Brand new(used) car, runs, has some issues, wondering what to do

I would like to preface this by apologizing for my lack of knowledge on cars, this is the first car I’ve ever owned.

anyways, i just picked up a used 2004 mazda 6 s (3.0L v6 manual) with around 146000 miles on it. the car runs and has gotten me from place to place as well as i could have hoped, but the engine makes a weird noise. the noise is almost like coughing or pop-corn popping(ive only noticed it when the car is moving, not at idle), from what i can tell it doesnt really happen around a specific rpm.

Another thing i noticed is that the car shakes when i turn on the ac, and one time when i was idling it stalled out just from turning it on.

A friend of mine said that “it could use a tune up” which im assuming they meant it could use new things like a new air filter, spark plugs, ignition coils or things, but im not sure.

I know this is probably confusing and not helpful enough, but I was wondering if its something simple like just swapping spark plugs, because thats something that I would feel comfortable doing on my own(after looking up how to do it)

Also i plugged in an obd scanner and didnt get any errors.

Note as many conditions as you can to make a replicatable connection. Guess number 1 ac problem.

Before you express confidence in your ability to changing spark plugs in this car , I suggest you google the procedure and watch the you tube video.


Most vehicles recommend spark plug change around 100,000 miles, yours may be overdue. Review, as suggested the YouTube video on changing in the plugs. Air filter change is relatively simple.
Before we go any further is your check engine light on?


Like @Purebred asks, is your check engine light on?

Does your check engine come on when you turn the key to “RUN” and then go off when car starts?

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The Idle Air Control valve may be dirty or defective.

The IAC valve controls the engine idle speed when different loads are imposed on the engine.

For example, when turning the steering wheel, the power steering pump imposes a load on the engine that could cause the engine to run rough or stall.

Same when turning on the AC, the compressor imposes a load on the engine that could cause the engine to run rough or stall.

You can try removing the IAC valve and clean it with throttle body cleaner to see if it eliminates the shaking/stalling when the AC is turned on.

Otherwise, it may need to be replaced.


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Since you know very little about cars, can you afford to find a good independent shop to diagnose and fix these initial problems?

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Nope no check engine light

What worries me is the popcorn noise. It could be knocking, just something loose rattling, or something really serious.

are you sure it is coming from the engine? you can have 2 problems. the running rough with A/C on and then the popping sound which could be from your suspension or a loose heat shield or something else that is loose and vibrating.

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If it’s actually “brand new used” and assuming that it’s a dealer sale, take it back immediately to the seller. and demand a refund.

It could be knocking, misfire, bad valves, etc, which could all cost you big bucks in the near future.

If it’s a private sale, find a reputable mechanic and get it diagnosed ASAP.

It depends on your State but my own State has a very dim view of “Curbstoners” who sell crappy cars as “Private Sales.”

I would hope that this coughing/popping is not a sign of low compression on a cylinder or two. That would be worst case scenario and could also cause an RPM drop when the A/C is on.

If the spark plugs come out I strongly suggest a compression test at that time. If there is a serious mechanical issue that is the time to find out about it; before loading up the parts shotgun.

I’m going to offer my opinion on the engine stalling when the ac compressor is turned on . . .

I think there’s a fair chance the ac compressor may be shot and about to let go in dramatic fashion

I can’t offer anything better but the popping sound should be heard by a mechanic. A code reader is fine but a diagnostic computer will take a look at the readings from the various sensors to see if anything is out of the ordinary. Might be worth the $125 diagnostic fee to have someone check, but for sure have the popping and stalling checked.

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