I have a 69 cutlass with a holley electric fuel pump. Is there any reason the previous owner would have plugged the 2 vent tubes coming out of the tank? It seems to work ok, but I've noticed that when I open the fuel cap there seems to be a large amount of vaccuum when I fill up. Also, seems this would cause the pump to work harder than necessary. I did have to replace the pump soon after I bought the car in 2004 and I wonder if it was working too hard. I've probably only driven the car 1000 miles or so since putting the motor in in 2004, at which time I replaced the dead electric fuel pump. I have my old 69 cutlass/parts car and it still has the stock tank venting set up. Should I scavenge these parts and put my driver back to normal?
Eventually I plan on going back to mechanical fuel pump but that will require new fuel and return lines that don't exist on this car. I hate the electric pump, it's really noisy.

 

I don't know if it make the pump go bad. All new cars use electric fuel pumps they are all sealed systems and require pressure in the tank to make them work, but they also have fuel return, and are vented, and use fuel regulator to slow the flow of gas at the injectors. Of matter of fact they vent when the motor is warmed up and when the rpm is raised above idle. I know I need to back on track. "I like mechanical fuel pumps better for cars with carbs". As fare as changing the tank you just have to see when you change it over. I would run a vent to the tank.

 

I may be asking for it but I'll give it a Mr. Science inspired guess.

If the tank isn't vented the pump is going to work harder and harder and will eventually burn out prematurely.
It's like sucking through a hose with the end blocked, either the hose collapses or in this case the tank builds a vacuum.
The pumps working way too hard building a vacuum in the tank that's pulling against the pump.
I've seen older cars with partially collapsed fuel tanks from having the wrong non-vented caps put on that had mechanical pumps.

I would put the vent back on if it had one originally.

 

The sound you here when open the cap up on the tank is pressure being released. The tank is building up pressure from hydrocarbons. Venting the tank would take care of this problem.

 

X2.
If this is a typical street driven car, I would also lose the electric pump and revert to the mechanical one...

 

Thanks for the responses. I did decide to take the original style venting components off the parts car and put them on the driver. I'm sure the fuel pump will thank me. And I'll be a step ahead whenever I do decide to get new fuel and return lines and go back to the mechanical pump. Maybe next year. I'm just happy to have the car off the jack stands where it sat with no front control arms for over 2 years. That wasn't much fun.

 

I don't know if I believe this. First of all the fuel is pumped out faster then the build up of any VOC. Secondly, and more importantly pressure in the tank can push against the cap's rubber washer and escape. So, I find it hard to imagine a scenario as you describe. Now on the other hand if there is low pressure in the tank it draws the cap in and in fact causes the rubber washer in cap to seal even tighter. Thus, low pressure in the tank would sustain itself.

And yes I do think lower pressure in the tank would make the fuel pump work harder.

 

That brings up another question for me. Is the cap vented? I have a 68 442 with a cap that has a double twist on. I don't know if it is vented or not, ans don't know if it's even the correct cap. What tyrp of cap would be on a 68 442 automatic?

 

I would think the cap is not vented; for several reasons.

-First my 70 tank cap is not and it is newer
-Because of the shape of the tanks, their location and the fact they are filled through the rear the makes the neck very low relative to the gas level. This means that if the tank were vented it would also be likely to leak gas when accelerating. Especially if the tank is full
-A vented cap would have one-way diaphram in it. If your cap is like mine it is too thin to contain a reasonable diaphram

You can check to see if your tank has venting tubes. For example my tank has the tubes on front of tank (that is side of tank nearest axle) on the passenger side. There are 2 short rubber hoses connected to each vent and the hoses go into a Y shaped plastic piece. The hose and plastic are safely above the hightest level of the tank so generally fuel cannot leak nor dirt get in. Nevertheless, on occassion with a full tank if I turn hard, stop hard or do anything else to cause fuel to splash around there can be small dribbles of fuel that do splash up and out of the plastic vent.

It seems like an incredibly stupid design to me because the splashed fuel drips down very close to one of the exhuast pipes. I can see that with the right splash and car movement during a drip the gas could land on the hot exhaust pipe.