Tips, Tricks & Trends

Why Do Car Engines Lose Power Over Time?

Besides 0-60 times, one of the most common ways you can compare two cars’ performance is horsepower. And owners often install mods like performance exhausts and turbochargers in an effort to give their engine more power. However, not only is your engine’s quoted horsepower not always accurate, but it’s also not static. Over time, no matter what kind of tuning you’ve done, your engine will lose power. But there is something you can do about it.

How do car engines make power?

Whether your ICE car burns gasoline or diesel, it makes power via combustion. Air is sucked into the combustion chamber and mixed with a spray of fuel. This gets compressed by the piston and ignited, either with a spark plug (gasoline) or further compression (diesel). The resulting explosion forces the piston down—and voila, we have horsepower and torque.

An electric car’s motor obviously doesn’t rely on combustion. However, it still produces power thanks to chemical reactions within its battery. And, like a combustion engine’s power output, it can be measured.

The device doing the measuring is the dynamometer or ‘dyno.’ There are 2 basic types, OnAllCylinders explains, a chassis dyno, and an engine dyno.

There are several kinds of chassis dyno, Hot Rod reports, but the most common is the ‘rolling-road’ drum-roller type. The car is first set into position with its drive wheels on the rollers. The tester floors the accelerator, which spins up the drum rollers. The rotation is used to calculate wheel torque, which is converted into horsepower.

Though a chassis dyno reports wheel horsepower, it doesn’t actually report crank horsepower. That’s where the engine dyno comes in. Instead of spinning rollers, the engine is bolted directly to the dyno. Based on how much force the dyno uses to hold the engine at a given speed, it calculates engine output.

As Motor Trend discovered when it dyno-tested the C8 Corvette, dynos aren’t infallible. However, they can validate manufacturer claims. Plus, they also help tuners boost performance. Or, see how much power has escaped from an engine over the years.

What causes power loss?

In using a chassis dyno, there’s already some engine power loss going on. The transmission, driveshaft, differential, and wheel hubs all absorb a portion of the engine’s output. The ‘traditional’ rule-of-thumb states the drivetrain causes a 15% power loss. Today, though, even AWD and 4WD drivetrains don’t cause more than 10% loss. And RWD ones cause closer to 1-2%, SuperStreetOnline reports.

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However, the engine power loss from time and wear is separate, Jalopnik, and Road & Track report. As we just discussed, a combustion engine needs air, fuel, compression, and a spark. If one of those 4 is missing, the engine loses power or stops working.

Some engine power loss is inevitable, Car Throttle explains. As engines get old, their seals get hard and brittle. Piston rings and valvetrain components wear down. This causes compression and horsepower to drop.

It’s a similar story with the throttle cable, Autoblog reports. It can stretch and fray, or even break entirely. When that happens, air won’t flow past the mass air-flow sensor, which won’t trigger the fuel injectors. A clogged air filter creates a similar problem.

Speaking of fuel injectors, they can clog over time. The MAF sensor, catalytic converter, spark plugs, valves, and pistons can clog, too, 2CarPros and Motorsport.com report. A caked-over injector can’t spray fuel into the combustion chamber. Carbon deposits prevent valves from sealing properly and spark plugs from firing.

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And on pistons, they can cause hot spots that lead to knock (spontaneous combustion). The engine then adjusts the ignition timing, preventing knock but also cutting power. Plus, if the catalytic converter is clogged, exhaust gases can’t exit as quickly, leading to less oxygen in the combustion chamber.

Can I reverse it with a tune-up or an engine rebuild?

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Some of these problems can be resolved with a tune-up, Kelley Blue Book reports. This usually consists of new spark plugs and ignition wires, fresh hoses, and a new air filter. Although, Cars.com points out that, if you’re following your car’s maintenance schedule, a tune-up is usually unnecessary.

In the case of a stretched throttle cable, that can be a DIY repair, ItStillRuns reports. There’s an adjustment screw located where the cable meets the throttle body, which connects the fuel and air systems. Just tighten the screw until there’s roughly ¼” of play. If you’re noticing delayed accelerator response or problems with the cruise control, it might be the cable, YourMechanic reports.

Removing deposits, though, is trickier. The MAF sensor can be easily removed and cleaned. However, doing the same to the valves, fuel injectors, and pistons may require fully- or partially-disassembling the engine. A high-mileage car can be better than a low-mileage one in this regard. that’s because regular driving extends seal life.

Mechanic tunes up an old Hyundai Galloper engine
A mechanic works on an old Hyundai Galloper engine | Seung-il Ryu/NurPhoto via Getty Images

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At that point, some may consider getting an engine rebuild. An engine rebuild involves cleaning the engine and replacing components, Autoblog explains. That includes the seals, piston rings, valve guides, and similar parts. Extensive rebuilds can even replace the camshafts, crankshaft, and pistons, leading to more engine power.

However, engine rebuilds are time-consuming and expensive. Unless you’re finding metal in the oil, CarsDirect reports, or seeing a lot of white smoke due to failing piston rings, it may not be your best option. Though, if your engine has failed completely, that’s a different story.

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