Manifold temperature sensors measure the temperature of the air inside the intake manifold. Other sensors measure the temperature of the air entering the engine or the temperature of the air in the surrounding atmosphere.
Manifold temperature sensors provide valuable information for several reasons. One reason is the Powertrain Control Modules (PCM) ability to calculate the amount of EGR flow (exhaust gas recirculation) into the engine by measuring the amount of temperature rise inside the intake manifold during EGR system operation. Another benefit is the PCM being able to accurately modify the amount of fuel being injected on turbocharged engines.
A failed manifold temperature sensor can cause several engine driveability issues.
Rough idle, lack of power, and high NOx production
A manifold temperature sensor that sends out a high temperature reading tells the PCM that the incoming air is very thin. The PCM in response reduces the amount of fuel to the engine so that the proper air / fuel mixture is maintained. The PCM unknowingly is creating a lean condition which can cause a rough idle. At freeway speeds, it can also cause a lack of power. Lean running engines also create high levels of NOx (oxides of nitrogen) which is a chemical component of smog.
Rough running engine, lack of power, and emissions test failure
A manifold temperature sensor that sends out a low temperature reading tells the PCM that the incoming air is very dense. The PCM in response increases the amount of fuel to the engine so that the proper air / fuel mixture is maintained. The PCM unknowingly is creating a rich condition which can cause your engine to run rough. Rich running engines also create high levels of carbon monoxide (CO) and hydrocarbons (HC) which are a chemical components of smog. These increased emissions levels will result in a failed tailpipe emissions test for your vehicle.
Another possible cause of a rough running engine or lack of power at freeway speeds created by a low temperature input is the perceived lack of EGR flow. If the PCM does not see a temperature increase in the intake manifold air during EGR system operation, the EGR valve may be opened more, allowing more EGR gas flow into the engine. This increase in EGR gas flow displaces more oxygen in the combustion chamber. This results in a highly diluted air / fuel mixture which makes less power and can create a rough running condition.
The trained technicians at YourMechanic can diagnose and, if necessary, replace your manifold temperature sensor.
This article originally appeared on YourMechanic.com as Symptoms of a Bad or Failing Manifold Temperature Sensor and was authored by David Smith.