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ECU relearns the idle. Anytime battery is disconnected everything is set back to factory. Problem is, car is no longer factory new. Needs to relearn what's up, fuel pressure, vacuum, all the things based on current conditions.
Okay that makes sense, but how does idling for a long time help the ECU learn, as opposed to just idling at stop lights? And what effect does blasting the AC have on the ECU relearning?
 

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I don't know man, just do it. I'm sure it just speeds up the process and the AC on full probably just introduces another environment to the system.
 

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I don't know man, just do it. I'm sure it just speeds up the process and the AC on full probably just introduces another environment to the system.
Kk, I'll try. But can I still do this procedure and relearn the ECU even though it's been like 2 weeks since I last disconnected my battery?
 

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So I did it. Turned on car, shifted to D, stepped on brakes, idled for 22 mins, then blasted AC, idled for 15 mins, then drove.

During the idle time, never had the rough idle. As soon as I started driving, rough idle popped up again.

dafuq
 

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I know this is an old thread, but I'm working on this similar problem right now on my 2004 mazda 3.

I've done everything you've done. I also replace the PCV a month ago. Today I took out and cleaned the EGR valve. Still have the rough idle though.

I'm thinking of taking out the upstream O2 sensor (easiest to get at) and soak it in fuel for a few hours.

After that, I'm out of ideas.
 

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2012 Mazda 3 GX MT5
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Starving the engine will cause rough idle ... When engine is rev high it has the ability suck more .. more air and flow
(Simple remedies) Air flow & fuel flow...
  • Dirty Fuel filter and /or lines (this can throw a code if flow is very restrictive)
  • Dirty fuel injectors
  • Dirty air filter
  • MAF sensor ( this will throw a code )
  • Throttle body and the solenoids (
 

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My problem seems to have went away with some fresh oil. I added seafoam to the oil and a bit to the gas tank. However, I'm thinking it was a stuck open EGR, and the problem went away after the EGR cleaning and driving a bit to work everything out.
 

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My problem seems to have went away with some fresh oil. I added seafoam to the oil and a bit to the gas tank. However, I'm thinking it was a stuck open EGR, and the problem went away after the EGR cleaning and driving a bit to work everything out.
Good !
On your next oil change think about doing a engine flush Engine Flush Plus
and drop some of this in the Pro-Line Gasoline System Cleaner.
 
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This is an old thread, but I wanted to chime in if it could help any body. I had a rough idle twice in the 15+ years I've had my 05' mazda3 with 140k miles.. the first one was due to the gas cap(the springloaded vent failed on the cap)-which was replaced under warranty.

The second time I had a rough idle some codes showed(can't recall at the moment), I noticed it was happening when the engine would warm up, turned out it was the thermostat being stuck so I replaced it. In addition, I put in some fuel injection cleaner in the gas tank and ran it hard/took it out on the highway for 15+ minutes, rough idling went away.

So for those who have tried what others have mentioned, you might want to consider your gas cap, thermostat(especially if it's happening once the engine warms up and your temps look wonky), or even some fuel injection cleaner(maybe with a fill of premium fuel too, I used Berryman B12 btw, great stuff) and running it on the highway.

As of now my driver side motor mount seems to be busted so I get engine vibration, but my idle is normal. I'll replace the mount soon, and eventually I'll probably do what others have done just to give the car a nice "tune up"(Purge selenoid valve, Throttle body cleaning, etc.).
 

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did you check your fuel pump?

probably it can be that is going bad? who knows..
i did, but it doesn’t work. I’m thinking of the timing belt on my car. And i need to try what he mentioned, wherein most people mention about it; to let the car turned on for 20-30mins and fully turn on the AC after for another 15-20mins.
 

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i did, but it doesn’t work. I’m thinking of the timing belt on my car. And i need to try what he mentioned, wherein most people mention about it; to let the car turned on for 20-30mins and fully turn on the AC after for another 15-20mins.
And i also changed the fuel pump as well. Nothing changed
 

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I have dealt with rough idle a few times during ownership. At first I used to wipe down the throttlebody with a rag soaked in carb cleaner. Worked ok for awhile. I have since started using a can of Seafoam in the gas. Two tanks of Seafoam mixed gas usually does the trick for at least 6 months.
 

