Engine sags

[vphouger]

I have a 1986 Chevy C20 2 wheel drive. The engine is a 454 with a new Edelbrock 750. The truck has dual tanks. The engine starts and idles fine with no issues. However, when I take out on the road after a few miles the engine loses power and seems to want to die, until I press the dual tank switch to the opposite tank, then I surges to life. After awhile it repeats the same behavior until I press the switch to the other tank.

I have replace the dual tank switch, the gas tank selector valve (that was a chore), and the fuel filter. The situation is a mystery to me. Any suggestions? Thanks.

 

[vphouger]

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[Geoduck]

Nice looking rig!
Check if the tanks are properly vented.
My 85 dual tank has the vent going to the charcoal canister.

 

[vphouger]

Thanks Geoduck, I'll check.

 

[Ricko1966]

Did they hook up the charcoal cannister purge when they changed the carbureator? @SirRobyn0 has put up excellent information on charcoal cannister and tank vent do a search. I'll bet you don't have the purge line hooked up

 

[SirRobyn0]

Did the hook up the charcoal cannister purge when they change the carbureator? @SirRobyn0 has put up excellent information on charcoal cannister and tank vent do a search. I'll bet you don't have the purge line hooked up

That could be a real possibility, either that or the canister has had lines off of it for so long it's totally plugged and nothing gets through it. Regardless it sounds to me like the evap system isn't working.

@vphouger See this thread >>>>> https://www.gmsquarebody.com/thread...should-be-set-up-and-other-information.37324/ <<<<<

 

[MidneightReider]

Thanks Geoduck, I'll check.

Easy way to check is to go for a ride with the gas cap removed on whichever tank you're using.

 

[vphouger]

Thanks MidneightReider. I'll give it a try.

 

[vphouger]

That could be a real possibility, either that or the canister has had lines off of it for so long it's totally plugged and nothing gets through it. Regardless it sounds to me like the evap system isn't working.

@vphouger See this thread >>>>> https://www.gmsquarebody.com/thread...should-be-set-up-and-other-information.37324/ <<<<<

Thank you SirRobyn0. Good information.

 

[vphouger]

That could be a real possibility, either that or the canister has had lines off of it for so long it's totally plugged and nothing gets through it. Regardless it sounds to me like the evap system isn't working.

@vphouger See this thread >>>>> https://www.gmsquarebody.com/thread...should-be-set-up-and-other-information.37324/ <<<<<

Just curious if there is any success in taking the canister a part to clean it and replace the filter?

 

[Grit dog]

So are your tanks pulling a vacuum? IE it runs fine with no gas caps?
Another possibility although likely not as perfectly repeatable with both tanks is both fuel socks could be plugged and tanks full of varnish and a short time of pulling fuel through clogs them. Then the varnish falls off once not being pulled to the sock on the end of the fuel pickup.

 

[SirRobyn0]

So are your tanks pulling a vacuum? IE it runs fine with no gas caps?
Another possibility although likely not as perfectly repeatable with both tanks is both fuel socks could be plugged and tanks full of varnish and a short time of pulling fuel through clogs them. Then the varnish falls off once not being pulled to the sock on the end of the fuel pickup.

I think something like this is more likely than a canister / evap problem. @vphouger should try a quick drive with the caps off. My bet is it won't make a difference but a little diagnostic test like this could save a lot of wasted time and money.

@vphouger Our units are sealed from the factory. I suppose a guy could cut the canister open careful and be able to put it back together, but if the filters clogged, the charcoal is destroyed IF it truly has a problem just replace it. First try the no cap test drive, to see if that is the problem.

 

[vphouger]

So are your tanks pulling a vacuum? IE it runs fine with no gas caps?
Another possibility although likely not as perfectly repeatable with both tanks is both fuel socks could be plugged and tanks full of varnish and a short time of pulling fuel through clogs them. Then the varnish falls off once not being pulled to the sock on the end of the fuel pickup.

Thanks, another thing to consider.

 

[vphouger]

I think something like this is more likely than a canister / evap problem. @vphouger should try a quick drive with the caps off. My bet is it won't make a difference but a little diagnostic test like this could save a lot of wasted time and money.

@vphouger Our units are sealed from the factory. I suppose a guy could cut the canister open careful and be able to put it back together, but if the filters clogged, the charcoal is destroyed IF it truly has a problem just replace it. First try the no cap test drive, to see if that is the problem.

Thank you.

 

[greyghost]

what type fuel pump using? in tank, electric, mechanical , pressure regulator?, even carbs need steady pressure flow, fuel in tank?, tank return line?, tanks vented?, caps venting?, bad gas?, check for water n debris use glass or see thru container, how old is coil? might be overheating n shutting down, same with ignition module.