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Discussion Starter #1
Before you get mad and think "another freaking post about the rough idle??!! Don't you search the forums first??!!" let me explain...

First, i have read probably every forum on here dating back 5+ years about the idle issue. And i've tried just about every suggestion i've come across, spare the major PITA ones. Second, i haven't crossed a description of the problem that quite matches what i've got yet, so i'm hoping someone sees something here that i'm not seeing.

Background -
just bought the car, 2008 Mazda 3i sport 2.0l with 119k miles, a few weeks ago and it's been doing this since i got it. But i've never owned one of these when it WASN'T doing it, so i'm not exactly sure what it's supposed to sound/feel like when it's running right.

Description -
When i start the car cold, it revs up to about 1500-2k rpms and slowly comes down to what i'd consider a normal idle of ~800-900 rpm's. After driving a bit and the engine warming up completely, whenever i stop the idle drops down to ~400-500 rpms and feels/sounds rough, no AC on. But it has never died on me, never thrown any lights or codes, and runs like a champ when you're actually driving and idle improves when in park or neutral.

What i've already tried -
New spark plugs (OEM replacements), new air filter, throttle body cleaning as per TSB instructions, purge solenoid valve clean with PB blaster, clean MAF sensor with CRC MAF sensor cleaner, ran bottle of injector cleaner through it, and did the disconnect the negative battery terminal let it idle and relearn process, dealer said there's no ECU reflash available for it.

Thoughts? I know it's been suggested to replace the PCV valve, but it's such a PITA to get to that i don't want to do it unless there's a good chance that's what it is. I'm about out of ideas though. Other than this frustrating idle issue, and needing to put new brake rotors on the front, i loveeee the car so far. Any help/ideas are appreciated!
 

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did you check your fuel pump?

probably it can be that is going bad? who knows..
 

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I've got the EXACT same problem.
Put it in Neutral or Park, idles at 700 RPM.
Put it in Drive and brake, idles at 300 and it feels like the car wants to stall.
When the car is cold, it's fine.
When it warms up, it acts up.
I think I'm going to try and sell the car during winter time.

Could it be the fault of the transmission? I don't know much about cars, but if the idling is fine when in N or P, but is crap when the engine is connected to the transmission, could it be somewhere there? I tried doing a transmission fluid flush but it didn't help.

I'll bump this threat with you until we both solved it.

Actually, I only did new air filter and new spark plugs, all OEM.

Can you tell me how abouts you went with the other stuff? You just did it yourself? Where did you find the resources to do it?
 

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I've got the EXACT same problem.
Put it in Neutral or Park, idles at 700 RPM.
Put it in Drive and brake, idles at 300 and it feels like the car wants to stall.
Since the brakes are vacuum-assisted, it makes me think that there is a brake booster vacuum leak that is letting unmetered air into the intake which is affecting the idle. Sometimes you can hear the leak by listening carefully as you press down on the brake pedal or hear it squeak by pressing hard and fast on the pedal. You don't need to be moving when you try this! Other times, you can find a leak of brake fluid below the master cylinder at the firewall (which can cause the booster to leak, but is ultimately also a worn master cylinder where the fluid is leaking past the seal at the pushrod).
 

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Since the brakes are vacuum-assisted, it makes me think that there is a brake booster vacuum leak that is letting unmetered air into the intake which is affecting the idle. Sometimes you can hear the leak by listening carefully as you press down on the brake pedal or hear it squeak by pressing hard and fast on the pedal. You don't need to be moving when you try this! Other times, you can find a leak of brake fluid below the master cylinder at the firewall (which can cause the booster to leak, but is ultimately also a worn master cylinder where the fluid is leaking past the seal at the pushrod).
so if this is true, my rpm should drop when I press on the brakes, regardless of whether I am in P D or N? And I don't know exactly what normal brakes sound like compared with a leaky one so I can't compare.
 

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Discussion Starter #8
I've got the EXACT same problem.
Put it in Neutral or Park, idles at 700 RPM.
Put it in Drive and brake, idles at 300 and it feels like the car wants to stall.
When the car is cold, it's fine.
When it warms up, it acts up.
I think I'm going to try and sell the car during winter time.

