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Honda Civic 2002: Why does it only drive without the petrol cap on?

My Son's 2002 Honda Civic has been very reliable until a few months ago. Whilst driving on a hot day the car just died while stopped in traffic. Luckily a guy jumped out to help him and suggested he open the petrol cap to release pressure. The car started after doing this. It seems to drive without stopping only when we drive without the petrol cap on. Any suggestions as to how to fix this problem? We are worried that it will stop at any moment on a hot day, and we would preferably like to be able to drive the car with the petrol cap on.

I think what’s happening when you open the fuel cap, Frank, is not a release of pressure, but rather a release of vacuum. As your car uses petrol, the level in the tank gets lower. If the tank can’t `breathe’, allowing air to fill that space in the tank, a vacuum is created. Eventually, the vacuum becomes so powerful that the car’s fuel pump can no longer drag fuel from the tank to the engine and the car stalls.

When you remove the fuel cap, hey presto, the vacuum is released and the fuel pump can do its thing once more. In modern cars, this is often caused by the charcoal canister (a part of the pollution-control equipment) becoming clogged and not allowing air back into the fuel tank. I’ve actually seen cases so bad that the vacuum has actually collapsed the fuel tank under the car (must have been a powerful fuel pump).

Driving around with no fuel cap on is both dangerous and illegal (it renders those pollution controls useless because it allows fuel to evaporate into the atmosphere) so the solution is to find out where the blockage is and replace the relevant parts. Like I said, I’d start with the charcoal canister in the engine bay. It could even be that dust or dirt is blocking one of the rubber hoses that link the pollution gear systems. It might seem like a big problem, but from the symptoms you’ve listed, I think it’ll be an easy fix.