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GMC V6 and V12 Engines Engine repair and rebuilding

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  #1  
Old April 19th, 2023, 07:18 PM
goodpal goodpal is offline
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Question Fuel supply loss when in gear

Hi all,

Wondering if any of you have any ideas on an issue I'm having with my 61' GMC 1000. End of last summer I had a sudden loss of fuel to the engine and ended up dead on the side of the road. Fuel pump looked fine so I replaced the fuel sending unit, gassed back up and all was good.

Fast forward to this spring and I'm taking my truck out of storage and don't make it more than 5 miles before a loss of fuel/power again. Put gas in the tank and no dice.

Had it towed back to the storage place (which also does classic car repair) and asked them to take a look. They said I was getting air in through the fitting and replace the fitting and built new fuel lines, called me and said all was good.

Today I made it about 15 miles back towards my house (5 to go...) and started experiencing a sudden and complete loss of power again with one or two kicks back on briefly. Pulled over, truck starts right back up now (which it didn't last time) and idles great. Putting it in gear I can make it another...50 feet maybe before I start to lose power. It now currently lives in the parking lot of a friendly tax accountant and I intend to tow it back to my house.

Any one have any experience or ideas on this one? I'm planning on checking/replacing the fuel filter, checking compression, air filter etc. but am curious if there are any other thoughts on this.

Thanks! Been a member for a while but my first time posting here,
Jeff
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  #2  
Old April 19th, 2023, 08:39 PM
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AZKen AZKen is offline
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Default Re: Fuel supply loss when in gear

Hi Jeff, too many people guessing. You need to do some troubleshooting/tests. Are you wanting and are able to work on it yourself? It's probably not a good idea to guess and buy more parts. When you say "sudden loss of fuel", is that a guess or do you have a reason to say that?
When you are ready and have it in a place/time to work on it. Let me know. If the shop did not fix it, they are not a place to go.

Not sure why you bought a new fuel sender? Also, how does a pump "look" fine?

Last edited by AZKen; April 19th, 2023 at 08:56 PM.
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  #3  
Old April 19th, 2023, 10:12 PM
lizziemeister'sV6 lizziemeister'sV6 is offline
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Default Re: Fuel supply loss when in gear

You got me guessing what a fuel sender is? Being a product of the 1960's the only "fuel sender" would be the fuel pump located on front of the engine. Stop guessing and stop buying parts you don't need. I would check the fuel filter - then the fuel pump - then see if the carb squirts fuel when you hand work the throttle while looking down the carb.
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Old April 20th, 2023, 02:12 AM
kknotts kknotts is offline
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Default Re: Fuel supply loss when in gear

I had a very similar issue with my ‘63 off and on over a several month period. Symptoms started with loss of power as if starving for fuel and finally ended with total shutdown just running through the gears. The diaphragm in the mechanical pump was too weak to supply fuel. Installed an electric unit and it is cured. Has never run better! On a side note I replaced the mechanical pump 3 times over a four year period.
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Old April 20th, 2023, 02:07 PM
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Default Re: Fuel supply loss when in gear

Hi all,

Sorry for any confusion and thanks for the feedback.

AZKen: yes, I'm generally mechanically competent and able to work on the truck myself, planning on getting it back to my house tomorrow. I would say sudden loss of fuel isn't a guess, the truck drives fine then suddenly the engine has all the symptoms of running out of gas, then comes back to life then "runs out of gas" again. The tank is fully topped up and the truck has been driven at 65 without any issues for 5+ miles at a go and the issue started to kick in both times this year going up hill so I'm 95% confident fuel getting to the engine is the issue here.

As for the fuel pump looking, "fine," I removed the fuel pump from the truck last summer and manually worked the pump and it shot gas without fail. I dismantled the fuel pump and checked the diaphragm for wear or damage and it looked brand new (the fuel pump has maybe 2000 miles on it).

