Under the Hood: What does it take to set a car on fire?

McClatchy-Tribune Information Services

Feb. 25, 2019

Q: Brad, I have a non-mechanical question. On my way down that steep hill, I noticed the hood of a car on the up side was engulfed in flames. Blazing! This was my first experience seeing an engine fire, and it made me wonder what could be burning. I know that engines are mostly metal, that oil leaks tend to smolder and that electrical shorts don’t flame without a nearby combustible. What is under the car hood that would cause this kind of a blaze?

—Marialis

A: Wow! You seem to know your fire stuff! I wonder if the “up side” hill climbing played into this or if it was just a coincidence.

The most likely cause of an engine fire has to be a fuel leak. This could occur due to a faulty fuel line connection, chafing or damaged fuel line/hose, or cracked fuel injector casing, among other factors.

Depending on a vehicle’s fuel injection vintage and section of its system, fuel pressure can range from about 40 to 3000 PSI, so a leak can really shoot fuel all over the place under hood!

Needed next is a source of ignition, such as a tiny spark from alternator or radiator fan brushes, a leaking ignition system, or a hot surface such as an exhaust manifold or turbocharger.

Another possible cause is an oil leak, but a really hot surface such as the exhaust manifold would be needed to get it going. A missing oil filler cap could result in quite a bit of oil slung around. I wonder if this could be the culprit on the fire you witnessed as the exhaust manifold temperature while climbing would be higher than normal?

When I was a tech at Pontiac many moons ago, we’d occasionally see a severe valve cover oil leak light up the 2.5L GM engines. In a case like this, prior to a possible fire, one would note a lot of smoke and odor, rather ominous signs of what might be ahead.

You’re spot-on regarding the electrical shorting, which according to stats is only about half as likely to cause a fire as the mechanical faults above. It’s possible for a wire’s plastic insulation, tape and tubing to catch fire, but the majority of circuits are protected (fused or similar) in cas a short to ground occurs, and additional combustible surfaces nearby would be likely be needed to result in a large fire. Aside from a plastic engine cover, air filter box, fuse box, rubber hoses, and additional wiring with plastic insulation, there aren’t a large number of combustible items under hood to burn.

The number of vehicle fires has steadily declined year over year, so that now about half as many occur each year as occurred during the 1980s and ’90s. In virtually all cases of vehicle fire, it’s recommended that passengers quickly exit the vehicle and get at least 100 feet away. Do not return for belongings!

Raising the hood or opening doors in an attempt to extinguish the fire often dramatically accelerates the fire, as more air becomes available. I used to carry a fire extinguisher but fell out of the habit. Unless the car was a cherished classic, I’d prefer to stand back and start imagining how nice the next one will be!