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  #1  
Old 08-02-2009, 11:24 AM
xanthous xanthous is offline
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Car idling around 600 RPM at stoplights- not good, right?

It shudders a little at this RPM and has stalled before at stoplights. I used a gas treatment a few months ago and it seemed like maybe it was a bit better after that, but I also don't drive it a lot. I did put in a few gallons at a Circle K last night- I don't ever get gas from there (I was on the road), and after driving highway for 1.5 hours, when I got into town, it was idling really low at stops again.

Not good, right? What might be the problem?

(it's a Mazda 3 hatchback, oh, and also- just got a 30,000 mile service recently which included transmission flush, engine treatment & other stuff, the stalling problem happened before that and feels like it's going to happen again)
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  #2  
Old 08-02-2009, 11:35 AM
pulykamell pulykamell is online now
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I'll be watching this thread carefully. I have a Mazda 3, manual, with 60K miles on it, and I've had similar issues recently. It usually idles at 700-800 rpm, but recently it's started ping-ponging between 500 and 1000 before settling at 700-800 rpm. A couple of times, though, it missed coming up from 500 rpm and stalled. The first time it happened (in December), I chalked it up to an odd tank of gas, but it's started happening again as of last week or so.
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  #3  
Old 08-02-2009, 01:44 PM
SanibelMan SanibelMan is offline
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I had a 2006 Madza 3i with automatic that had similar symptoms. It would shudder on occasion at stoplights. I mentioned it to the dealer and they didn't seem to think anything of it. I turned it in at the end of its lease with about 40,000 miles on it. Sorry I can't be more helpful, but maybe there is something about the Mazda 3 engine. Mine had the 2.0 liter, not the 2.3, if that narrows it down.
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  #4  
Old 08-02-2009, 03:13 PM
Gary T Gary T is offline
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Sometimes a bottle of Techron fuel system cleaner will help with this type of symptom. It's not a sure bet, but it's the easiest and cheapest thing to try first.

A fairly likely cause is carbon build-up in the throttle body. If carbon deposits are present there, a throttle body cleaning is called for. This is one of the first things I look for when dealing with idle quality issues.
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  #5  
Old 08-02-2009, 03:41 PM
Nunzio Tavulari Nunzio Tavulari is offline
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I'm a home mechanic but not a professional.

Depends on what you mean by "not good". If your car is using 1 quart of oil every thousand miles or it sounds like there's a woodpecker under the hood, that's "not good".

There are several possible reasons for your problems, none of which can be diagnosed without having the car here. Modern cars are sealed systems from the air cleaner to the tailpipe. No air that goes into the intake is

There may be a vacuum leak. One of the rubber hoses on the engine may be cracked or loose. There is a sensor just beyond the air cleaner that measures how much air is entering the engine. The car's computer calculates the correct ratio of air to fuel. If air leaks in beyond that sensor the calculations are wrong.

There is an Idle Control Valve on the side of the intake manifold. The simple explanation of this device is that it calculates how much air to feed the engine based on whether the engine is cold and how much load is on it. If you have the A/C on, or the car is in Drive, there is more load on the engine. As part of the sealed system it bleeds off air depending on the needs of the moment. Since it's Summer, I assume you have the A/C on. This valve can get gummed up with fuel residue and the valve isn't able to function correctly. It has to be taken off and sprayed with fuel system cleaner.

These are two of the more frequent causes. Your engine isn't in dire shape and the cost to repair depends on how quickly the mechanic can find the cause.

If you just had the 30,000 mile check done by the dealer, a check of the vacuum hoses is usually listed but rarely done. They last beyond 75,000 miles in most conditions, (Viton replacements will last the life of your car) and unless they heard the hiss of a loose one they wouldn't spend more than a minute on them. They may have also said they did a fuel system cleaning (usually poured a can of cleaner into the gas tank) but that would not have done much for the Idle Valve. They'll deny any negligence and charge you to fix it. Bring it to an independent garage (if there are any left near you).

As an aside, all brand name gasolines have detergents in them. This mainly benefits the combustion chamber. Fuel deposits will still accumulate in the intake system and around the valves but it is much less than what used to occur. You don't need Techron or any additive for the gas tank. They will not do anything.

There are other additives that can be added into the intake system that reportedly clean deposits. I have no proof for or against those claims but I personally feel better when I feed Seafoam through my intake every 30,000 miles.
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  #6  
Old 08-02-2009, 05:22 PM
Gary T Gary T is offline
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Quote:
Originally Posted by Nunzio Tavulari View Post
You don't need Techron or any additive for the gas tank. They will not do anything.
That is not true as a blanket statement. There are cases where Techron will make a noticeable difference.
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  #7  
Old 08-03-2009, 12:24 AM
xanthous xanthous is offline
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Nunzio, thanks for all your great info. I did have the A/C on for the trip. I'll have to do some driving without it on and see if i notice anything. I do have an extended warranty on my car, so hopefully this is something the dealership can look into without great cost to me.
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