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So i have a unrelated question about block difference so my buddy blew his speed up (shocker i know) we're gonna pull a NA 2.3 and use the block as a donor. Will everything oretty much be the same all his stuff is going onto that block the only thing we are using is the bare block.
 

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I have dealt with rough idle a few times during ownership. At first I used to wipe down the throttlebody with a rag soaked in carb cleaner. Worked ok for awhile. I have since started using a can of Seafoam in the gas. Two tanks of Seafoam mixed gas usually does the trick for at least 6 months.
Purchased a 2007 Mazda 3 early in 2020 and had that rough idle when in drive and stopped (foot on brake). It would stop if you put it in park. My mechanic could not solve the issue after replacing MAF sensor, purge valve assembly and cleaning throttle body. I joked with him after getting an oil change whatever part was causing this would suddenly fail and that would be the only way to fix the idle issue. Well....the NEXT day it did just that! The car was barely drivable and had a hard time getting it over to him. My OBD2 scanner returned a P0401, but strangely the mechanics reader picked up no codes? After he tried the MAF sensor again, and another throttle body clean, he finally replaced the EGR valve. Car now drivable and rough idle gone!
 

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Purchased a 2007 Mazda 3 early in 2020 and had that rough idle when in drive and stopped (foot on brake). It would stop if you put it in park. My mechanic could not solve the issue after replacing MAF sensor, purge valve assembly and cleaning throttle body. I joked with him after getting an oil change whatever part was causing this would suddenly fail and that would be the only way to fix the idle issue. Well....the NEXT day it did just that! The car was barely drivable and had a hard time getting it over to him. My OBD2 scanner returned a P0401, but strangely the mechanics reader picked up no codes? After he tried the MAF sensor again, and another throttle body clean, he finally replaced the EGR valve. Car now drivable and rough idle gone!

when you say rough idle, what was that like for you? I have one that feels like it gets hiccups. Manual Transmission.
 

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when you say rough idle, what was that like for you? I have one that feels like it gets hiccups. Manual Transmission.
The odometer needle would jump back and forth between 600 - 400 rpm and when it hit 400 the engine would vibrate and at times feel like it was close to stalling, but never did. Eventually it would jump back to 500-600 and the vibrating would stop. It would do this dance at almost every stop. I tried to get used to it and was told it was because the car is old with lots of mileage (240km). I did not share this opinion and common sense told me there had to be some reason for this, which turned out to be true. Now it settles on 500 and does not move. There is a constant barely detectable vibration which I am comfortable with as it is not “jumping around”. This I would agree is due to it being and older car with mileage!
 

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The odometer needle would jump back and forth between 600 - 400 rpm and when it hit 400 the engine would vibrate and at times feel like it was close to stalling, but never did. Eventually it would jump back to 500-600 and the vibrating would stop. It would do this dance at almost every stop. I tried to get used to it and was told it was because the car is old with lots of mileage (240km). I did not share this opinion and common sense told me there had to be some reason for this, which turned out to be true. Now it settles on 500 and does not move. There is a constant barely detectable vibration which I am comfortable with as it is not “jumping around”. This I would agree is due to it being and older car with mileage!
ahh man mine is hovering around 700rpm but i feel small jumps. It's the fact that it's not consistent that bothers me when i'm just sitting in my car haha. A lot of people recommend changing PCV which is really cheap and easy to do. I did mine already but it didn't fix my issue. will check for a vacuum leak soon.
 

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ahh man mine is hovering around 700rpm but i feel small jumps. It's the fact that it's not consistent that bothers me when i'm just sitting in my car haha. A lot of people recommend changing PCV which is really cheap and easy to do. I did mine already but it didn't fix my issue. will check for a vacuum leak soon.
I forgot to mention my car is an automatic. It does pop up to 700rpm when put in park (which I think it how it is supposed to run). My MAF and purge valve c/w lines were replaced. The mechanic also completed a “smoke test” with no results.
 
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