An idler pulley is basically a pulley housing with an integral sealed bearing pressed in. The purpose of the idler pulley is to facilitate the serpentine belt routing – either to route it around an engine obstruction or to wrap the belt around an accessory pulley more fully in a way that improves its bearing surface and grip.
Some engines have more than one and some with fewer driven accessories will not need an idler pulley. If present, the idler pulley is usually attached to the front of the engine on the timing cover or a bracket.
Replacement is normally simple; involving moving the serpentine belt out of the way and undoing one or two bolts. The carmakers who set labor times sometimes figure an idler pulley replacement as additional time on top of serpentine belt replacement.
Also, sometimes, they figure it as a shorter operation in which the belt doesn’t need to be fully removed. In any case, the difference is small.
Cost of Coolant Temperature Sensor Replacement
To illustrate, here are estimates of the coolant temperature sensor price on some common vehicles using a labor rate of $100 per hour:
- 2010 Ford Fusion 3.0-liter engine – it usually takes half an hour of labor to replace the sensors. A GDP replacement sensor is about $19 which makes a total job cost of about $69.
- 2013 Journey 3.6-liter engine – the labor time for sensor replacement is 0.6 of an hour. An OE Mopar sensor can be purchased at around $39 and a Standard replacement sensor is about $18. Job completion would cost around $99 using factory parts and about $78 using an aftermarket sensor.
- 2006 Volkswagen Jetta 2.5-liter engine – the estimated labor time for sensor replacement for this vehicle is 0.6 of an hour. A factory replacement costs around $45 while an NTK replacement sensor is about $22. You would end up paying about $105 using an OE sensor and around $82 with aftermarket parts.
- 2010 Toyota Camry with a 2.5-liter engine – the labor time for sensor replacement is 1.6 hours. An OE Denso sensor will set you back at around $76 and an ACDelco replacement sensor is about $26. The total ECT sensor cost will be about $236 using factory parts or around $186 using an aftermarket sensor.
One cost saving thing to keep in mind is that the main wear item on the idler pulley is a roller bearing, and car manufacturers typically don’t make roller bearings. Which is to say – there’s not necessarily a good reason to pay extra for a factory replacement part, as it may have the same bearing in it as a name-brand aftermarket part.
Other Cost Considerations
When the sensor is removed, the coolant is lost. Sometimes, the coolant can be drained into a clean container and re-used, but this isn’t always practical and the draining process itself can introduce contaminants. It’s normal for a charge to be added for replacing coolant as necessary.
Depending on how the sensor is placed, the quantity might be a small amount or it might be as much as a gallon. Coolant costs vary from about $7 a gallon to as much as $50 a gallon for some factory brands.
Before a sensor is replaced, it ordinarily must be diagnosed and most shops have a flat one-hour diagnostic charge for that. Sometimes, a sensor can trigger a trouble code and turn on the engine light.
In the event of a failed or inaccurate coolant temperature sensor, the generic OBDII code P0166 would be triggered. This still has a diagnostic procedure to verify the failure and it is generally most economical to go through the diagnostic procedure to rule out other possible causes and verify the failure before replacing parts.
What is Coolant Temperature Sensor For?
The coolant temperature sensor is usually located near the top of the engine; either in a cylinder head coolant passage or at the thermostat housing. Its role is to inform the engine management system of the engine temperature which uses that information to adjust fuel mixtures and timing, as well as to know when to turn on the cooling fans.
The temperature information from the sensor is also displayed on a dash gauge or sometimes, just triggers a warning light in the event of an overheating condition.
There are a couple of other coolant sensors that this shouldn’t be confused with. Many vehicles have a coolant level sensor (usually on the coolant reservoir) which can trigger a dash light or warning if the coolant level gets below a certain point.
Some vehicles also use a coolant switch – most often in the radiator housing – to directly control the cooling fans.
What Happens When the Coolant Temperature Sensor Fails?
Most engines have a thermostat that is designed to maintain the engine temperature at about 200 degrees once the vehicle is warmed up. If there is a gauge on the dash, typically, that would be right at the halfway mark.
If the thermostat is operating normally but the coolant temperature sensor isn’t, then, the dash gauge would be off in one direction or another.
Once the engine has reached operating temperature, the cooling fans will cycle on and off to maintain that temperature. They rely on information from the coolant temperature sensor to do that. If the fans don’t come on as they should or if they stay on excessively, that can indicate a faulty sensor.
One of the more important jobs of the coolant temperature sensor is to inform the engine management system of the engine temperature. When the engine is cold, the fuel mixture must be richer to allow smooth firing and power output.
When the engine is hot, the mixture must be made leaner to prevent detonation and plug fouling. These require accurate coolant temperature data. Rough running when cold is one possible symptom. Poor performance and loss of power when warm is another, as is generally poor fuel mileage.