fuel vent issue stumped need help

[bedwards]

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 last year. 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

 

[bedwards]

Well... it ended up being the tank selector valve. My guess is that it was partially switching or leaking internally. A 69$ injection and couple hours on the shop floor and it runs good again.

 

[JoeR Jr]

I don't see how a bad selector valve could cause dieseling, but I suppose most anything is possible.
If it still diesels after this, check to make sure it's not running a little hot, and check the timing.
If it's not either of those try some higher octane gas for one tank.
Joe

 

[bedwards]

I can live with the occasional dieseling. I think it was from the over-rich condition. But opening the cap on the gas tank and having a couple gallons of gas pour out is pretty hair raising. I drove it 10 miles after fixing it without a problem. I am going home tonight to drive it again to make sure before I trust it on a longer trip.

 

[gotyourgoat]

Always glad to see a positive and relatively simple outcome. Just trying to understand what was happening and how you had so much fuel past the pump.

Guessing the switch was frozen partway open in one direction and fuel was being pushed into the one tank from the return line. Then the tank was overflowing back through the vent lines to the charcoal canister and up into the carb from there?

So you didn't use half a tank of gas getting home. It was just the stuck switch transferring fuel to the other tank, and everywhere else, making it look like fuel was being consumed.

Am I close or way off on this?

I dont have dual tanks but I do question my evap canister, especially now.

 

[bedwards]

I think that is exactly what happened. I did have to clean the plugs as they were carbon fouled after all the flooding. I've driven it to work 2 days this week and so far so good.

 

[Wildjoe330]

After putting the factory tank back on, it was off for a while, Looks like I am having issues of factory passengers side tank overflowing 3 lines on the tank 3 on the fuel pump one on the add on custom auxiliary tank. I replaced my switch over valve a year or two ago so I was thinking that it should be ok, and we know it could have gone bad so I might just try to bypass it for a while to use some fuel and see what is does. Could be anything but not what was suspected. It's good to have you guys here for some help talk.

 

[C10MixMaster]

Well... it ended up being the tank selector valve. My guess is that it was partially switching or leaking internally. A 69$ injection and couple hours on the shop floor and it runs good again.

If it was pushing liquid fuel through the charcoal canister, the canister is ruined you need to replace it.