Vapor lock?

[Matt88s]

Howdy all.

Q. for ya.

A friend of mine had been having occasional trouble starting her car after work around 5pm.
Usually it will start after a few tries or if you leave it for a bit and come back.

So I happened to be there the other day when it was doing it so I took a look. It cranked energetically, no problem there. Since I knew it usually did this on hot afternoons, I thought, well, maybe the fuel system isn't venting and its vapor locking. So I went back and took the gas cap off. Whoosh. Definitely a problem there. Went back and tried starting it again, car cranked a bit, then fired up. :2tsup:

However I was expecting air to be sucked in when I took the cap off if it was vapor locked per say, but it appeared to be pressurized, like I suppose it would be after sitting in the sun all afternoon, but why would that effect fuel uptake? I can see a vacuum on the tank effecting uptake but pressure?

Can someone explain this to me?

[Honorary Bloke]

You don't say what year model it is or where you live, so it is a bit tricky. A newer car is very unlikely to develop vapour lock because the fuel pump is in the petrol tank and surrounded by liquid petrol--no vapour. But if the fuel pump is mounted outside the tank, up toward the engine, it could well be vapour lock if the weather is very hot. :)

[Matt88s]

It's an early 90's honda, probably a 91/92.

I don't think it was "vapor lock" per say, as cars of the last two or three decades don't really suffer from "vapor lock", but it did have what seemed like an excessive amount of pressure in the tank, pressure that should have been vented away by the tanks venting system, and it started right after I open the cap and releaved the pressure, so I was trying to figure out why having pressure in the tank might keep it from starting. :?

It might be totally unrelated I suppose, but it would be rather coincidencial.

[Barry_White]

I haven't heard of vapour lock occurring for years. It only ever seemed to occur on old pommy cars that where built for a cold climate.

I have experienced the whoosh of air into the tank after a long trip but it has never caused the car not to start.

I would think if the car was sitting in the sun it would rather build up pressure in the tank rather than a vacuum and I would think that it would be forcing fuel into the pump rather than constricting it.

Maybe the pump is slowly dying or the filter is blocked.