I saw an ad on facebook and it got me wondering. I know if your gas tank is not vented, you need a vented cap or it could collapse on itself. A vented tank can use a non vented cap and a non vented tank can use a vented cap. But can a vented tank use a vented cap without issues?

Gas tanks are not really vented. The vented style cap, or charcoal canisters ect. do the venting. A vented cap should work on any tank if the gas cap fits the fill tube.

With old Mopars I think the fuel tube and gas cap tangs have to be a matching pair? I have a car with the ECS emission system and other types of non ECS gas caps simply don't fit right - either not at all, or too loose ect. They likely did this on purpose to keep the right cap on the car so people didn't accidentally vent an unvented system.

It would be fine. Only issue would be if you have a charcoal canister to scrub the fumes then another vent in the system would bypass it.

Got to call shenanigans on a tank collapsing under vacuum. At least a steel tank wouldn't. Motor would just be starved for fuel.

The aftermarket "vented cap" that was on my 51 when I bought it would not release the vacuum or pressure for that matter, the tank would oil can. When I went to fill it up and opened the cap you could hear the whoosh and that oil can sound. When it sat int eh sun and heated up you could hear the tank oil can as it expanded.

I ended up buying a new aftermarket cap that did the same thing, so I had to mod it to work properly. At no time did the fuel ever stop flowing though.

That is wild. I've pinched the vent line before and never had a steel tank collapse before. Made one hell of a whoosh sound though opening the cap. Was this an electric or mechanical pump?

What vehicle? If it has EVAP then the cap should be non-vented. The venting to atmosphere is done by the EVAP system via a (usually) vacuum valve when the vehicle is running after the initial 300 second charcoal canister purge. If the cap IS vented and it is a current EVAP vehicle then it will show a code or warning that there is a leak in the EVAP system. When the vehicle is not running the entire fuel tank, fuel lines, intake system, (carburetor if it has one) is totally sealed and "vented" to the charcoal canister until the engine is started. At least that's the way I understand how EVAP systems work.

x2 on if it's modern and has an evap system, the cap needs to be non-vented.

I'm not sure how the newest stuff works, but in the mid 2000's on many cars they would determine if there was a small leak in the evap system by closing the canister vent and purge valves after the engine is shut off, wait 20-30 minutes, and then the ECU would wake back up and measure the vacuum in the fuel tank.

Mechanical pump and this was after running around town for a while, something I am not sure could be easily replicated in the shop. Not sure if it would have stopped fuel flow if I was on the highway doing a banzai run though,

Usually it would starve for fuel but I have seen tanks get sucked in as well.

IMO I don't think there could ever be enough vacuum to collapse a fuel tank if it is a permeation compliant fuel tank as they are subjected to thousands of pressure/vacuum cycles as part of the compliance testing with extreme pressure and vacuum during that testing.

Here is the CARB procedure and it states 10,000 60-second cycles of pressure and vacuum.

Pressure Test
A pressure test shall be performed without fuel and prior to any other
portion of the durability demonstration or preconditioning of the fuel tank.
(a) Determine the fuel tank system’s design pressure and vacuum
limits under normal operating and storage conditions considering
the influence of any associated pressure/vacuum relief
components. To do this, measure the pressure limits using a fuel
tank from an evaporative emission control system that is not used
for any other portion of this test procedure by installing a
pressure transducer in the fuel tank. With the exception of the
use of the pressure transducer and connection to a carbon
canister, as applicable, the fuel tank and fuel tank configuration
used for these pressure measurements shall be identical to those
used in the remainder of this test procedure. Using compressed
air of no less than 21 °C, pressurize the fuel tank with compressed
air, seal the fuel tank, and measure the pressure every second for
5 minutes. Use a vacuum pump to draw a vacuum in the fuel tank,
seal the fuel tank, and measure the pressure every second for
5 minutes. Record the maximum and minimum pressure
measurements on the test report. Subsection (b) of this test is not
required if the fuel tank pressure does not exceed a gauge
pressure of + 1.0 kPa for at least one minute when pressurized
and the fuel tank vacuum does not exceed a gauge pressure of
– 1.0 kPa for at least one minute when a vacuum is drawn in the
fuel tank.
(b) A pressure test shall be performed by sealing each fuel tank and
cycling the pressure between + 13.8 and − 3.4 kPa ( + 2.0 and
− 0.5 psig) for 10,000 cycles at a rate of 60 seconds per cycle. If
normal operating or storage conditions cause pressure changes
greater than + 13.8 or – 3.4 kPa to accumulate in the fuel tanks,
cycle the pressure in the fuel tanks between the actual high and
low pressure limits experienced during normal operation or
storage. The tank pressure test shall be performed in a 49 ± 3 °C
environment with compressed air of no less than 21 °C.

A12, do you know when that regulation came into existence? This could explain why this is seen with older tanks and not the new stuff.