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350matt, Engineer
Category: Mazda
Satisfied Customers: 21622
Experience:  Mechanical Engineer BEng Hons22 years experience
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Howdy--have a challenge 05 mazda 3, 2.0 auto rough idle only

Customer Question

howdy--have a challenge for you
05 mazda 3, 2.0 auto
rough idle only when at a stop, in gear, with foot applied to brake, around 500 rpm
when i pull on emerg brake, let off foot brake, rpm goes to about 700 and the rough idle subsides.
mazda forums are full of this phenomenon for mazda 3
I think it just comes down to a resonance frequency that this car does not like as I have tried a number of things which dont make much diff:
14 volts across terminals with everything on, including in drive with brake depressed
replaced motor mounts
cleaned throttle body (2-3x)
replace evap solenoid
other stuff
what I would like to do is disengage the idle control component which drops the rmps when the foot is on the brake-just leave the rpm idle at 700, dont drop it to 500. obviously it is just a fuel efficiency thing for those in stop and go traffic--I would be happy with this as otherwise the car is fine. but the rough idle thing at 500 is very annoying.
how do i disengage just that?
people note that when they pull a fuse for horn and brake lights the idle does not dip but obviously one needs horn and brake
Submitted: 1 year ago.
Category: Mazda
Expert:  350matt replied 1 year ago.


If you haven't already then I'd check for an airleak after the airflow meter, any air dragged in here isn't 'seen' by the ECU and so not compensated for and can lean the engine out and can also allow the engine to rev up when not desired causing rough running.

As its a mechanical fault it tends not to turn on the fault light and you can sometimes hear a 'hissing' noise with the engine running.

Check the hose clips for tightness and inspect the trunking for any cracks or splits and also all the vacuum system, the small bore pipes and fittings for cracks and missing parts.

The best way to locate a leak is to have the engine running and warm and then spray lighter gas /propane around each joint in turn. If the engine rev's up you've found your leak.

Now you might think that spraying lighter gas around a hot engine isn’t wise, however the flash /ignition point of gas is about 400°C so you need a naked flame or spark to set it off and I’ve used this method for many years without incident.

Work your way through each possible joint one at a time and you should find it. I use a slightly flattened piece of brake pipe and some rubber hose from the can of lighter gas to provide a spraying 'wand' and allow a direct blast of gas into each area, especially those difficult to reach with large implements.

Customer: replied 1 year ago.
ok- so that is the mass airflow sensor-maf?also, are there any vacuum systems on the Left side of engine--viewing from front of car?
Expert:  350matt replied 1 year ago.


yes the MAF = mass airflow sensor thats part of the air intake

and I think there may be an crank case breather on the LHS that runs back to the inlet

Customer: replied 1 year ago.
I went around all the joints with electric motor spray which is trichloroethane and did not get an a change in rpm . I checked it with it in gear with the weight on the break. Idle was a bit rough this is the only place where it is a bit rough and I can't detect a vacuum leak. Battery is pretty new and seems to crank well can weak battery affect Idle under this condition?
Expert:  350matt replied 1 year ago.


if the battery isn't charging then yes you will get all sorts of errors so if you have any doubt over it then you can do a quick check just with a voltmeter to measure battery voltage with the engine off and then measure it again with the engine running

the 2nd reading should be higher than the 1st and about 13-14V

Customer: replied 1 year ago.
No vacuum leaks found and the volts are about 13.9 with engine running and all accessories on. From my understanding there is an electrical component which drops the idle from about 700 to about 500 when the foot is on the brake. Can this be disengaged?
Customer: replied 1 year ago.
The only time this is an issues when I pull up to a stop light and then it's just annoying I would rather have the adle just stay at 700 the engine is butter smooth at 700 .thanks for your help
Expert:  350matt replied 1 year ago.


the brake pedal switch will both drop the idle speed / put the ECU into an alternative mode

so if you take a look in the drivers footwell at the top of the pedal there should be 2 switches - 1 is for the brake light the other for the control / ECU so try disconnecting them 1 at a time to see what changes

Customer: replied 1 year ago.
There's a blue connector that I removed but looks like it needs to be plugged in and in place in order to release the gearshift so obviously the shift lock. Are there different electrical connectors? closer to the firewall?