By Mike Allen, the Saturday Mechanic
One question I’ve gotten with some regularity from readers for many years usually comes around this time of the year, as people begin to travel for the summer season: “Why does my car run so poorly in the mountains?”
Simple answer–it’s the air. Specifically, air gets thinner the higher you get. And your car runs on more than gasoline; it runs on air as well. For every pound of gasoline (about 21 liquid ounces) your engine burns, it needs to ingest 14.6 lbs of air, or nearly 1600 gallons. This air is pushed into the engine past the throttle by atmospheric pressure, which by coincidence is around 14.7 psi.
At 5,000 feet above sea level, atmospheric pressure is only 7/8 as dense as on the beach, and is not pushed into your engine as rapidly. Consequently, your car or truck–already laden with people and luggage or a trailer–will be down on power. That can make driving around at elevation problematic. Climbing hills will require more throttle input, and may require downshifting.
Automatic transmissions that are well-behaved at lower elevations may downshift unexpectedly, or start to hunt between two adjacent gears. This happens when the engine won’t make enough power to climb a hill, triggering a downshift to a lower gear. A minute or two later as the vehicle speeds up and reduces the load on the transmission, an upshift occurs. This cycle can continue to the top of the hill unless you manually keep the transmission in a lower gear or toggle of any overdrive switch. It’s annoying, but not damaging.
The reduced air pressure can also make an engine already starved for air by a plugged air cleaner element even worse off, bad enough to trigger a CHECK ENGINE light in extreme cases. In the days that cars used carburetors, the dirty air cleaner would make the engine run with an excess of gasoline, creating thick, black acrid-smelling smoke. Modern fuel-injected cars simply correct for the reduced airflow by injecting less fuel—which of course reduces power. I recall helping an elderly couple who were stuck in the parking lot on top of Pike’s Peak, their heavily-laden Honda Accord too anemic to back slightly uphill and out of the parking space they had pulled into. A slight push was all they needed.
A second problem caused by thin air is the lack of cooling. Thin air carries less heat away from the radiator and brakes. If your cooling system is not in good shape, you may experience overheating while climbing hills or brake fade while descending them. The cure, in both cases is to slow down and use lower gears.
There is one sunny side to higher altitudes—thinner air is less likely to cause engine knock, so you can safely reduce the octane rating of the gasoline you buy. In fact, you may have no choice—gas stations in Denver and other high-altitude cities may simply not have 93-octane gas, and the regular may be marked as 85 on the pump instead of the more normal 87. While at altitude, you’ll be fine, but avoid filling up on the cheaper grades just as you leave town. Otherwise, as you descend to closer to sea level, you may have a lot of low-octane fuel to burn off before you can refill with something more appropriate.
Fortunate enough to have a turbocharged or even supercharged engine? Then most of this advice doesn’t apply to you, as the turbo will force air in at sea-level rates even at very high altitudes. The performance drop caused by a clogged air cleaner will be just as profound as on a normally-aspirated car. And the potential for overheating is worse, as the engine’s ability to make its normal amount of power will be unchanged, even though the cooling system is still breathing the thin stuff.
Most cars should weather the altitude found on any public road without incident, but only if they’re running properly to begin with.