Car idle reduction

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Car idling at normal and low RPM


A car's idle is typically set at a level which will ensure it can never stall. This is desirable for the general public but better efficiency can be gained at the expence of a small risk of stalling or poorer drivability.

A reduction of idle will reduce the amount of fuel used when the car is idling as the engine rotates at fewer RPMs and therefore less friction and less fuel. The most gains will be observed for driver stopping frequently at traffic lights.

Note: This mod may result in increased engine wear as less oil is pumped at lower RPMs for cars with oil pumps powered directly from the engine

Forum thread for discussion about this wiki page

Instructions for mod

See your cars workshop manual for directions to adjust your particular cars idle.

Typically the steps are as follows

- The car is warmed up to operating temperature by idling or reving the engine for some time.

- Some electronic controlled engines require a diagnostic connection to be bridged or connected to 0v. The connection informs the ECU that the user is adjusting the idle so that the ECU doesn't compensate for lower idle.

- A set screw is rotated to adjust the idle typically the set screw is found on the throttle control module (connected to the air intake).

- For electronic controlled engines the diagnostic connection bridge is removed

For some cars the ECU will always attempt to keep the idle above a safe limit. This feature can often be overridden by locating the diagnostic connection, bridging it or connecting to 0v when a low idle is desired. This can be achieved by a switch placed in the cabin. (Note: Some cars can only have the idle adjusted by having the ECU reprogrammed.)

User experiences

Please enter your user name and any relevant data in the table

User data
User Name Car Make, Model, Year Cost of Mod Time to Perform Mod MPG Before Mod MPG After Mod MPG improvement guess Instruction Link
Saand Mazda, 626, 1991 $5 2 Hour (Due to time for installing switch to tell ECU to stop compenating for low idle) 19.9 ml per minute 15.4 ml per minute 22.7% reduction at idle (based on back to back tests using home made scan gauge)
0 % reduction while accelerating or coasting
Saand results

Problems / Consequences of mod

Reduction of engine idle may result in increased engine wear as less oil is pumped at lower RPMs for cars with oil pumps powered directly from the engine.

The car can stall at a low idle, which is a higher risk as the idle is reduced lower from the factory set idle.


Forum thread links

Forum thread for discussion about this wiki page

External links