Could it be the fault of the transmission? I don't know much about cars, but if the idling is fine when in N or P, but is crap when the engine is connected to the transmission, could it be somewhere there? I tried doing a transmission fluid flush but it didn't help.

I'll bump this threat with you until we both solved it.

Actually, I only did new air filter and new spark plugs, all OEM.

Can you tell me how abouts you went with the other stuff? You just did it yourself? Where did you find the resources to do it?
It's super frustrating, isn't it?! I'm determined to figure it out. Selling the car isn't an option for me, so i've gotta get it resolved. As far as doing the other stuff goes, yes i just did it all myself. I found explanations/tutorials for it all on these forums. The throttle body clean was easy, there's a TSB that gives you step by step instructions. MAF sensor clean was easy, just remove the wire clip and 2 screws. Getting the Purge solenoid valve off was kind of a PITA, but definitely doable.

Since the brakes are vacuum-assisted, it makes me think that there is a brake booster vacuum leak that is letting unmetered air into the intake which is affecting the idle. Sometimes you can hear the leak by listening carefully as you press down on the brake pedal or hear it squeak by pressing hard and fast on the pedal. You don't need to be moving when you try this! Other times, you can find a leak of brake fluid below the master cylinder at the firewall (which can cause the booster to leak, but is ultimately also a worn master cylinder where the fluid is leaking past the seal at the pushrod).
I've been wondering if there could have been some kind of vacuum leak, and this would absolutely make sense considering it only happens in D and R when the brake pedal is engaged. I'll look into this closer when i get some free time. Would you recommend just starting at the master cylinder and following the vacuum line? I've read spraying carb cleaner is a good way to check for vacuum leaks. Any other methods?
 

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It's super frustrating, isn't it?! I'm determined to figure it out. Selling the car isn't an option for me, so i've gotta get it resolved. As far as doing the other stuff goes, yes i just did it all myself. I found explanations/tutorials for it all on these forums. The throttle body clean was easy, there's a TSB that gives you step by step instructions. MAF sensor clean was easy, just remove the wire clip and 2 screws. Getting the Purge solenoid valve off was kind of a PITA, but definitely doable.



I've been wondering if there could have been some kind of vacuum leak, and this would absolutely make sense considering it only happens in D and R when the brake pedal is engaged. I'll look into this closer when i get some free time. Would you recommend just starting at the master cylinder and following the vacuum line? I've read spraying carb cleaner is a good way to check for vacuum leaks. Any other methods?
Do you have a gas bbq?
You can use propane to check for vacuum leaks. https://www.google.com/search?client=safari&rls=en&q=using+propane+to+check+for+vacuum+leaks&ie=UTF-8&oe=UTF-8
 

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Would you recommend just starting at the master cylinder and following the vacuum line?
The Vac line will be at the booster which is in between the master cylinder and the firewall. That is one source of the leak, but the other is internal to the booster which contains a rubber diaphragm.
 

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Discussion Starter #11
I've got a small propane torch i could use, obviously not lit. You think this is a better option than carb cleaner?

The Vac line will be at the booster which is in between the master cylinder and the firewall. That is one source of the leak, but the other is internal to the booster which contains a rubber diaphragm.
Is this an issue you've had and found that the problem was the booster and/or vac line?
 

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I've got a small propane torch i could use, obviously not lit. You think this is a better option than carb cleaner?



Is this an issue you've had and found that the problem was the booster and/or vac line?
I haven't had this issue specifically with the Mazda 3, but I have had loose booster vac lines on other cars, and certainly the failed booster problem on other cars. A failed diaphragm in a booster is pretty much the only reason they would be replaced.

You can test the booster by removing the booster line and securely plugging the line end (you can leave the booster end disconnected) and see if the problem goes away. I suppose you can test the vac line the same way by capping the fitting on the intake where the line connects.

The brake pedal will become very difficult to press, so don't drive the car this way.
 

