Fuel smell, but no leaks.

[lilpup]

Hi all,

I've got an 86 Chevy Silverado RWD with a 305 and an Edelbrock Quadrajet. On some of the warmer days I've caught it vapor locking after a longer drive.

I've noticed recently that the truck has a little bit of a fuel smell when walking around it. I have been checking for leaks, but have not found any. Maybe I'm getting ahead of myself here, but I was thinking maybe the fuel lines are starting to get old? And possibly contributing to the vapor locking?

What're your thoughts?

-Taylor

 

[74 Shortbed]

Check lines and connections to the charcoal canister.

 

[1987 GMC Jimmy]

X2
Those lines rot sometimes and will exude a gas vapor smell. And they could have disconnected.

 

[Georgeb]

Is your fuel smell only after shutting down when it's warm? What do you have to do to get it to start when it "vapor Locks"?
Mine carb used to boil over when I shut down hot. I would hold the pedal down at 1/4 throttle when starting warm and she would light right up.....

 

[highdesertrange]

being a 86, it's probably time to do the rubber hoses. x2 on the canister hoses and maybe the canister itself. highdesertranger

 

[chengny]

If there is a deteriorated/cracked hose in the evaporative emissions system - and the engine is turned off - fuel vapor will escape on hot days. That might explain the occasional gas smell. But the OP also mentions a vapor lock problem.

A gas smell (with no evidence of leakage) along with an intermittent vapor lock condition can be caused by a leak in the fuel pump supply line - specifically in the rubber hose that connects the steel tubing to the nipple on the tank sending unit.

With the engine running, the fuel pump creates a negative pressure in the supply line to draw the gas out of the tank. And the lower the tank level, the greater the vacuum required to lift the gas out of the tank. That's why vapor lock - due to air leakage in the supply line - occurs more often when the tank is low. If the truck is driven long enough to significantly lower the gas level in the tank, the pump will tend to get air bound - more so than at the beginning of a trip when the tank is full.

Also, since that hose is at the highest point in the supply piping to the pump, a leak in that area won't normally show up as liquid gasoline on the ground.

If the leak were anywhere else in the supply line it would be obvious. When the engine is killed (and the fuel pump stops pumping) negative pressure in the supply line is lost. Any part of the supply line below the tank sender level then becomes slightly pressurized - due to the head pressure of the gas in the line. The volume of gas in the supply line (below the sender and up to the pump suction) is allowed to drain out onto the driveway - and dissolve your tar.

But since the leak is at the tank top level, when the pump stops nothing leaks out to atmosphere. There is only the small volume of gas in the hose (and a short section of adjacent tubing) that could leak out. But since there are no restrictions in the pick-up tube, the gas quickly drains back into the tank. Not enough gas leaks out to show on the ground.

And, because the rest of the supply line (from the high point to the pump) remains packed with gas, the pump easily develops suction at the next start-up. No long cranking times.

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[lilpup]

Thanks for the help, y'all. I'm going to take a look at the lines and canister tonight.

Could one get a good charcoal canister at a salvage yard? There's a few in my area, and it would be nice to save a buck or two.

 

[Oldrider]

If you replace all of the rubber lines and still have the gas smell after shutting the engine off, try replacing the fuel pump itself.