Help with fuel problem in 1970 521

[Gabriel]

Hello everyone. Since I got my 1970 521 a few months ago, I’ve been an avid reader of this forum. It’s helped me a lot. Thank you all. 
This is my first post. I hope you can help me figure out what’s wrong with my truck. I’m running a L20b engine that was installed before my time. The truck had been running fine for a while, but the fuel pump was leaking oil so I changed it. After that, I started running out of gas every time the gas tank had one gallon less than full. That means it only ran when the tank was completely full. I tested it by driving around with a small plastic gas container in case I ran out of gas. And like clockwork, once it went below about a gallon, the truck would run out of gas. So I checked the tank and replaced the hoses and concluded I had a faulty new fuel pump. So I replaced it. It didn’t work. Once the gas went below about a gallon, it did it again. I’m going to replace the fuel pump again, but what do you guys think? What are the chances of two faulty pumps in a row?

thanks

 

heres my truck

[thisismatt]

I would either pump/siphon gas from the tank at the pump to see if you are still getting fuel, or pull the fuel gauge sender at the tank to check the pickup tube.

[thisismatt]

Also check your evap hoses. Have you noticed if you get a rush of air when you open the gas cap after it has died? 

[banzai510(hainz)]

when it does it why dont you take the hose off the out put side of pump and point it in a safe direction and see if gas shoots out..

 

check the hoses real good at the ends for cracks in hose maybe it gravity feed when full but after that its sucking air before the fuel pump as you said you swapped 2 fuel pumps.

i had a no gas situation only to find out when I swapped the fule pump the hose had a crack in it the intake right where hose went on.

[Gabriel]

2 hours ago, thisismatt said:

I would either pump/siphon gas from the tank at the pump to see if you are still getting fuel, or pull the fuel gauge sender at the tank to check the pickup tube.

Thanks. Yes. After it started running out of gas, I sucked on the hose (no pun intended) and gas came out no problem. To me it meant that the pump didn’t have enough sucking power when the gas was farther/less pressure from a full tank. That’s how I decided the 1st new pump was bad. 

[Gabriel]

2 hours ago, thisismatt said:

Also check your evap hoses. Have you noticed if you get a rush of air when you open the gas cap after it has died? 

This one I haven’t checked. This could be it. Thanks. 

[Gabriel]

1 hour ago, banzai510(hainz) said:

when it does it why dont you take the hose off the out put side of pump and point it in a safe direction and see if gas shoots out..

 

check the hoses real good at the ends for cracks in hose maybe it gravity feed when full but after that its sucking air before the fuel pump as you said you swapped 2 fuel pumps.

i had a no gas situation only to find out when I swapped the fule pump the hose had a crack in it the intake right where hose went on.

I will try this and double check the hoses. Thanks 

[banzai510(hainz)]

long as everything is stock on the fuel vent system from crankcase or carb I only think it has to be a plugged line or cracked line.  Like I said check the out put of fuel pum. then start truck and see if gas comes out.

[Gabriel]

Will do. Thanks!

[datzenmike]

Hope you kept the original fuel pump. They don't have oil in them but are splash oiled. Usually one of the two gaskets on the plastic pump spacer leak. That's an easy fix. Very rarely oil can leak at the pivot pin the pump arm turns on. Just degrease and a small dab of RTV seals it.

 

 

 

Your old pump didn't have this problem so changing it, or the act of changing it, has added the problem. So put the original pump back on there is nothing wrong with it that you can't fix. Either the new pump is crap, that's very likely these days, or the fuel line at the pump from the tank is loose or cracked. Pump will suck air easier than it will lift fuel up to the engine.

[banzai510(hainz)]

MIke I had the org fuelpump spray mosre a mist out of that vent hole.  I would guess the diphrame is cracking and leaking a little.  I never had pump go bad where gas was in the oil.

[bilzbobaggins]

hole in the pickup tube?  

[datzenmike]

Ran fine till fuel pump was replaced. Seems to me the last thing done is the place to start looking for an introduced problem..

[banzai510(hainz)]

23 hours ago, Gabriel said:

had been running fine for a while, but the fuel pump was leaking oil so I changed it.

yep mike your right

[Gabriel]

22 hours ago, datzenmike said:

Hope you kept the original fuel pump. They don't have oil in them but are splash oiled. Usually one of the two gaskets on the plastic pump spacer leak. That's an easy fix. Very rarely oil can leak at the pivot pin the pump arm turns on. Just degrease and a small dab of RTV seals it.

 

 

 

Your old pump didn't have this problem so changing it, or the act of changing it, has added the problem. So put the original pump back on there is nothing wrong with it that you can't fix. Either the new pump is crap, that's very likely these days, or the fuel line at the pump from the tank is loose or cracked. Pump will suck air easier than it will lift fuel up to the engine.

 

[Gabriel]

Thank you all. So this is the update. Yesterday I got yet another pump. This was a more expensive one from Nappa, $60 versus the $35ish from auto zone. I installed it, also changed the filters. And still no success. Then unplug the intake hose from the pump and put it on a small container with gas while I ran the engine dripping gas into the carb. Nothing. I did that to the other pump as well and nothing either. I’m suspecting it may be the camshaft that’s not moving the pump? I’m gonna remove the valve cover and see what I see. also, I double checked the lines and the seem fine. Unless there’s a micro crack somewhere, they’re fine. I’m even able to siphon gas from the hose connecting to the pump easily. 
Datzenmike. The old pump doesn’t look like it was the original and it leaked from one of the little holes for the pivot pins. But I still have it. That will be my next step after the looking at the cam. 

whats your opinion on an electric fuel pump? Is there a model that could work that you know of?

