1987 K10 350 TBI Vapor Canister (Charcoal Canister) full of gas

[gweber71]

Hi Everyone. I have a 1987 Chevy K10 with a 350 and TBI Fuel Injection. Recently the Vapor Canister has been filling full of gas. Does anyone know why this would happen?

Truck has over 236,000 miles on it.

Truck has dual 16 gallon fuel tanks with original fuel lines, return lines, vent lines, and fuel tank selector valve.

 

[Wes Harden]

Are you topping it off when fueling ?

 

[Bent77]

Sweet. The only other time I’ve seen this, the tank vent was plugged and the heat of the day was pushing the fuel up the back line

 

[gweber71]

I thought maybe topping it off was it. The first time it happened the gas pump did not shut off and I had fuel come out the neck. This was on the left tank. I was running on the right tank with the left tank maybe over full for about 3 days. It has been hot here too. The truck would start and idle fine, but when you stepped on the gas it would stall. I could not get the truck to accelerate. I let the truck idle while I was calling a tow when I noticed gas running out of the Vapor canister. I took the canister off and turned it upside down to drain the fuel. I put a new canister on and paid more attention to filling the tanks. I have always filled the tanks to when the gas pump shuts off and never had a problem. I ran the truck again for a couple of weeks and the canister filled with gas again after I filled the left tank and was running on the right tank again for 3 days. The Vapor canister has 2 hoses on it. One is a vent that comes from the tanks. The second one runs up to the TBI. The TBI is supposed to suck in the fuel vapors collected from the tanks when the truck is above idle. If the canister fills with gas, the TBI is sucking in additional fuel which is too much for the engine to handle. At least I think this is what is happening.

Where is the tank vent? Is that the small hose running from the tank filler neck to the tank? I noticed the tank filler hoses and vent hoses connected to the filler necks are original and not looking great.

 

[Bent77]

There’s another on the top of the tank

 

[dyeager535]

Hi Everyone. I have a 1987 Chevy K10 with a 350 and TBI Fuel Injection. Recently the Vapor Canister has been filling full of gas. Does anyone know why this would happen?

Truck has over 236,000 miles on it.

Truck has dual 16 gallon fuel tanks with original fuel lines, return lines, vent lines, and fuel tank selector valve.

Given that the EVAP canister *is* the tank vent, seems to me we know the tank side of the EVAP is clear, no? Also assuming everything worked fine, then it started this, with no changes from you OP...

Do these setups use a solenoid to control EVAP purge? I believe the vent line fitting has a check valve on the inside of the sending unit, not sure how fuel gets in the vent line without being chronically overfilled, or that check valve failing.

Maybe if there is a purge solenoid, and it's open, pulling some vacuum on the vent line at all times, it could assist picking up fuel from the tank? I didn't think the canisters held vacuum, but if they hold fuel, apparently they are better sealed than I thought.

If the tank is severely pressurized (or holding a good vacuum) with those later 16PSI vented gas caps, when you open them you'll hear the tank pop as the metal goes back to its original shape.

 

[gweber71]

Given that the EVAP canister *is* the tank vent, seems to me we know the tank side of the EVAP is clear, no? Also assuming everything worked fine, then it started this, with no changes from you OP...

Do these setups use a solenoid to control EVAP purge? I believe the vent line fitting has a check valve on the inside of the sending unit, not sure how fuel gets in the vent line without being chronically overfilled, or that check valve failing.

Maybe if there is a purge solenoid, and it's open, pulling some vacuum on the vent line at all times, it could assist picking up fuel from the tank? I didn't think the canisters held vacuum, but if they hold fuel, apparently they are better sealed than I thought.

If the tank is severely pressurized (or holding a good vacuum) with those later 16PSI vented gas caps, when you open them you'll hear the tank pop as the metal goes back to its original shape.

There is no solenoid on the EVAP canister. I see the same in the 1987 GM Service Manual. The EVAP canister has 3 connections on it. One hose from the vent line from the tanks, one hose running to a open port on the front bottom of the TBI, and 3rd connection is labelled Air Cleaner and has a factory plug on it.