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you are not alone with that issue. i was suppose to get this fixed over the summer, but been too busy lol.



For me, this is what i did;
- swap out all the 3 mounts to ROCA (didnt help, bit firmer than stock)
-change the Purge Solenoid Valve (didnt fix, but increased my mpg) :)
-cleaned maf (didnt help)
-cleaned throttle body (didnt help)
-new factory plugs ( didnt fix )

I deff know it is not the fuel pump.

Know with all of that being said;

i have been doing alot of research & the issue could be electrical.
I got accessed to Mazda's official service documents for the 2004-2005 mazda 3
and it had a whole suit of things to test/check for rough idle/ or if car vibrates when in drive.

I notice one thing in the service bulletin that kinda stood out, and it was about the BRAKE SWITCH (don't remember the exact wording but had something to do with testing for correct voltage in the brake switch unit).
This leads me to believe it has something do with the brake switch behind the pedal (or whatever its called).
Now looking back , i remember reading somewhere about another person removing the fuse responsible for the brake lights & horn in his 3 hence idles perfectly smooth.

Here was the brake switch they were referring to in the service manually (here)
 

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you are not alone with that issue. i was suppose to get this fixed over the summer, but been too busy lol.



For me, this is what i did;
- swap out all the 3 mounts to ROCA (didnt help, bit firmer than stock)
-change the Purge Solenoid Valve (didnt fix, but increased my mpg) :)
-cleaned maf (didnt help)
-cleaned throttle body (didnt help)
-new factory plugs ( didnt fix )

I deff know it is not the fuel pump.

Know with all of that being said;

i have been doing alot of research & the issue could be electrical.
I got accessed to Mazda's official service documents for the 2004-2005 mazda 3
and it had a whole suit of things to test/check for rough idle/ or if car vibrates when in drive.

I notice one thing in the service bulletin that kinda stood out, and it was about the BRAKE SWITCH (don't remember the exact wording but had something to do with testing for correct voltage in the brake switch unit).
This leads me to believe it has something do with the brake switch behind the pedal (or whatever its called).
Now looking back , i remember reading somewhere about another person removing the fuse responsible for the brake lights & horn in his 3 hence idles perfectly smooth.

Here was the brake switch they were referring to in the service manually (here)
if it was the brake switch, then it should rough idle whenever you press the brake right? For me, it only rough idles when i brake in D, but not P or N.
 

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Discussion Starter #16
if it was the brake switch, then it should rough idle whenever you press the brake right? For me, it only rough idles when i brake in D, but not P or N.
Same here. My car was actually idling fairly well this weekend. I didn't drive a ton, but when i did it was staying closer to the 600rpm idle range vs. 400ish. Not sure if it was just luck or if it's adjusting to all the stuff i've cleaned. Thinking about doing another idle relearn tonight, but that seemed to make it worse last time so i'm hesitant to do so. Still need to check my vacuum lines.
 

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We have some fairly specific symptoms. No one can pinpoint what the problem is?
I checked my fuel consumption and I got 9.48L / 100 KM, or 24.81 MPG. Those numbers make me depressed. Is it part of our idling problem?

I also noticed that after I refilled it to full, when I idle, the tachometer drops to 300 rpm, then after like 10 seconds it gets bumped back up to about 600 rpm.
 

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For all those with idle problems after many attempts at fixes try this simply, free idle relearn.

Turn the car on and let it idle for 20-25 minutes. Make sure all accessories, lights, etc are off.

Now turn AC on full blast and let the car run for 15-20 minutes.

Go for a chill drive. Hopefully your idle is now better. Do this relearn anytime battery is disconnected.
 

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For all those with idle problems after many attempts at fixes try this simply, free idle relearn.

Turn the car on and let it idle for 20-25 minutes. Make sure all accessories, lights, etc are off.

Now turn AC on full blast and let the car run for 15-20 minutes.

Go for a chill drive. Hopefully your idle is now better. Do this relearn anytime battery is disconnected.
What is that suppose to do to the car?
 

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What is that suppose to do to the car?
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.
 
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