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?
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.
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.
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!