As for the fuel sender the guy I bought the truck from a few years back had it in a box of parts he gave me. When I first ran into issues with it and didn't see anything wrong with the fuel pump I pulled the fuel sending unit out of the gas tank and it was falling apart. I hear what you are saying since it is basically just a tube that pulls fuel so my best guess was an air leak in the line or corrosion, looked like it was original to the truck and I don't know how it was stored by previous owners before it came into my possession.

Lizziemeister: This is what I mean by fuel sending unit

https://www.partsgeek.com/hshh47n-gm...QaApxjEALw_wcB

kknotts: thanks, I've been weighing the value of better reliability by retrofitting parts vs keeping the truck original, good to know you've had good experiences with an electronic fuel pump.

My plan is to get the truck back to my house, check the fuel pump for functionality and/or a leak/oil spray then replace the fuel filter. I'll keep folks posted with my luck, or lack therof!

Best,
Jeff
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  #6  
Old April 20th, 2023, 02:35 PM
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ilvracn ilvracn is offline
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Default Re: Fuel supply loss when in gear

I also had a similar problem with my 66 1500. fuel pump check valve check valve came apart inside pump. these trucks seem to be hard on pumps. or pumps are poor quality. i carry a spare electric pump, in case of trouble. on my third pump in this truck.
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Old April 20th, 2023, 03:23 PM
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jmfurgason jmfurgason is offline
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Default Re: Fuel supply loss when in gear

Goodpal: I experienced a similar problem with gas, has the gas tank been removed and cleaned? residule build up in tank caused the same issues on my truck, old gas and sitting to long may be the issue clogs up fuel line. LouieLouie
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Old April 20th, 2023, 05:22 PM
goodpal goodpal is offline
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Default Re: Fuel supply loss when in gear

jmfurgason: I haven't removed the gas tank and cleaned it and it is original to the truck with 60k some odd miles on it. When you pulled your tank did you scope it first or just go for it?
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  #9  
Old April 20th, 2023, 05:33 PM
Jim A Jim A is offline
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Default Re: Fuel supply loss when in gear

I have had similar problems with other vehicles over the years, but never with my '60 1500.
The culprit in some of those was a build up of debris, sludge and other gunk in the pickup screen in the tank. I am not sure what ours look like, though.
A fuel filter is a cheap and easy diagnostic/fix and never a bad idea.
Blowing back into the tank may also be a temporary relief; do it from before the filter.
The pic you sent is the fuel level sender; I have no luck getting mine to work after a fuel thief messed mine up forty years ago. A stick or dowel down the filler makes a great alternative.
Good luck. Keep posting your progress.
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Old April 20th, 2023, 05:36 PM
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Default Re: Fuel supply loss when in gear

The throttle squirt observation test needs to be done immediately after stalling. This will tell you if carb bowl is empty. if no carb squirt, hand squirt gas/dribble gas from a bottle down throat of carb after stalling. If that starts the truck, then it's a fuel delivery problem obviously.
From what you say, It seems that something is happening gradually. It seems it starts right up when you get ready for a ride. That indicates bowl is full. Something recoups after wait period.

So causes are:
1. Gas cap not venting. Vacuum occures. No fuel flow. So drive with cap off next drive.
2. Debris in tank, lines, filter. Debris blocks then releases....
3. Bad/weak fuel pump. Can't keep up.
4. Sticking/slow flow float valve/needle valve
5. The only way to test a fuel pump is an on the truck flow test. Disconnect center distributor wire. Disconnect the hose or tube at carb inlet. Insert that hose/tube into a bottle. Crank motor for 15 seconds. Observe good pulsing stream of gas into bottle.
6. Bad accelerartor pump seal
These tests will give clues to the cause. It's hard to catch a problem that occures not in the garage but after a drive. Then restarts after awhile.

Buy a new fuel pump. Mechanical, not electric. In this case, you have to have a baseline. The pump is the baseline. Only one filter is necessqary. Install it in the line before pump inlet.

The fuel pick up tube part of the sender has a sock on it. The tank would have to be pretty bad for that new sock to be clogged. Check all the rubber sections of the fuel line for collapse or degradation. fuel line size and hose ID should be 5/16 or 3/8.

Last edited by AZKen; April 21st, 2023 at 08:56 AM.
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