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Discussion Starter #1
Hey everyone, hoping to get some input.
2008 Mazda 6 i4. current mileage is almost 122k. Engine light is on with a code that starts with P, but I cant remember what it is exactly.

About a week ago the car started shaking back and forth when idling, sometimes enough to cause motion sickness to my wife. Its pretty violent sometimes. This only happens when the RPM is just below 750. When the AC compressor kicks on, the RPM goes just above 750 and the shaking stops, then the compressor shuts off and the RPM falls to just below 750 and shaking begins again.

I read it might possibly be the spark plugs which haven't been changed since i bought the car 3 years ago at 89k miles. (oops). I changed them this weekend and the shaking appears to be less but is still there.

Also, the sparks plugs to cylinders 2 and 3 (the middle two) had what looked like gas getting passed the threads on the plugs and had some residue on the spark plug wire boot.

Any ideas what this could be?
 

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Hey everyone, hoping to get some input.
2008 Mazda 6 i4. current mileage is almost 122k. Engine light is on with a code that starts with P, but I cant remember what it is exactly.

About a week ago the car started shaking back and forth when idling, sometimes enough to cause motion sickness to my wife. Its pretty violent sometimes. This only happens when the RPM is just below 750. When the AC compressor kicks on, the RPM goes just above 750 and the shaking stops, then the compressor shuts off and the RPM falls to just below 750 and shaking begins again.

I read it might possibly be the spark plugs which haven't been changed since i bought the car 3 years ago at 89k miles. (oops). I changed them this weekend and the shaking appears to be less but is still there.

Also, the sparks plugs to cylinders 2 and 3 (the middle two) had what looked like gas getting passed the threads on the plugs and had some residue on the spark plug wire boot.

Any ideas what this could be?
I would say a mis-fire. That will cause significant shaking, especially on worn motor mounts. If you feel the exhaust with your hand and there is a rhythmic puff then you are not igniting at least one cylinder.
I am a little confused that you say spark plug wires as this vehicle should have coil on plug and not wires. The last year in the US that had that was 2005.


I would replace the plugs (NGK platinum) and coils (Denso or NGK) and PCV valve. If you are ordering online it may be a good time to check your air intake filter and cabin filter. Check to be sure you don't have any intake vacuum leaks (accordion tube) and give the MAF a cleaning with the special spray.


If you grab hold of the motor (not running) at a solid point and pull forward and rearward and it moves significantly then you have some motor mounts to replace (secondary to a poorly running engine)


If it is indeed a mis-fire then you must fix it soon or it will destroy the catalytic converters.

This is my best guess based on your brief symptoms presentation. If you did have the DTC (I'm guessing a P0300) that would be it for sure.


Good luck.
 

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I would say a mis-fire. That will cause significant shaking, especially on worn motor mounts. If you feel the exhaust with your hand and there is a rhythmic puff then you are not igniting at least one cylinder.
I am a little confused that you say spark plug wires as this vehicle should have coil on plug and not wires. The last year in the US that had that was 2005.


I would replace the plugs (NGK platinum) and coils (Denso or NGK) and PCV valve. If you are ordering online it may be a good time to check your air intake filter and cabin filter. Check to be sure you don't have any intake vacuum leaks (accordion tube) and give the MAF a cleaning with the special spray.


If you grab hold of the motor (not running) at a solid point and pull forward and rearward and it moves significantly then you have some motor mounts to replace (secondary to a poorly running engine)


If it is indeed a mis-fire then you must fix it soon or it will destroy the catalytic converters.

This is my best guess based on your brief symptoms presentation. If you did have the DTC (I'm guessing a P0300) that would be it for sure.


Good luck.
I apologize on my wording regarding the spark plug wires/coil. I didn't know the correct terminology.

Yesterday (Sunday 7/28/19) I replaced all 4 spark plugs with NGK Platinum (pics to prove it). That is what helped reduce the shaking. I will try replacing the coil on plugs or whatever the correct term is and see. I will also check the motor mounts.

My new thought/question is, would the residue from the middle two spark plugs that got on the coil on plug cause it to malfunction and thus cause the misfire? If so, then changing them should fix this problem.
 

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My new thought/question is, would the residue from the middle two spark plugs that got on the coil on plug cause it to malfunction and thus cause the misfire? If so, then changing them should fix this problem.
Maybe. If it caused the plug's firing current to short out in part it will cause a misfire, yes.

