Large spark in CMS motor

Ok, and I'll end with this, I think. What keeps the improbable scenario from exploding? Those improbable scenario's actually do not explode.

No. They are typically flat and generally tilt towards a low end of the tank for all the gunk to collect at. The pump is suspended by the tank unit up from that position. There are ridges for rigidity on some tanks but those ridges generally run in a direction so as to drain to the low spot of the tank and the low spot is typically the forward end facing towards the front of the vehicle. This allows for more ground clearance under the back of the vehicle. That area is also the most vulnerable and typically will be the damaged/ruptured area when a driver drives over an obstacle.

Are there not formed

I would say that a pint of fuel could collect, maybe depending ont he damaged area but if the damage is located on the front bottom it is likely very very little will remain. Keep in mind a puntcture on the bottom of the tank will act exactly like a can of juice. Hold the juice can and poke a hole in it from the bottom. Juice will come out and as it comes out air will replace the leaking juice otherwise the fluid would not drain. Gasiline works the same way.

Yeah I am sure there is a secondary back up measure there are countless others in the fuel lines and in the old days the carburetors had preventatice features to limit fuel flow.

And there in lies the probable answer as a fowled spark plug will spark but not necessarily ignite the fuel mixture. Gasoline running past the brushes keeps every thing cooled down under normal operating conditions. And with electrical wiring running into the fuel tanks even for the tank units to measure the level of fuel you have to know that sooner or later there is going to be a short and that does not seem to present a problem either. that has veen a fact long before electric fuel pumps wer even being used. There is absolutely air in the tank otherwise the tanks would colapse or fuel would not exit. Fuel vapor may be too dense to burn unless the conditions are right. Vapor literally does not exist with out air, vapor is simply difused matter suspended in air.

Either way I believe your explanation that the spark in not hot enough is the stop gap safety measure. That is indeed a constant vs air and vapor conditions around the fuel pump.

Leon wrote: ...

Not necessarily--there are other gaseous materials other than O, N, etc., that are the constituents of air.

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It might seem so, but that is not the case.

Gasoline vapor is heavier than air. With air given a vapor density of 1, the vapor density of gasoline is between 3 and 4.

In an undisturbed tank, vented to the outside, as gasoline evaporates, the vapor will displace the air. In the case where all of the gasoline has been drained and air has entered the tank, the remaining vapor will be concentrated at the bottom.

Disturbed, I suspect that a shaken tank would not qualify as undisturbed. There is an awful lot of shoshing around going on in there when the vehicle is moving and if there is a hole in the bottom of the tank from a damaging impact the vapors will "grain out" after the fuel.

Or, more visible way to think of it, if were to start w/ a liquid in a closed container w/ a movable wall and if retract that wall creating a vacuum there will become a new equilibrium of two phases--liquid and gas. The concentration will be dependent on the specific liquid. There's no air needed...

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And some fool will try to weld the 'empty' tank ....

Sadly, it does happen.

Many years ago a young neighbor of mine was using an oxy-acetylene torch to cut open a fuel tank which he had emptied some time beforehand and thought was safe. It exploded, leaving him with serious permanent brain damage.

How could anyone tell?

Sorry, that was thoughtless and cruel, but it's the mood I'm in at the moment.

Friend of mine was working on a gas tank from a 1979 Ford Bronco washed the gas tank out several times was trying to solder the tank and all of a sudden there was a big bang what had happened the tank actually expanded,he finished soldering it but we had a hellova time getting it back onto the truck as it had expanded so much. Dan

Trouble with that is that water is viscous.

Best thing for cooling them is an internal flow of hydrogen, but that really is a pain to work with.