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#22 · (Edited)
I, also, have trouble with a pressurized (NEW) fuel tank, on a 1970 El Camino (350). I have changed the fuel cap MANY times and still it pressurizes. My guess is that the 10% ethanol/gasoline is what's doing it. I hope to try 90 octane un-ethanol gasoline ASAP, to see if that helps. I, too, drilled a small hole in a new cap. The fuel just flew out of the hole on RH turns. I have a vented/non-surge OE cap on the fill pipe and STILL get a gas smell on RH turns.
 
#23 ·
the new aftermarket caps are the problem.as long as the tank has another mean of venting they work fine such as a 71 or later with the charcoal cannister. but a system with no external vent like the 70 and earlier will build pressure.the action of a mechanical pump produces a pressure pulse on the inlet side of the pump during a complee pump cycle and eventually pressureizes the tank. ive seen them bad enough to cause carb flooding. you have to vent the tank either by modifying the cap or adding a vent to the tank. unless you can find an original cap
 
#24 ·
heflinhunter said:
I, also, have trouble with a pressurized (NEW) fuel tank, on a 1970 El Camino (350). I have changed the fuel cap MANY times and still it pressurizes. My guess is that the 10% ethanol/gasoline is what's doing it. I hope to try 90 octane un-ethanol gasoline ASAP, to see if that helps. I, too, drilled a small hole in a new cap. The fuel just flew out of the hole on RH turns. I have a vented/non-surge OE cap on the fill pipe and STILL get a gas smell on RH turns.
Click to expand...

I also think it's the gas.
 
#25 ·
I had this same issue with the 65 Elky to the point that it would shut off the engine because the fuel pump couldn't pull gas through. The accelerator pump was shooting blanks after it stalled. No fun setting in Atlanta rush hour traffic in 95 degree weather with all Yuppies behind me pissing and moaning. Regardless of pressure or vacuum (my issue was vacuum) - I drilled a couple of 1/8" holes in the top of the cap and 2 more offset from them through the bottom layer, inside the gasket ring - problem solved and no gas running out in hard right corners- and a new fuel pump for insurance. I don't keep it full either - I want to be able to add a few gallons of new fuel every time I take it out so the mix is always fresh. FWIW, Ed
 
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#26 ·
my 66 elcamino had a loose fitting gas cap when purchased. i later got a vented cap and it built pressure immeaditely. the cap almost flew out of my hands at fill up time. after i found out that vented meant only in. i researched here and ended up drilling holes like the other poster did. that fixxed the problem. now the issue is i can smell gas in the garage on a hot day so i open the door and let it vent out before start up.

jim
 
#27 ·
I have a stock 71 350/350 (with charcoal can and no damage to lines I can see.)and don't understand the venting system.

My Japanese cars all built pressure and/or vacuum in the tank. (For lowest emissions, it would make sense to allow some level of both pressure or vacuum before the relief valve(s) in the cap (or elsewhere in the system) open.) They made sucking or blowing noises to different degrees, under different conditions. I don't really know suck from blow sounds.

Stock cap is in storage, and doesn't look like it vents pressure or vacuum...but I'm not sure. Aftermarket locking cap has black rubber gasket. When the gasket got cracks, the car smelled gassy. I emailed the cap company. They mailed two replacement gaskets in a little white fabric bag. No charge.

What I don't understand is why, if the system is sealed, and vents/sucks what it needs to through the charcoal can, why does the gas cap never hiss? Did early EVAP systems vent and suck everything on either side of atmospheric pressure? Anyone got a stock system with charcoal can of that vintage that builds pressure or vacuum in the tank?
 
#28 ·
dogsridewith said:
What I don't understand is why, if the system is sealed, and vents/sucks what it needs to through the charcoal can, why does the gas cap never hiss? Did early EVAP systems vent and suck everything on either side of atmospheric pressure? Anyone got a stock system with charcoal can of that vintage that builds pressure or vacuum in the tank?
Click to expand...
My 71 has the original tank, cap, charcoal can, etc. Non-vented cap, and I've never had any issues with pressure/vacuum in the tank.

(For all I know though, the tank may have gaping holes in the top, I don't think it's ever been dropped and I know the seal on the sending unit lockring needs replaced, it's on the to-do this summer)
 
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