I have an idle RPM drop and rough idle issue with my Mazda 3 1,6 2005.

When it's cold enough outside, below 2-5 celsius, the engine has idle RPM drops after it's been driven so that it has reached its operative temperature.

Let's say I drive 15-20 kms and stop. After 15-30 seconds the idle RPM drops shortly to approx 500 RPM, the engine almost feels like it will stall but then it bounces back. Then half or full minute or this kind of longer period goes with rough idle and then RPMs drop shortly again. But this time the drop is not so rough, it's smoother. Then it gets better.

This happens with or without my foot on the brake pedal. This happens without the A/C compressor clutch or the big fan in the front engaging.

Usually the issue goes away if I drive a long distance (trip taking like 1,5-2 hours). But it comes back the next day after the engine has cooled down again and after it has warmed up to the operative temperature.

After that 15-20 km of driving and letting the engine idle for couple of minutes I can reproduce the issue just by reving the engine to let's say 3000 RPMs for like half a minute. Then I let it idle for 15-30 seconds and whoosh the idle RPM drop occurs once again.

When the RPMs drop the engine sounds kind of like someone is choking. I hear like gasping, no hiss or anything like that.

Coolant, oil, brake fluid, all are within limits. Fuel cap is tightly secured, tank is at least half full. I think engine starts fine, cold or warm.

The battery was changed little over an year ago at the Mazda dealership. Don't know if they did throttle body calibration but the engine didn't have this issue immediately after battery swap. I also used a bottle of STP ultra 5 in 1 an year ago but the issue didn't begin immediately after that either.

I did have CEL light twice approx an year ago on two wet days. First it went away after couple of hours. The next time it didn't and gave me the code P0171 - System Too Lean. I just cleared that and hasn't seen CEL after that. I did the check the fuel trim percentage and it fluctuates between 0 and 10%: it gradually raises to 10% and then drops to 0 again.

To my knowledge nothing has been done for these parts in the car in the past:

• Throttle body
• Air sensors
• Intake hoses, manifold and gasket
• Fuel filter, pump and injectors
• Engine mounts

Because idle settles after couple of minutes but idle RPM drops reappear once again after load, could it be PCV or EGR valve or some other part that is engaged soon after engine goes from load to idle acting up? Wouldn't vacuum leak or dirty enough throttle body cause constant issues while idling?

Could some fuel system additive help? Bottle of that and then some intensive highway driving? I could also disconnect the battery and reset the throttle body calibration. Can it be done without battery disconnect?

I'm not comfortable cleaning up throttle body or something like that myself. I could take it to a mechanic but the mechanic should come see it immediately after I drove the car to the dealership and it should be cold enough day because the issue won't manifest after engine cool down or in warm dealership premises.

 

Let the car idle 20 mins without load then 15 mins with A/C on.

Checked some OBD data as well.

Gas pedal fully pressed: throttle body 75,7%
Shouldn't it be 100?

STFT raises to 25% at idle and stays there.
But vacuum is approx -18 in Hg with A/C on and approx -22 - -23 with A/C off.

This time no rough RPM drops but temp was 4 Celsius.

 

Now STFT was 0-10%, LTFT within same range, with approx average 5%

O2 sensor 1 fluctuates between 0.1 and 0.9 V but some periods remain 0.1 or between 0.6 and 0.9 V when AC kicks on.

O2 sensor 2 is sometimes steady 0.45 V but may fall down and stay there for a while to 0.1 or raise up to 0.9 V and stay there for a while.

Vacuum was approx 25 in/Hg but when AC kicks on it may shortly drop to 19 but then raise to 23 before raising back to 25 when AC compressor turns off.

A puff sound can be heard from intake when AC kicks on and vacuum drops shortly.