2021 BMW M4 Drive Belt Idler Pulley Replacement Cost

While the engine is running, the crankshaft pulley is constantly spinning. This provides drive to other mechanisms by means of a belt. The belt will be tightly wrapped around the crankshaft pulley and the accessory pulley, so the belt is unable to slip. Manufacturers use idler pulleys to route the belt around various engine accessories.

If the drive belt idler pulley fails, there will be a very audible chirping noise from the front of the engine. This is because the idler pulley bearing is no longer smooth, and rotating causes metal to grind at high speeds. If it is not changed immediately, there is a chance it will become dislodged, or allow the drive belt to slip off. If the belt falls off, the accessory that it drives will no longer function. For instance, if the alternator belt idler pulley fails, the alternator may stop functioning. Also if the pulley begins to wobble, allowing slack in the belt, it will make an extremely loud squeaking or screeching noise, especially on cold starts. This is the belt slipping on the crankshaft pulley, and will require belt replacement once the idler pulley is replaced.

Depending on which belt breaks, driving the car may be hazardous or impossible. The power steering belt and air conditioning compressor belt will not disable the vehicle, but the vehicle may be unpleasant to drive. If the engine cooling fan belt breaks the engine may overheat, causing further damage. If the alternator belt is broken or missing the vehicle will need to be towed, as battery power will only last a few short minutes.

Idler pulleys tend to last quite some time. However, driving style and operating environment may affect part longevity. The majority of vehicles will surpass 100,000 miles before idler pulley replacement is needed.

If the pulley is making sufficient noise to be suspected, it will be physically inspected to verify the diagnosis. The drive belt will be relieved of tension, and the pulley turned by hand to feel for vibration, resistance, grinding, or looseness. If any of those symptoms are noted, it will be replaced.

When replacing a drive belt idler pulley, the remaining parts of the pulley system will be inspected to ensure they are free from defects that will lead to damage of the new belt or idler pulley. If there are no defects, the belt tension will be released, and tightened once the new idler pulley and belt are in place. Proper belt tension is mandatory to prevent belt failure.

When one drive belt component fails, it is typical to notice the others in need of replacement as well. The belts should all be removed and inspected, as cracking or fraying may be hard to detect while on the vehicle. While the belts are removed, the idler pulleys and belt tensioners should be inspected and replaced if faulty. If the drive belt is broken or missing, all pulleys and components for that belt should be inspected for proper alignment and functionality.

There are many manufacturers of drive belt idler pulleys. They tend to range in price, but they are not typically expensive. Higher quality belts and pulleys may last longer, run more quietly, and resist breaking. At a minimum, all replacement parts should be OEM grade or better.

The novice DIYer would be perfectly comfortable replacing an idler pulley, but a torque wrench with small enough increments must be used to avoid damaging the engine accessory mounting bracket, or timing belt cover, whichever the particular vehicle uses. If a torque wrench is unavailable, do not attempt this repair, as the damage caused by over-tightening, or under-tightening can be very costly.