 

thank you all for taking the time to help me. 

[thisismatt]

Yeah, maybe eccentric lobe is loose

[datzenmike]

Boo on after market mechanical pumps. Bet the pump arm ground the eccentric down. Never happens with the factory originals that are close to $90. For $35 you get $200 of trouble. If the lobe was loose the sprocket would shortly walk off the end of the camshaft.

 

You want not much more than 4 psi. These are a little pricey but reliable as hell. Anything under $100 is Chinese knockoff, just throw in garbage and save the aggravation. An original '81- 720 truck is this design if you have a wrecking yard near you.

 

https://www.ebay.ca/itm/164012020320?fits=Model%3A720|Make%3ANissan&epid=1511264616&hash=item262fe0d260:g:Z6AAAOSwo49eClmb

 

What ever you get, mount as low and as close to the tank as you can. Put an inline filter between tank and new pump. Wire to ignition switch. 

[banzai510(hainz)]

so did you put the out put in safe direction and see if gas squirts out??????  you keep saying in put???

 

you can pull the pump out and use your hand to pump this and see if gas squirts on the OUTPUT side.    Pretty simple to do. if works then take the valve cover off to see if eccentric. once carb is full the pump arm will float or go limp(if not return line.

 

I dont believe the eccentric lobe on the end of the cam would go bad as its prety hard and oil splases on it

 

but orginal statement was it was working till I changed the fuelpump cause it was leaky

Edited July 23, 2021 by banzai510(hainz)

[Crashtd420]

1 hour ago, datzenmike said:

Boo on after market mechanical pumps. Bet the pump arm ground the eccentric down. Never happens with the factory originals that are close to $90. For $35 you get $200 of trouble. If the lobe was loose the sprocket would shortly walk off the end of the camshaft.

 

You want not much more than 4 psi. These are a little pricey but reliable as hell. Anything under $100 is Chinese knockoff, just throw in garbage and save the aggravation. An original '81- 720 truck is this design if you have a wrecking yard near you.

 

https://www.ebay.ca/itm/164012020320?fits=Model%3A720|Make%3ANissan&epid=1511264616&hash=item262fe0d260:g:Z6AAAOSwo49eClmb

 

What ever you get, mount as low and as close to the tank as you can. Put an inline filter between tank and new pump. Wire to ignition switch. 

Mike dont the 720s have a return line? would this output to much psi deadheading at the carburetor? 

[datzenmike]

The pump is regulated to 4.2? PSI. The return line is not a pressure regulator. It has a restriction with a tiny hole in it because if it was fully open gas would just take the easy path back to the tank. The restriction allows the pulp to push against it and build pressure. Even with the hole the pressure is 4.2.

 

Used on a 521 without a return line, once 4.2 psi is reached the pump stops till the carburetor uses enough to drop the pressure and restarts the pump.

[Gabriel]

5 hours ago, banzai510(hainz) said:

so did you put the out put in safe direction and see if gas squirts out??????  you keep saying in put???

 

you can pull the pump out and use your hand to pump this and see if gas squirts on the OUTPUT side.    Pretty simple to do. if works then take the valve cover off to see if eccentric. once carb is full the pump arm will float or go limp(if not return line.

 

I dont believe the eccentric lobe on the end of the cam would go bad as its prety hard and oil splases on it

 

but orginal statement was it was working till I changed the fuelpump cause it was leaky

No, I put a short hose dipped into a small container with gas in the INPUT side of the pump while I had the truck running and it didn’t suck any gas. But today I’m gonna do what you say when I get home. 
thank you 

[Gabriel]

6 hours ago, datzenmike said:

Boo on after market mechanical pumps. Bet the pump arm ground the eccentric down. Never happens with the factory originals that are close to $90. For $35 you get $200 of trouble. If the lobe was loose the sprocket would shortly walk off the end of the camshaft.

 

You want not much more than 4 psi. These are a little pricey but reliable as hell. Anything under $100 is Chinese knockoff, just throw in garbage and save the aggravation. An original '81- 720 truck is this design if you have a wrecking yard near you.

 

https://www.ebay.ca/itm/164012020320?fits=Model%3A720|Make%3ANissan&epid=1511264616&hash=item262fe0d260:g:Z6AAAOSwo49eClmb

 

What ever you get, mount as low and as close to the tank as you can. Put an inline filter between tank and new pump. Wire to ignition switch. 

Thank you for the advice. I’m gonna give one last try to the mechanical Pump but if it doesn’t work, I’ll go with this one. 

[Gabriel]

https://drive.google.com/file/d/1G9AhqpZFDccMbR7JhR2dCtWphBi_x_v4/view?usp=drivesdk

 

here’s the answer. The fuel pump leg isn’t touching the cam. And it looks like it got sanded off a small chunk. Which now that I look at the other pump, it also lost a small chunk. Any guesses what is going on.    
 

sorry for the link. I’m not sure how to add images from my phone. 

[datzenmike]

Have you got your old pump still????

 

Find a factory used L series pump or go electric. Those pumps are crap, obviously.