Low idle speeds can be caused by a number of culprits, and the troubleshooting can
be extensive. Typical culprits can be as varied as: intake vacuum
leaks, too little fuel (lean), too much fuel (rich), bad spark (ignition), etc.
Best suggestion is to get a basic tune-up done first to eliminate most of the
FC3S Pro - FAQ, Basic Tune-Up
Before trying anything, make sure the engine is in good shape. A compression
check is highly recommended to ensure you're not messing with a hurt motor.
Low compression will cause a bad or low idle.
FC3S Pro - FAQ, Compression Testing
In most cases, it is intake vacuum leaks that is the most likely source of the
problems. For a more detailed description of intake vacuum leak
troubleshooting, please click on this link:
FC3S Pro - Problems #1, Engine, Intake Vacuum Leaks
For possible fuel problems, check the fuel system to make sure everything is
properly working. Fuel injector cleaning is highly recommended if
fuel injector condition is unknown. Replacing fuel filters is also
recommended if condition is unknown. Zenki FC turbos have an idle mixture
adjust screw that should adjust fuel delivery at idle speeds. This
idle mixture adjust screw is located on the passenger strut tower in the
engine bay. It is labelled "R" (for rich) and "L" for lean.
Adjust the screw until idle speed is highest.
Moving to the ignition system, bad spark plugs and spark plug wires can cause
bad ignition spark, which will affect idle quality. Bad or dead coils
(packs) will affect idle quality. Ignition timing off of spec will affect
idle - usually too retarded. Ensure spark plug wire position
is correct! The L1 and L2 spark plug wires are the bottom
positions; swapping leadings front to rear does not change anything - both fire
at the same time. The T1 and T2 spark plug wires need to
be in the correct positions.
Proper procedure for idle adjustment:
- Allow engine to warm up to normal operating temperature
- Adjust ignition timing to Mazda spec at 750RPM - prop throttle if necessary
- Adjust TPS to Mazda spec -
FC3S Pro - TPS and throttle stop adjustment
- For Zenki FC3S turbo, adjust idle mixture adjust screw
- Adjust idle adjust screw on BAC valve, if available
- If necessary, adjust throttle stop screw on throttle body
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