I have not been able to determine where the fuel is coming from when the canister fills up. I am running the truck without the canister hooked up. For a while, I hooked up a brake fluid catch jar which is somewhat clear to see which hose the fuel might be coming from. I did not see any fuel in the jar. Recently I have been running the truck with the vent line unhooked and pointed under the truck and the TBI hose capped. I saw some fuel come from the vent line once, not sure if was just left over from the last time the canister was full. I also saw a little bit of fuel come out of the TBI hose when I uncapped it on Sunday.

The Service Manual says the EVAP Canister holds the fuel vapors from the tanks while the engine is not running. When the engine is running and moves above idle, the TBI sucks in the vapors from the canister using vacuum. I have felt the vacuum on the TBI line while moving the engine off idle.

The TBI unit has never been apart or rebuilt with new gaskets. Do you think there could be something wrong with the TBI that would push fuel down the TBI hose to the canister? The truck runs good but sometimes I experience intermit surging at idle and then it stops and idles fine. I have looked down into the TBI when it's running and see a nice fan spray pattern and no dripping / leaking fuel from the injectors.

Recently I feel a slight hesitation intermittently when accelerating. The truck also cranks longer when I am running off the left tank compared to the right tank. It has been this starting this way for years.

 

[dyeager535]

I think you can't really go wrong making sure all the TBI gaskets and seals are in good shape. Some of them are prone to failure especially at this age.

If the line to the TBI pulls vacuum, then I see no way for fuel to make it's way in there *from* the TBI.

IIRC the later tank switch valves were exceptionally complex. How did GM vent both tanks on that setup? If it's through the valve (I imagine both would have to be open all the time, no switching of vent) I wonder if it's possible that fuel is leaking inside the switching valve?

Does it do this regardless of which tank you are running off of?

Theoretically I suppose the longer crank times could be associated with the problem, but they could also just be due to a weaker pump or something.

 

[gweber71]

It's been a while since I posted about this issue. My latest observation is when running the truck off the passenger side fuel tank, gas is leaking into the left fuel tank. Seems to occur when it's hot and humid out. I did not see this occur much over the fall, winter, or spring months. It's easier to see if I run off the passenger side tank twice in a row while leaving the driver side tank at 1/4 filled. After running the passenger side tank down to 1/4 tank, then switching the tank to the drivers tank, it's full, without me actually filling it at the gas station. I think the fuel tank selector valve needs to be replaced, it's the original factory installed valve from 1987. Has anyone experienced the valve leaking from one tank to the other? I have seen discussions about the valves leaking fuel on the ground but nothing from one tank to the other.

Is there a trick to getting the old valve off the frame of a K10 long bed truck? It's in a tight location between the valve, the frame, frame support and truck cab. To make it better, both the nuts and the bolts are spinning together when trying to loosen them....

 

[imiceman44]

It's been a while since I posted about this issue. My latest observation is when running the truck off the passenger side fuel tank, gas is leaking into the left fuel tank. Seems to occur when it's hot and humid out. I did not see this occur much over the fall, winter, or spring months. It's easier to see if I run off the passenger side tank twice in a row while leaving the driver side tank at 1/4 filled. After running the passenger side tank down to 1/4 tank, then switching the tank to the drivers tank, it's full, without me actually filling it at the gas station. I think the fuel tank selector valve needs to be replaced, it's the original factory installed valve from 1987. Has anyone experienced the valve leaking from one tank to the other? I have seen discussions about the valves leaking fuel on the ground but nothing from one tank to the other.

Is there a trick to getting the old valve off the frame of a K10 long bed truck? It's in a tight location between the valve, the frame, frame support and truck cab. To make it better, both the nuts and the bolts are spinning together when trying to loosen them....

I haven't seen it on the square body trucks but I had a freightliner that had that problem, it would even overflow if the tank was already full and on those it's not a manual selector, it's supposed to keep both tanks balanced.
Changed the selector valve and it was fixed

 

[gweber71]

Yes that was happening to me too if the driver tank was full. It would back up through the vent line all the way to the vapor canister and overflow the canister.

 

[imiceman44]

Yes that was happening to me too if the driver tank was full. It would back up through the vent line all the way to the vapor canister and overflow the canister.

The funny thing was, if we let sit engine off overnight it would equalize, but then drive one hour and the right tank would be empty.