rough idle, poor gas mileage, bouncing RPM... whats going on? - Mazda3 Forums : The #1 Mazda 3 Forum
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post #1 of 3 (permalink) Old 09-21-2011, 02:16 PM Thread Starter
Go-Kart Driver
Join Date: Aug 2007
Posts: 77
rough idle, poor gas mileage, bouncing RPM... whats going on?

i have a 2005 Mazda3 2.3 that i have been having some issues with that i think might be related.
First, is rough idle. its not a rhythmic bounce. it more like it lightly hopes around, (like 100-200rpms) also seems to idle kinda low when stopped and in drive.
second, poor gas mileage. i used to always get over 30MPGs, now no matter how i drive i always seem to get 25MPGs driving around town, going 60MPH on the express way, going 80MPH on the express, always yields 25MPG. These MPGs are calculated from fill up to fill up. so i am sure they are accurate.
third, this one is odd, if i take the car up to about 35 MPH and remove my foot off the gas pedal and just coast, as the car slows down to about 30 MPH the RPMs will drop to about 1000. Then bounce up to 1500, then back down to 1000, and back up to 1500... it will do this about 3-5 times before the settles on about 1200RPMs at about 27 MPH. it will do the same thing when breaking, but only bounces 1 or 2 times.

I dont have a check engine light.
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post #2 of 3 (permalink) Old 09-30-2011, 04:17 PM
Moped Rider
Join Date: Sep 2011
Location: Wilmington, NC
Posts: 25
Re: rough idle, poor gas mileage, bouncing RPM... whats going on?

The first time I ever disconnected/reconnected the battery on my '05, I had the same problem with the bouncing idle & rpms. After much research & stress, I figured out that after disconnecting/reconnecting the battery, the computer had to re-learn the throttle position. This is the whole procedure I go through and it has worked every time: disconnect your negative battery cable, pump the brake petal several times to clear all the juice out of the computer, then reconnect your battery. Start the vehicle and let idle until engine is warm, and then shut it back off. Wait 10-15 minutes then start the vehicle, let idle until warm & then shut back off. After going through that whole procedure, go for a test drive and see if there is any improvement in the idle. It might help to clean your throttle body & change your spark plugs too. Hope this helps...
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post #3 of 3 (permalink) Old 10-03-2011, 01:23 AM
Short Track Racer
Join Date: Jun 2006
Location: San Antonio, TX
Posts: 148
Re: rough idle, poor gas mileage, bouncing RPM... whats going on?

Sounds as though the transmission doesn't know which gear to settle in? May be going inbetween 2 and 3rd gear. Just a though.

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