Smell of Gas in a Radiatorby Richard Rowe
Engines consist of four basic fluid systems, most designed to remain out of contact with the rest. Water- and oil-carrying passages should -- ideally -- remain completely isolated, doing their thing without interference from each other. The air and fuel systems do join in the cylinder head, but anytime you get a crossover of anything but air and fuel then you're looking at serious trouble in the engine. A good head-sealing compound in the coolant might help, but expect major repairs at some point in the future.
The Basic Problem
Your car's fuel and cooling systems are two completely separate entities, and where one goes the other was never meant to tread. Gasoline only comes close to your engine's cooling system in a few places, which cuts down the possible number of problem areas. The main problem here is that gas goes in through the air stream and cooling is supposed to stay in the block. Since the coolant is always under pressure, a simple coolant leak in the manifold will put water into your cylinders. So, the problem is far deeper than that.
Pressure and Leaks
Gasoline, in the intake stream, will never come under enough pressure to overcome that of your cooling system. Even highly turbocharged and supercharged engines won't reach one-fifth the pressure of a cooling system. The only time a fuel-containing mass comes under that much pressure is in the combustion chamber and in the exhaust, during and after the combustion process. This area also happens to coincide with a number of water-carrying channels and gaskets, all of which offer some interface between hot combustion gases and the water jacket.
You're looking at either a blown head gasket or a head cracked in the exhaust port or around the top of the engine's combustion chamber. If your engine hasn't experienced massive overheating in the recent past, then you're probably the victim of a blown head gasket. Overheating will also blow head gaskets, but normal operating temperatures will almost never crack a head. Consistent knocking or pinging and certain fuel additives can crack heads.
Blown head gaskets offer a panoply of opportunities for fluid system crossovers, particularly where the water jacket is concerned. A blown head gasket will typically send coolant into the combustion chamber while the piston moves downward, partially or completely quenching the combustion event and sending a cloud of white steam from your exhaust. If you've got exhaust gases in the radiator and a cloud of gray and white smoke coming from your exhaust pipe, then it's likely a blown head gasket. Fuel smell in the radiator without a cloud of steam usually indicates a head cracked in or around the exhaust port, since this area is almost always under some kind of pressure and won't allow much -- if any -- water to enter the exhaust stream.