Fuel tank pressure build up??

[blueburban87]

Hello,
I'm hoping someone has experienced this before.
I'm running a Blueprint vortec engine in my 87 suburban with holley EFI setup. Just today I smelt this sweet tea smell chemical smell in the cab.
I had my local shop look at it with me and as I popped the hood and cranked the engine fuel cam out the top of the Charcoal canister. I opened the fuel tank cap and had the pressure sound so we know its not getting a clean release at some point.

Any ideas where to start looking?

 

[someotherguy]

Due to emissions regulations, your fuel tank is not allowed to vent to the atmosphere, so the cap is pressure-tight. Pressure that builds up goes through the small line off the sending unit (vent) and travels up the frame rail to the charcoal canister. A line from the canister then goes to the TBI unit (in the stock setup) and the excess fumes are burned with the air/fuel mixture passing through the TBI.

For liquid fuel to be in the canister, that usually comes from overfilling ("topping off") the tank instead of stopping when the pump lever clicks off automatically. That liquid fuel then gets pushed up the vent line as the pressure builds in the tank, instead of just fuel vapor. Once the canister gets soaked in fuel it's usually done and needs replacement. I've seen them split open, not good, considering that's fuel vapor and potentially liquid fuel under your hood with sources of ignition nearby.

How long has your conversion to the Holley system been in operation? If it's recent, I'd start to wonder if someone routed the fuel pressure, return, and vent lines correctly.

Richard

 

[blueburban87]

Due to emissions regulations, your fuel tank is not allowed to vent to the atmosphere, so the cap is pressure-tight. Pressure that builds up goes through the small line off the sending unit (vent) and travels up the frame rail to the charcoal canister. A line from the canister then goes to the TBI unit (in the stock setup) and the excess fumes are burned with the air/fuel mixture passing through the TBI.

For liquid fuel to be in the canister, that usually comes from overfilling ("topping off") the tank instead of stopping when the pump lever clicks off automatically. That liquid fuel then gets pushed up the vent line as the pressure builds in the tank, instead of just fuel vapor. Once the canister gets soaked in fuel it's usually done and needs replacement. I've seen them split open, not good, considering that's fuel vapor and potentially liquid fuel under your hood with sources of ignition nearby.

How long has your conversion to the Holley system been in operation? If it's recent, I'd start to wonder if someone routed the fuel pressure, return, and vent lines correctly.

Richard

all really good points. What i can tell you is it has a Warbro intank pump to work with the sniper. The engine did not come in the truck it was a Crate Blueprint, Its been in the vehicle since 2021. Even when the tank is low when i remove the gas cap the sounds of pressure release. I think your 100% correct, the charcoal can has probably been bad for a long while and I didnt notice or the smell wasnt bad enough since I normaly drive with the windows down. What caused me to notice was i had my AC charged yesterday and the smell was so bad I thought i had a R134 leak. thats when we found the canister over full. To be safe I left the suburban at the shop that did the AC as they are a general mech as well. HE was going to push air through the vent line to see if its pluged then bypass the canister. After reading this i'm not sure he is doing the right thing?
Thoughts?

 

[someotherguy]

I wouldn't bypass the canister (although it is likely bad and needs to be replaced, from being soaked in fuel) - because the tank needs to vent. Without the canister, you'll just be venting into open air under the hood. Older systems (pre-emissions) had gas caps that would vent directly to the atmosphere, but you may be hard pressed to find one that will fit your filler neck. IF you are able to, then you could safely cap the vent line from the sender.

Richard

 

[blueburban87]

I wouldn't bypass the canister (although it is likely bad and needs to be replaced, from being soaked in fuel) - because the tank needs to vent. Without the canister, you'll just be venting into open air under the hood. Older systems (pre-emissions) had gas caps that would vent directly to the atmosphere, but you may be hard pressed to find one that will fit your filler neck. IF you are able to, then you could safely cap the vent line from the sender.

Richard

if i cannot find a cap to vent( not likely) i assume the investigative work to see whats causing the system not to vent is what has to be done and will for sure need to replace the canister?

 

[Schurkey]

Bypassing the charcoal canister is nuts.

Replace the canister, verify the tube from tank to canister isn't plugged; and then assure that the purge valve works.

GM had a problem with purge valves in/around '84--'85, the canister would fill with fuel; and there could be charcoal bits 'n' pieces lodged in the carburetor or TBI unit. So purge-valve problems are not new, and can cause the canister to fill with fuel.

 

[blueburban87]

I appreciate the feedback. My setup is the crate engine with Holley efi, there is no valve hookup from the factory tbi hooked to the ecu pulling vapors into the throttle body to burn. So the vapor filled the canister making it useless but no factory purge to mange it. I was going to put a vented pressure valve tank screw in style in and redirect the line to the canister into the vapor line that feeds back to the fuel neck.
Any issues you see with that?

 

[Schurkey]

Yeah. You're venting hydrocarbons directly into the atmosphere. This is illegal for good reason, and silly.

GM has already worked-out the engineering to fix that problem, all you have to do is replicate it.

 

[blueburban87]

I have the purge solenoid removed from the engine builder. I can replace the canister, issue will be pulling clean air in that the purge solenoid would have done. Any ideas?

 

[L31MaxExpress]

Only the really late model TBIs had a purge solenoid, the rest used ported vacuum. I hooked my purge canister up to manifold vacuum lacking a ported vacuum fitting through a Dorman 47311 inline restriction. Works great although my engine is in closed loop within 10-15 seconds after startup. The resticted fitting allows a slight and continuous purge on the canister while the engine is running. I do not have fuel fumes even recently with it sitting in a 100F shop with a near full tank for 2 weeks.

Mine is a 1987 G20 with a Proflow 4 injected cammed L31 350. Your 87 suburban should have the same setup. Canister on the front core support with 3 fittings. One to the tank vent, 1 to a breather cap (looks like a cap but it is a screened vent) and 1 that ran to ported vacuum from factory.