source of vacuum. I wonder if plugging the evap line would allow the pump to draw better, since perhaps it is causing some sort of vacuum in the tank that is fighting the fuel pump?

Is there a way to trouble shoot a sending unit for flow? when I had it out of the tank I blew compressed air both ways thru both outlets, and it flowed well.

Also, is there a way to diagnose a faulty pushrod short of replacing the pump? I have already had two brand new pumps on it, so I'd like to avoid throwing parts at it and retain the mechanical fuel pump system as well.

 

[Georgeb]

I can agree with @1987 GMC Jimmy that there could be an issue with the pump and or pus rod/cam lobe but I am stuck on the fact that it ran with an auxiliary can connected directly to the pump inlet. If you have the cap off the tank there shouldn't be any vacuum building there. What if you attach to the fuel line close to the tank and see if you can draw fuel from there to eliminate the possibility of a leak inn your steel line outside the tank. just to be sure you are connected to the 3/8" line on the sender for the source to the pump correct?

 

[1987 GMC Jimmy]

GeorgeB-Thanks for the response! I will hook the pump back up to a gas can again when I get home today and verify it will draw from a can. I'd just hate to drop the tank again and not resolve the issue.

87 Jimmy- I do have a fuel pressure gauge in my box, but the issue is that it won't even draw up enough fuel to the pump inlet to provide a reading. I do know that it is pulling a marginal amount of fuel from the tank since the clear inline filter is halfway full.

Also, I primed the float bowl one more time and watched the clear filter. You can see that the pump is pulling vacuum but it is almost as if the vacuum from the pump is fighting another source of vacuum. I wonder if plugging the evap line would allow the pump to draw better, since perhaps it is causing some sort of vacuum in the tank that is fighting the fuel pump?

Is there a way to trouble shoot a sending unit for flow? when I had it out of the tank I blew compressed air both ways thru both outlets, and it flowed well.

Also, is there a way to diagnose a faulty pushrod short of replacing the pump? I have already had two brand new pumps on it, so I'd like to avoid throwing parts at it and retain the mechanical fuel pump system as well.

I don't think plugging the EVAP line would help, but I'd try anything in the spirit of experimenting to fix your problem. I say this because you mentioned running it with the gas cap off. That would do the same thing. Other than what you did, I doubt it as far as the sending unit goes. There are several ways to go about the pushrod. I'll assume that your pump is not damaged since it worked out of the can, but the pump's gonna have to come off. You could try getting a buddy to bump the engine over and watch the pushrod to see if it pushes out. Or you could take the backing plate off, remove it, and inspect for scarring or bending. There's nothing definitive with this, but I suppose you could take the valve cover off and run the engine for a moment too see if the valves that proximally correspond to the fuel pump move like they should. That would give you the worst case scenario clue about a wiped cam lobe, maybe. I don't know which valve to watch, or I'd tell you.

 

[Polyphemus287]

GeorgeB- That is a good idea, I will place the a fuel can near the location of the fuel tank once I get home and see if it draw fuel from there. When I originally did the fuel can test it was close to the fuel pump itself.

87Jimmy- If GeorgeB's suggestion to place a gas can near the tank location doesn't work, then I will pull the pump and inspect the pushrod. I am unsure what you mean when you say backing plate. Could you elaborate on this some more? To my knowledge the truck's fuel pump mounts directly to 250 inline's block. It just baffles me that it would draw vacuum and the cam lobe be wiped out. However, I suppose anything is possible. Hopefully, the cam lobe isn't wiped, because at that point I am unsure if it would be worth it to install a new cam etc.

I'll report back once I have run some more tests. Thanks again for the help fellas!

 

[1987 GMC Jimmy]

GeorgeB- That is a good idea, I will place the a fuel can near the location of the fuel tank once I get home and see if it draw fuel from there. When I originally did the fuel can test it was close to the fuel pump itself.

87Jimmy- If GeorgeB's suggestion to place a gas can near the tank location doesn't work, then I will pull the pump and inspect the pushrod. I am unsure what you mean when you say backing plate. Could you elaborate on this some more? To my knowledge the truck's fuel pump mounts directly to 250 inline's block. It just baffles me that it would draw vacuum and the cam lobe be wiped out. However, I suppose anything is possible. Hopefully, the cam lobe isn't wiped, because at that point I am unsure if it would be worth it to install a new cam etc.

I'll report back once I have run some more tests. Thanks again for the help fellas!

Yeah, I would definitely do George's suggestion first. I've actually never worked on a 250, but I'm assuming it's exactly the same as the others. The pump mounts to a thin metal mounting plate which bolts directly to the block. I don't think the rod will come out without the mounting plate off of there. Here's what it looks like. I doubt that'll end up being the problem, but like you said, anything's possible. And theoretically, it may not be completely wiped, just enough to halfway push the diaphragm in the pump.

 

[Polyphemus287]

GeorgeB & 87jimmy- I disconnected the lines at the fuel tank to metal line connection where line connects with tank and ran a rubber hose into a gas can. No love. The pump did not pull any gas from the can either. Primed the float and let it run for a minute each try. After theses failed attempts I placed finger on the end of the rubber line to see if the pump was sucking any vacuum. Felt no suction.

From this point I pulled the pump from the block. I saw no pushrod, but I was able to place a finger on the cam lobe. it felt smooth as far as i could tell (i.e. no scarring.) However, i cannot get a visual on the lobe itself. I bumped the engine over a few times to see if there was a pushrod flopping about, or if something else was wrong. I saw no pushrod, but saw nothing questionable either.

I compared the contact points on both mechanical pumps I have tested and they appear identical. No metal shavings or the like, just a little painted scrapped to bare metal where contact was made. The pumps are identical expect one is a napa pump and the other is from orielys.

What gives? At this point i may just have to wire up an electric pump, but I would rather avoid this. I have never wired an aftermarket fuel pump, and I wonder if take this route would a return line to the tank be necessary?

 

[Georgeb]

Well what the frick! I don't think it uses a push rod because the cam is very close to the side of the block. Could you post up a pic of the pump installed and plumbed?

 

[1987 GMC Jimmy]

Damn. I wish I knew more about the 250. Did you see the mounting plate? If you take it off, the pushrod should be clear to come out. I searched for it on O'Reilly, and they showed a pushrod for the 250. I'm not trying to conflict with anyone, but it's hard to imagine the cam lobe scrubbing on the pump arm. The pump looks just like all the other ones. Can I also see a picture of the pump off? Have you tried taking pictures in there to see if the cam lobe is for sure directly in contact with the pump arm. If the plate comes off, and no arm comes out, then the cam lobe being worn from rubbing on that arm definitely comes into play. I'm not trying to doubt anyone. I'm just extremely vexed by your predicament, and I wish I knew more about the Inline 6.

 

[Georgeb]

I wonder if @Geektrucker could help?

 

[1987 GMC Jimmy]

Ooh, good idea.