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GMC V6 and V12 Engines Engine repair and rebuilding

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  #1  
Old April 12th, 2021, 04:34 AM
geasson geasson is offline
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Default Is it my fuel pump?

My 1966 GMC 1-ton has been sitting way too long and so today I went to work on it. It has a new tank, good gas in the tank but it is NOT getting gas to the carb. When I disconnect the line from the carb and just crank, I get no gas. BUT when I removed the pump and operated it by hand, it squirted gas with no problem Do these V-6s have a rod that pushes the pump, or do they directly touch the cam shaft?

Thanks

Should I just go with an electric pump??
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Old April 12th, 2021, 05:33 AM
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Default Re: Is it my fuel pump?

I think they touch the cam, but since yours has been sitting a long time the needle might be stuck in the carb. That's what I'd check since you obviously have gas at the pump.
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Old April 12th, 2021, 08:04 AM
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Default Re: Is it my fuel pump?

But if the problem is the needle in the carb, he should et fuel 'at' the carb when the hose is off the carburetor. He did not.
I am sure I recall mention of small differences here are small differences in the arms on fuel pumps on the 401's and 478's. It had something to do with the effective stroke of the arm. I have not experienced it myself.
Is it possible the fuel pump was replaced and never ran properly after that?...probably not, but I had to ask.
My guess is that the installed stroke is not enough to get fuel up to the carburetor ( maybe one of the check valves in the fuel pump has stuck open) but with a longer stroke (supplied by you), you are getting better pumping action.
Beyond that, I have no other suggestions.
Good luck, and let us know what you ultimately find o be the problem.
Rod J
Issaquah, WA
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Old April 12th, 2021, 03:13 PM
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Default Re: Is it my fuel pump?

Quote:
Originally Posted by WE7X View Post
But if the problem is the needle in the carb, he should et fuel 'at' the carb when the hose is off the carburetor. He did not.
Issaquah, WA
I sure didn't see that. you're right!
Most fuel pumps today have a lifetime warranty. You can probably take it back and get it replaced for free. I just did that here at OReillys myself.
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Some people are like slinkys. Not worth much but funny as heck when pushed down stairs.
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Old April 13th, 2021, 08:01 AM
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Default Re: Is it my fuel pump?

geasson, You did do a good test, but need more info. If you still need help, please answer in order and completely:

1. What V6 is it (cu in and letter designation)? I see a 305
2. Did you have motor running before parked long ago? When was last time it ran?
3. Have you replaced fuel pump? When?
4. What is part number of pump you have? you didn't say if it was new.
5. No, you don't want an electric pump.
6. You say you "went to work on it"......why? Did it not start or what? Are you trying to start the motor?
7. What is the history of this motor? I read your posts, only 5 posts in 7 years.
8. Were both the crank test and the hand test done with the truck gas tank hose/line attached to pump inlet?
9. Were both the crank test and the hand test done with the truck outlet hose/line still attached to pump outlet?

10. Where are any and all fuel filters?

Damage can be done to any motor that has been sitting a long time, without using correcting start-up procedures. Be careful.

Last edited by AZKen; April 13th, 2021 at 11:19 AM.
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Old April 14th, 2021, 03:16 AM
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Default Re: Is it my fuel pump?

AZKen,
The motor is a 305 E, sorry for not saying that sooner. The truck has not run for a couple of years and the fuel pump was new last time it ran. When I got the truck, the tank was full of crud and rust, so I just replaced it with a new tank and got it running, rough, but running. I replaced the pump before I discovered that the tank was full of rust and so it was a new pump. I compared the profiles of the pumps and they look the same. I disconnected the fuel line from the carburetor and cranked the motor and got nothing in the bottle on the line. While the line from the tank was disconnected, fuel was leaking to the extent that I had to plug that line with a bolt. The hand pump test was done with nothing connected; only the fuel in the pump was displaced, not a lot of fuel, but I could feel the vacumn when putting my thumb over the input line. I could also feel pressure when putting my thumb over the output line. The motor will run when gas is poured into the carb, but only until that fuel is burned. Thanks for your help.
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Old April 14th, 2021, 03:21 AM
geasson geasson is offline
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Default Re: Is it my fuel pump?

Life has gotten in the way of my intentions to get the truck restored. I have too many projects, too much work and not enough time. The truck has been sitting outside and I have built a new garage to put the truck under cover, while I try to finish my 1948 CJ2a restoration. It had not started in quite a while and Sunday was a beautiful day to crawl under the hood. Fuel filters on the frame and an added inline filter have all been replaced. The bottom line is that I have fuel to the pump, just not past the pump.
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Old April 14th, 2021, 04:09 AM
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Default Re: Is it my fuel pump?

I can't tell from your answers if your new fuel lines were used while tank was rusty. By that I mean fuel line from tank to pump inlet and the line from pump outlet to carb. So if those lines (both of them) are clear, Do the following: be SURE there is gas in tank, install pump, install line from tank, install line on pump outlet and run the line into bottle and crank for 20 seconds. Don't guess, use a clock. Report. No more hand tests. No more tests with tank line disconnected. Bottle should start to fill and should see strong pulses of gas squirting. As you know as a rebuilder and restorer, there are several possibilities. I intend for us to find which one it is.

When I asked about filters: I wanted to know where they are in the system. Before pump, after pump or both.
I know you have done tests already and have made some judgments. I understand you said you are getting fuel to pump but not out. Please understand I can not factor in any of that. I have to have you troubleshoot using my method, the way I would do it. There are lots of methods. Please don't vary from my methods even though you don't agree or think you have already done them. Please be sure lines are clear, real sure. As you report results, I will have a few little other tests. If you don't have patience for any of this, I understand. Just let me know ASAP. All I know is you have asked for help and I am very pleased to help.
I realize it's pretty easy for me to say "buy another pump". But if you are like me you want to be sure. It would be really nice if you could find a part number or any markings on the pump you are now using.

Last edited by AZKen; April 14th, 2021 at 06:21 AM.
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Old April 15th, 2021, 02:44 PM
geasson geasson is offline
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Default Re: Is it my fuel pump?

THanks for your input. I did not replace the lines when I replaced the fuel tank, but I did disconnect everything and blow out the lines. There is a filter before the pump AND after the pump, both are new. With the pump installed and the line disconnected from the carb, I cranked the engine and got no gas into the bottle on the end. I may not have cranked it for 20 seconds, I will try a longer time tonight.
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Old April 15th, 2021, 10:01 PM
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Default Re: Is it my fuel pump?

OK, I will assume when you blew out the lines you made sure you got good air thru to the other end and you did not blow thru the filters.

Also, on this pump test tonight, if you don't get and flow out of pump, try again with a new hose and no filter on the outlet side. If no luck there either, take off gas cap and try one more time.

You don't need, and should not have, a filter after pump. Only one before pump.

During the sequence of events before and after you discovered tank was rusted, it's possible a line, or a filter or the pump got clogged or partially clogged. It's also possible the pump is bad.
The gas tank can also have a vacuum lock due to it not venting properly. That would prevent pump from sucking gas. I know you say you are getting gat to pump but you can't be sure of that. A partial block or venting problem will allow enough gas to see a "drain out" like you saw. But that is not prove of much. Only way to prove good flow from tank is let gravity cause fuel to flow out the end of line into a container to see the volume. For now, just do the outlet test per above instructions. All tests no less than 15 seconds with good battery and good starter speed. Take out plugs if you want more speed.

Last edited by AZKen; April 15th, 2021 at 10:22 PM.
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