10 Reasons Why Engines Lose Power Over Time

A viewer’s engine ran smooth, started fine, and seemed simply low on power, until he pulled a worn camshaft and found some valves barely opening at all. Engineering Explained’s Jason Fenske breaks down the four things every gasoline engine needs to make power, and why each one quietly fades with age. One fix turned into a bigger power gain than anyone expected. What’s actually happening inside an engine that feels “just old”?

An engine can idle smooth, start every time, and still be lying to you. That’s the trap one viewer describes falling into: a car that ran fine by every obvious measure, just quietly down on power, and he assumed that was simply how the engine was supposed to feel. It wasn’t until he pulled the camshaft to install performance parts that he found out why, and what he saw explained a problem he never even knew he had. Jason Fenske’s Engineering Explained channel built an entire video, watched over 3.4 million times, around the four things every gasoline engine needs to make power and how each one quietly erodes with age. But it’s the story buried in the comments, about a cam worn so badly some valves were barely opening at all, that shows just how deceiving a “fine-running” engine can actually be. What was really happening inside that engine the whole time?

Four Ingredients, One Question: What Makes Power?

Fenske frames the entire video around a deceptively simple question: what does a gasoline engine actually need to make power, and what happens when any of those ingredients degrades? The answer breaks down to four fundamentals, air, fuel, compression, and spark, and the video works through how each one erodes gradually enough that most drivers never notice the decline until it’s severe. That framing is what makes this video different from a typical maintenance checklist. Rather than listing symptoms, Engineering Explained treats engine wear as a systems problem, showing how a shortfall in any single ingredient forces the other three to compensate, and how that compensation is exactly why an engine can still start, idle, and drive normally while making meaningfully less power than it did when new. Commenters have praised the density of that structure repeatedly, with one summarizing it as ten distinct reasons delivered in roughly five minutes flat.

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The Cheapest Fix Hiding In Plain Sight

One of the clearest examples in the video is also one of the easiest for any owner to check: a dirty or clogged air filter. An engine relies on pulling in a specific volume of air to combine with fuel in the correct ratio, and a filter caked with dust and debris restricts that airflow, forcing the engine to work harder to pull in the volume it needs. That added resistance translates directly into lost power, and unlike more invasive causes of power loss, it’s also one of the cheapest and fastest to fix. The video uses this example as an entry point into the broader air-fuel-compression-spark framework specifically because it’s relatable, most viewers have neglected an air filter at some point, before moving into causes that are far less visible and far more consequential, like the internal wear happening inside the valvetrain that most owners never think to inspect until something forces them to look.

Valves That Were Barely Opening At All

The most striking story in the comments comes from a viewer whose engine exhibited none of the obvious warning signs. It ran fine, sounded smooth, and started without hesitation, the kind of car most owners would never suspect had a real mechanical problem. He simply assumed the car was low on power because that’s just how it was, an assumption plenty of owners make about aging engines. It wasn’t until he installed a performance camshaft and cylinder heads that the real story came out: pulling the old camshaft revealed wear so severe that some valves were barely opening at all. That’s a profound finding, because a camshaft controls exactly how far and how long each valve opens, which directly governs how much air and exhaust can move through the engine on every cycle. A cam that worn doesn’t announce itself with a check-engine light or a rough idle, it just quietly strangles the engine’s breathing, cycle after cycle, for however long it’s been degrading.

A Power Gain Bigger Than Anyone Expected

What happened after the swap is the part of the story that sticks. Once the viewer reassembled the engine with the new performance cam and heads, the power increase was described as incredible, far beyond what he expected from bolting on performance parts alone. That gap between expectation and result is the real lesson: he went in expecting the kind of gain a cam and head upgrade normally delivers on a healthy engine, and instead got that gain plus the recovery of everything the worn-out original cam had been silently costing him for who knows how many miles. It’s a scenario that likely applies to plenty of engines currently running with wear nobody has diagnosed, simply because a worn cam doesn’t present the way a blown gasket or a dead cylinder does. The engine just gets a little weaker, a little at a time, until the owner forgets what it originally felt like.

The Fuel Filter Nobody Thinks About

A separate comment raises a stressor that gets far less attention than air filters or worn cams: the fuel filter. A clogged fuel filter doesn’t just restrict flow to the engine, it forces the fuel pump to work harder to push fuel through the restriction, and that added strain can shorten the pump’s service life considerably. It’s a quieter failure chain than a worn camshaft, but it fits the same pattern the video establishes: a component degrades gradually, something downstream compensates by working harder, and the eventual failure feels sudden even though the underlying cause had been building for a long time. Fenske’s four-part framework of air, fuel, compression, and spark gives viewers a mental model for catching these problems early, rather than waiting for a component to fail outright and turn a preventable maintenance item into an unplanned repair bill.

The V6 Owner Who Made His Engine Stronger Than New

Not every comment describes decline, one details how to fight it. A viewer running a 3.4-liter GM V6 says the engine is more powerful today than when it left the factory in 2004, crediting a specific and disciplined maintenance regimen: switching to Mobil 1 synthetic oil immediately, installing a K&N air filter, running dual exhaust with pipes a half-inch larger than stock, replacing the factory plug wires, upgrading to Bosch Iridium spark plugs, and changing the automatic transmission fluid at 60,000 miles. None of those changes are exotic or expensive individually, but applied consistently over more than two decades, they add up to an engine that’s avoided exactly the kind of gradual decline the video describes. It’s a useful counterpoint to the worn-cam story: where neglect quietly erodes an engine’s output, disciplined, unglamorous maintenance can apparently hold that decline off entirely, or even reverse it relative to a factory baseline.

Ten Reasons, Five Minutes, No Padding

Part of why this video has resonated with over 3.4 million viewers is its pacing, and commenters have said as much directly, with one describing it as ten reasons delivered in five minutes at a rate of two reasons per minute. For a channel built around explaining engineering concepts clearly and quickly, that density is the whole appeal, there’s no padding, no lifestyle footage, just a direct walk through how compression bleeds off through worn rings, how spark degrades through fouled or worn plugs, and how fuel delivery suffers through clogged filters and injectors. For classic car owners in particular, the video functions less as a warning and more as a checklist, since engines built decades ago are exactly the kind most likely to be running with exactly this kind of invisible, accumulated wear hiding behind an engine that still starts, idles, and drives like nothing is wrong.

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