Can a Bad Tensioner Cause a Loss of Power? [Answered]

When it comes to engine performance, even small components can have a significant impact. One such component is the tensioner, which helps keep the serpentine and engine’s timing belt or chain in place. But can a bad tensioner really cause a loss of power?

To get straight to it, yes, a bad tensioner can cause a loss of power, regardless of whether it is the serpentine belt or the timing belt/chain tensioner. To briefly explain (in more detail below), if a tensioner is worn and the bearing stops it from turning freely, it becomes a drag on the engine, reducing its performance.

What Is a Tensioner?

There are two tensioners fitted to most modern vehicles one for the serpentine belt, which is responsible for turning the alternator, engine-driven fan, ac pump, and steering pump, and the other is for the Timing belt/chain. However, ultimately they both have the same job.

A tensioner is an extra pulley, usually smooth, so it doesn’t grip the belt with any teeth. There are different designs you might see an automatic spring-loaded or a manually adjustable tensioner. Its job is to take the slack out of the belt, so it remains under constant tension, hence the name! An automatic spring-loaded tensioner constantly adjusts itself as the belt wears, whereas a manually adjusted tensioner (usually only found on serpentine belts) requires adjusting when necessary.

If cars didn’t have tensioners, the belts would slip, squeal, and fall off the pulleys as they stretched and became weaker with age. If you’ve heard a serpentine accessory belt squealing before, it’s usually because the belt has stretched so far that the tensioner is past its maximum adjustment. When this happens, the belt slips on the pulley as it is forced around, making horrible screeching sounds.

How Does a Bad Tensioner Affect Engine Performance?

Serpentine belt tensioner

The serpentine belt tensioner doesn’t drastically reduce engine performance, but it does sap some of the power by increasing the engine’s load if the tensioner is faulty. Because the crank pulley turns a serpentine belt, if the tensioner is seized or its bearing is faulty, it will become a parasitic drag on the crank pulley, just taking a couple of horsepower away from it, also reducing fuel efficiency, but hardly noticeable. A bad tensioner would be more problematic for an alternator, ac compressor or the power steering pump than it would be for engine performance.

Timing belt tensioner

The timing belt tensioner can cause a reduction in the engines performance in a few different ways:

  • Reduced fuel efficiency – If the belt becomes loose because the tensioner is loose or too tight, the timing may be out, which affects the air and fuel being combusted in the engine. The MPG will suffer if the motor is no longer burning fuel efficiently.
  • Reduced power – There are two reasons for a power reduction. One is if the tensioner seizes, it becomes a drag on the engine, forcing it to work harder. The other is because the tensioner stops the belt from slipping and keeps the camshaft and crankshaft synchronized; if not operating correctly, the engine cannot fire on time and may combust prematurely, meaning there will be a significant loss of power and could even cause the car to misfire.
  • Increased emissions – Because, again, the belt will not be under the correct tension, the timing may be slightly out, causing an increase in the amount of fuel being burnt in the engine, which will lead to a rise in the emissions. It can cause the car to fail an emissions test.
  • Increased noise – Although not specifically affecting the engine’s performance, an increase in noise is quite common from a bad tensioner. Especially noticeable at low speeds or idle, the timing chain/belt vibrates against the tensioner.
both belt tensioners

Signs the Tensioner Is Worn

Apart from the noticeable reduction in engine performance a tensioner for either belt or chain can give some noticeable signs its worn and needs replacing or requires adjustment if possible. They include:

  • Screeching noises from the belt -Belt is usually slipping because the tensioner is worn.
  • Broken serpentine belt Can break because the bearing falls apart causing the tensioner pulley to wobble and the belt to catch and fray on other pulleys because of the distortion.
  • Charging light on the dashboard – The tensioner can seize and cause the serpentine belt to drag reducing the rate at which the alternator generates it’s power, the car may detect this and put the battery charging light on to indicate an issue.
  • EML light on – The engine management light can sometimes come on because the car can detect an issue that there is a potential minor misfire or a reduction in performance. All because the timing belt tensioner is causing a problem. However, this isn’t a common thing to happen.
  • Engine locked up and won’t start – Although this is catastrophic and the engine is a complete write-off, if the timing belt snaps because of the tensioner, the car will not start again, and it will lock up if it snaps when driving.

Is It Safe to Drive With a Bad Tensioner?

Driving with a worn serpentine belt tensioner is not so much of a problem however with almost everything its never a problem until it becomes a problem. At best the car may not charge correctly, the air con may not work and the power steering might be intermittent. But if the belt does break as a result, the car may shut off until it’s replaced.

However, driving with a bad timing belt/chain tensioner is far more problematic; although the car may run right, the engine will lock up if the belt slips or breaks. That could be the engine’s end, or worse, if driving at speed and the engine locks up, the car will be sent into a skid which could cause a road traffic accident.


To summarise in one sentence: a worn tensioner can cause an increase in the workload for a car’s engine, reducing the car’s performance and efficiency.

Read Timing belt vs serpentine belt – Know the difference next if you aren’t 100% sure about the differences and the jobs the two belts mentioned in this article do.

My name is Tom although my friends call me Tommy. Messing around with cars and bikes has always been a hobby of mine even from a young age. So I made it my day job 17 years ago. I am a fully qualified mechanic as you would expect. I've worked in all different areas of the motor trade, valeting, panel beating, engine repairs, I'm sure you get the idea. I enjoy sharing my wealth of knowledge and experience with others, which is the reason I spend a lot of time here writing for this website.

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