What the other respondent said above is definitely a "must" too -- misfires are very bad on cat equipped cars as they cause a lot of unburned fuel to dump into the cat where it is burned and will cause the cat to overheat and be destroyed. On some of these vehicles there's a "pre-cat" in the exhaust manifold and if THAT overheats and disintegrates the pieces can go back into the engine through the exhaust valves and trash the engine, so this is definitely not something to let go for any length of time at all.
 

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I apologize on my wording regarding the spark plug wires/coil. I didn't know the correct terminology.

Yesterday (Sunday 7/28/19) I replaced all 4 spark plugs with NGK Platinum (pics to prove it). That is what helped reduce the shaking. I will try replacing the coil on plugs or whatever the correct term is and see. I will also check the motor mounts.

My new thought/question is, would the residue from the middle two spark plugs that got on the coil on plug cause it to malfunction and thus cause the misfire? If so, then changing them should fix this problem.

The 6cyl seems to destroy coils in around 80-100K miles.
Most manufacturers recommend something around 100K mi for these components.
I would replace all 4 coils, not just 1 or 2.
 

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Coils should NOT fail. Period.

With that said they often do. Usually when they do it's because of crap design (they're mounted in such a way, and made of such a material set, that they can't take the heat); the easiest way to find out if that's what's going on is if you suspect a misfire on one cylinder move the coil from that cylinder to another and see if the misfire moves. If it does, well, you have your answer.

Throwing parts at a problem gets expensive fast and if there are four coils (4-cyl engine) it's extremely uncommon for all of them to be compromised at once. Make the effort to isolate the problem lest you wind up replacing perfectly good parts.
 

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Coils should NOT fail. Period.

With that said they often do. Usually when they do it's because of crap design (they're mounted in such a way, and made of such a material set, that they can't take the heat); the easiest way to find out if that's what's going on is if you suspect a misfire on one cylinder move the coil from that cylinder to another and see if the misfire moves. If it does, well, you have your answer.
Good advice in general.
Throwing parts at a problem gets expensive fast and if there are four coils (4-cyl engine) it's extremely uncommon for all of them to be compromised at once.
While I generally agree with this, there are times when it is not ideal. Say replacing headlights. If you have to take the entire bumper and headlights (halogen) out I would do both low beams. They have always failed within three weeks of one another in my experience and old bulbs loose luminescence over time.. Why do the labor twice. Or say brake pads, the inboard ones may be at replacement but the outboard ones have some life left. Replace them all.
Electrical parts fail with repeated heating and cooling and all the coils heat and cool the same. If one has failed as a result of environment it is a reasonably safe bet that others are moving along the same path. Given the OP has two plugs with un-combusted fuel on them we would be looking at a 50% failure rate. You can of course gamble and save the extra $60 but the notoriously low reporting rate of misfire for the 1st gen car combined with the price of catalytic converters compels me not to gamble with this one.



This will depend on your particular frugality. Some replace their shoes before the soles fall off. Others just continuously tape them back on.


Make the effort to isolate the problem lest you wind up replacing perfectly good parts.
Yes indeed. Make every effort to isolate the problem
 

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Well, headlamps are a different issue. If you have one blow, and inspect the other, you'll find that filament compromised. So you change it. (PS: On many cars that allegedly require removing the bumper cover to change them you actually don't need to. In fact it's the exception -- in a big way -- rather than the rule that you do. Including the Gen3 Mazdas.)

Brakes are a matched set per-axle. But if my fronts are gone, I don't automatically do the backs. If the INSIDE pad is bad and the outside is not then something's hanging up as that should not happen; you have something there to fix.
 

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Discussion Starter #10
Here is an update for everyone. I get a new set of 4 coils, and installed them. That caused a DEFINITE misfire from the coils. I replaced the original coils and the car is back to how it is. I am going to send the cruddy coils back. I am going to attempt to figure out if it is the MAF or something air related next. I havent messed with it since I bought the car, but who knows.

By the way everyone, I thank you for all your help with this.
 

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Let us know how things continue to develop and we'll do our best to help.

I was with Talon on this one too. Make sure you check the PCV Valve and Hose, check your throttle body, and make sure the MAF is clean.
 
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