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Discussion Starter #1
I'm totally stumped by this phenomenon. As the title implies the engine (2006, 3.0L S) stutters and occasionally backfires when accelerating but only after it warms up and presumably goes "closed loop" (correct me if my terminology are off, I'm not a mechanic by trade).

I replaced the fuel pump last week with a used pump but that changed nothing. I replaced the MAF (mass airflow sensor) yesterday again with zero improvement. It has thrown zero codes so no check engine light illuminates.

I feel like it's constantly running rich which to me explains why I get two to three minutes of perfect function in the morning when I pull it out of the garage and then it goes back to it's devilish ways. If I drive aggressively (heavy foot, higher than normal revving) it is fine. Furthermore, I'm getting terrible gas mileage, which again points me to a rich mixture. Now I'm leaning towards O2 sensors now but honestly at this point I'm just buying parts blindly.

I'm close to my technical (and financial) troubleshooting limit on this one right now and am desperate for some advice!
 

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Sounds very similar to what happens to my car; runs great at first when started then starts to backfire and stutter occasionally unless i drive agressively at higher rpm's.

I have a 2004 Mazda6s 3.0 with the CP-E mafci CAI, 163,000 kms (approx 101k miles). At about 125k kms (78k miles) misfires started to occur throwing a CEL for cylinder 3. Replaced all 6 plugs. About 2-3 months later CEL came back for #3, replaced ignition coil. And about 3-5 months later CEL came on for #1 cylinder. Replaced coils #1 and #2 since the manifold was off, figured i would have the back 3 all replaced. This was approx 1 year ago and it ran good but not great up until the last 2-3 months.

Like i first said, its missing and stuttering but not throwing any CEL's. Still have the original coils in the front bank (4,5,6) since no codes have come up. Should i replace the front 3? Wait for a CEL? Check/replace my o2 senors? Someone had mentioned to me about replacing all the spark plug wires?

I have a trusted friend that is a mechanic do the work for me. He says to just wait for a CEL but its been going on like this for a while.

Any advice? thanks!
 

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Discussion Starter #5 (Edited)
Well after months of dealing with the symptoms and finally breaking down and visiting two mechanics and still finding no solution. I decided to go back to the basics before trying any more specific repairs. In the beginning of this mess I checked the front three plugs and tested each of their coils (with a known good one) to no avail since they're so accessible. The back three plugs however are so damn deep and the front three plugs were in pretty good condition that I didn't bother with them. Until two days ago.

I rolled up my sleeves and started working my way to them intent on replacing the three back coils. I was pretty shocked at what I saw when I pulled out the back plugs. I attached a photo of what they looked like after 60,000 miles. They're Bosche Platinums and the plugs in cylinder's one and three had none of the prongs left. They were just burned away. I replaced all three of course and since I was all the way back there and I already had the new coils I replaced them too, though I'm sure they were fine.

In any event, it runs like a million bucks now and I couldn't be happier! Thought I should update this thread with the solution.
 

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For future reference: grab an OBDII scanner and laptop and take a snap-shot of when the engine is RUNNING GOOD! You'll see the fuel trims, ign advance, O2 data, TPS, speed, gear, calculated engine load, correlate open loop vs closed loop operations (cold start vs WOT) plus seeing some of the triggers that make emissions events happen. It's WAY better than guessing although that is still very likely to happen (guessing) but now you can see if your efforts changed it.

Those plugs up there look like detonation tore them off bit by bit. You might read up on the EGR being stuck and find a way to test the knock sensor(s). If they are defective, you could break rings and pistons. Yes, I am new here but have plenty of time on the Yoda line of V6s.

EDIT: according to the emission standards an engine is allowed X number of misfires before its recorded and generates a CEL. To all the people who notice the poor running engine (but its not bad enough to trigger the CEL), often times it spans months of misfire events with bad coils, bad plugs, egr faults, O2 sensor getting lazy, etc before it gets so bad that the engine has a continuous misfire event in a single run on the SAME cylinder and FINALLY the ECM says the misfire count is TOO high and trigger a CEL. This is mandated so the vehicle meets emission standards... Stumble, hesitation, sluggishness and high fuel consumption are all tell tale signs it needs attention. Here the scanner will help you see this before the ECM posts a CEL.
 

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Don't every run the Bosch plugs, especially the +4 platinum plugs. Bosch plugs are garbage and cause CEL's and other problems like no ones business. Stick to NGK iridiums or the platinums, or OEM motorcrafts, or the densos if youve got $$.
 
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Sounds very similar to what happens to my car; runs great at first when started then starts to backfire and stutter occasionally unless i drive agressively at higher rpm's.

I have a 2004 Mazda6s 3.0 with the CP-E mafci CAI, 163,000 kms (approx 101k miles). At about 125k kms (78k miles) misfires started to occur throwing a CEL for cylinder 3. Replaced all 6 plugs. About 2-3 months later CEL came back for #3, replaced ignition coil. And about 3-5 months later CEL came on for #1 cylinder. Replaced coils #1 and #2 since the manifold was off, figured i would have the back 3 all replaced. This was approx 1 year ago and it ran good but not great up until the last 2-3 months.

Like i first said, its missing and stuttering but not throwing any CEL's. Still have the original coils in the front bank (4,5,6) since no codes have come up. Should i replace the front 3? Wait for a CEL? Check/replace my o2 senors? Someone had mentioned to me about replacing all the spark plug wires?

I have a trusted friend that is a mechanic do the work for me. He says to just wait for a CEL but its been going on like this for a while.

Any advice? thanks!
OK! This is just like MINE 05 Mazda 6s
However backwards.
when I drive aggressively is when I have the problem and also after it warms up. I baby foot it keeping rpms Low it’s ok
 
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