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73-87 Chevy _ GMC Trucks => Fuel Systems and Drivability => Topic started by: bedwards on March 29, 2019, 07:21:09 AM
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I've been trying to diagnose a issue that has steadily been getting worse.
1985 c10 with the 5.0 and only 37000 original miles. It is bone stock. It started out as dieseling when you shut the truck off, but now it tried to strand me yesterday. It bogged down and died coming home from work. Got it started but it was coughing black smoke and I just barely could keep it running but got it home. It used a half a tank of gas to get home.
Carb looks fine was rebuilt 2 years ago. Been running nothing but non-ethanol. Thought I had an ignition problem but tonight while it was idling in the garage, it started flooding and smoking and popping again. I shut it off and took the breather off to check the carb and ignition again. I could literally hear liquid flowing. I traced it down to raw gas coming out of the canister vent on the side of the air breather. It was weeping around the canister also which is full of gas. But the strangest thing is somehow the truck has pumped gas from one tank into the other, enough that when I took the cap off the passenger side it ran a couple gallons out of the filler neck and I caught it in a bucket. It is somehow pumping gas through the vent lines from the driver side tank into the passenger side tank.
Just changed the oil 2 weeks ago and there was no fuel smell in the oil tonight.
Leaning towards bad fuel pump??? It is the original 32 year old 3 line pump. Doesn't it have a vent line? Could the diaphragm go bad and pump fuel into the vent?
any thoughts would be appreciated
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Check the routing of the 1/4" fuel return line from the fuel pump back to the fuel selector valve under the cab. The line from the pump should connect to one side of the switching valve. A 1/4" line from each tank should also connect to the switching valve on the opposite side of the valve relative to the line from the pump. The valve is supposed to switch the returns along with the supply lines as you select tanks. If the lines are connected correctly to the valve, replace the valve. And, you will need to replace the vapor canister since it is now saturated.
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the lines are factory, so what your saying is the valve is not switching right?
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That's the indication from the symptoms you posted. The OEM valve is motorized and can stick mid-position.
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It literally put gas into the passenger side tank while running on the driver side tank and it somehow filled up the vapor canister until it was flooding the carb.
So the extra fuel pump line is a return and not a vent?
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Agree Switching valve is likely problem. When tank got over full it migrated to the canister. Defiantly will have to replace canister. Once saturated it is ruined. You cannot dry it out.
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Have the valve ordered now, what happens if you don't replace the canister / :-\
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You will encounter bouts of fuel odor, more pronounced on warmer days, and the potential for periods of running rich.
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Thanks for all your answers. Hopefully will have it fixed tonight after work.
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Be sure to lower the fuel level in the tank that is overfull before plumbing in the replacement canister or you may flood it too.
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I drained 4 gallons out of it last night :(
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Well, its fixed. What a PITA to get in and out. You were right. Thanks very much for the advice.