Idler pulleys are engine pulleys that are responsible for guiding and tensioning the engine drive belts. The engine drive belts are routed in a specific manner around the various engine accessories, such as the alternator, water pump, power steering pump, and AC compressor. The idler pulley is meant to provide another smoothly rotating point for the engine belt to loop around, so that the desired routing can be achieved. Most engines use one idler pulley, and one tensioner pulley, though some designs do use more than one idler. Over time, idler pulleys will eventually wear out and need to be replaced. Usually a bad or failing idler pulley will produce a few symptoms that can notify the driver of an issue.
1. Visibly worn pulleys
One of the first symptoms of an issue with an idler pulley is visible wear on the pulley. Over time, as the pulley spins against the belt, both components will eventually start to wear. This may lead to visible scoring marks on the surface of the pulley, as a result of contact with the belt. Eventually the pulley and belt will wear to the point of reducing tension, which may lead to the belt slipping.
2. Belt squealing
Another common symptom of a possible issue with the idler pulley is squealing from the engine belts. If the surface of the idler pulley wears, or the pulley seizes or binds it may cause the engine belt to squeal as a result of it rubbing against the surface of the pulley. In some instances a bad pulley may bind or slip and cause the belt to squeal when the engine is first started. The issue will eventually worsen as the pulley continues to wear.
3. Damaged bearing or pulley
Another symptom, more visible, symptom of an issue with the idler pulley is a damaged bearing or pulley. In more serious cases, the bearing, or pulley itself may wear out to the point of breaking or cracking, and coming apart, or seizing. This can upset the rotation of the belt and lead to all sorts of problems. A broken or seized pulley can quickly lead to a torn belt, or in less serious cases, the belt falling off of the engine. An engine without a belt can quickly run into issues such as overheating and stalling, as it’s the drive belt that allows the engine accessories to function.
Idler pulleys are a common component found on the majority of road going vehicles that will eventually require replacement, especially in high mileage vehicles. Any of the engine pulleys are very important to the overall operation of the engine, as it’s the serpentine belt and pulleys that allow the engine to properly function after it has been started. If you suspect that your idler pulley may be having an issue, have the vehicle inspected by a professional technician, such as one from YourMechanic, to determine if the pulley should be replaced.
This article originally appeared on YourMechanic.com as Symptoms of a Bad or Failing Idler Pulley and was authored by Eduardo Ruelas.