“Know your car” Series #7


Mazda MX-5 Idle Dip ... Solved


The Problem

 I have noticed idle dip on my wife’s NA 1.6 litre when it comes to a stop, with the engine running ... like you do at intersections and traffic lights. After taking your foot off the accelerator pedal the revs will drop, perhaps as low as 400 rpm, and then go up to around 1,000 rpm (or more). This cycle may occur just once or continue for 2, 3, 4 or more times before the engine achieves a steady state around 900 rpm.


I have noticed that idle dip can also occur when at rest and you switch on (or off) the heater fan, air conditioner or even the lights (wherever the electrical power load changes). There may be many causes but here is a solution that worked for me ...

On the MX-5, the car's computer (ECU) regulates the idle speed. However, when you come to a stop it takes a few seconds for the ECU to take over and regulate the idle speed. In the intervening period the mechanical idle setting is in control. Let's call that the 'ambient' idle setting for purposes of explanation.


If the ambient idle setting is lower than the ECU-controlled setting (850 to 900 rpm) the car will commence idling at ambient (low) level and, within a second or two, take up the proper ECU setting. The cycling of the idle dip is as if the car senses the increase in rpm and is tricked into thinking that the car is not at rest any more and switches off the ECU idle. The revs then drop down again to the ambient level, the ECU kicks in again, the revs increases, and so on. The car finally settles and idles as if nothing is the matter.


The Solution


The solution is to reset the ambient idle setting


This is easily done … all you need is a piece of wire (like a straightened glider clip) and a screwdriver.


When facing the engine with the bonnet up you will see a little black box on the top of the passenger side wheel arch, with the words "DIAGNOSIS" embossed on it. Open it up by pushing on the top of the side clip. There will be a number of cells inside, some with terminal points. The lid of the box contains the key to these cells. When you insert one end of your wire into the TEN cell and the other in the GND cell, it causes the ECU to be bypassed and the ambient idle setting will be used.


Follow the black air intake tube which runs in front of the engine block to where it meets the intake plenum on the driver's side of the engine - that’s the throttle body. On the top left side of the throttle body you will see a small raised cylindrical section, sometimes with a black plastic covering, which contains a large Phillips head-topped screw. This is the idle adjustment screw. It turns anti-clockwise to increase the ambient idle speed.


So now here is the procedure.
1) Make sure that the engine is warm, but not hot.
2) Turn off all electrical items (fan, radio and lights)
3) Start the car and after it settles into a regular idle notice where the needle is on the tachometer. It should be just over the graduation mark below the "1". (around 850/900 rpm)
4) Bridge the TEN and GND cells
5) You should notice the idle speed change after a few seconds.
6) Have a look at the tachometer. If the ambient idle speed moved then you have discovered the problem.
7) Slowly turn the idle screw until the revs come up to just over the graduation mark on the tachometer.
8) Do this slowly, as there is a slight lag in screwing and response.
9) If the radiator fans come on during the process then stop adjusting and wait for it to go off.
10) When you have it idling where you think it should be remove the bridging wire. In a few seconds the ECU idle setting will kick in. If the engine remains at the same rpm then you have matched the ambient idle to the ECU setting.
11) If not, then replace the bridge and adjust the idle screw,
12) Repeat steps 10 and 11 until the idle speeds are matched.


Safe journey


Rob (Techno) Spargo

Mazda MX-5 Club of Victoria