Corner Wrench: The keys to escaping vapour lock

Article content

If you’ve tinkered with more than a few classics or done some performance mods, chances are you’ve run into vapour lock. It occurs when liquid fuel vaporizes before it gets to the point of delivery into the intake; carburetor or injector. While it’s much more common with carbureted engines, it can occur on fuel-injected models as well.

Article content

It’s all about the heat. Excessive heat in the engine bay or exhaust system can do the trick, and extremely hot weather doesn’t help. In slow or stop-and-start traffic, a lack of air flow around the vehicle will definitely make things worse. In addition, winter-blend fuels and higher altitudes can both bring lower boiling or vapour temperatures. Fuel pumps can’t move vapours, and the symptoms of vapour lock include stalling, low power, and hard restarting with excessive crank times.

We apologize, but this video has failed to load.

Try refreshing your browser, or
tap here to see other videos from our team.

Corner Wrench: The keys to escaping vapour lock Back to video

If you get hit with the vapours on the road, there are a few things you can do to hopefully get the engine restarted. If it’s a hot sunny day, get the vehicle off the black-top and into some shade. Pop the hood, and if you’ve got some cold water available, pour it over the mechanical fuel pump and lines. After letting things cool down, try to restart with the gas pedal partially depressed to help push any remaining vapours out. You may notice some sputtering after it fires up and the last of the vapours are expelled.

Article content

Finding and implementing a cure can be a multitask job. If your engine is still running bare metal fuel lines, either wrap them with an approved insulator wrap or switch to braided black flexible line which can provide its own insulation. Reinforced fuel injection hose can do the trick, but plan its routing carefully; as stiff as it seems, it can still kink when bent too far. You can also get a non-metallic carburetor spacer to help reduce the heat transfer from the engine, along with insulation shields. Electric fuel pumps (either in-line or in-tank) can eliminate the risk of vapours up to the point of a mechanical pump, but not beyond it. Pressurized fuel is harder to vaporize in transfer lines.

Article content

Check clearances between any fuel line or component and exhaust manifolds and pipes. Some of those factory heat shields might have looked tacky enough to remove, but they do serve a purpose.

Some automakers faced with this problem have gone to great lengths for a solution. Back in the 1980s, when Jaguar was equipping their low-profile XJ sedans and 2-doors with V-12 engines, they ran into excessive heat problems on their newly-designed fuel injection rails. Previous models used individual lines to each injector, and they tried different configurations of fuel rails with limited success. The eventual solution was to create a new, longer air-conditioning line and mount it to the fuel rail to provide enough cooling to keep things running smoothly.

Finally, as if ethanol didn’t cause enough problems for classics, it can also increase the risk of vapour lock. So get your classic off the sauce and on to a pure 91!