Misfire, plugs 2,4,6 w/carbon 1,3,5 fine

Automobile is 1997 Mazda Millenia with 2.5l V6 24 DOHC (not Super Charged)

Summary:

1) Was getting oxygen senor codes(P1170 and P1173)-replaced all 4 sensors

2) Noticed oil around plug wires-changed valve cover gaskets, plugs and wires

3) Started getting multiple cylinder misfire code (P300)-replaced rotor and cap, ran 2 bottled of injector cleaner through and changed fuel filter

4) Continued getting P300-pulled plugs and noticed front plugs (cylinders 2,4,6) were fouled with carbon and rear plugs (1,3,5) were fine. Figured plugs got fouled during oxygen sensor problems. Replaced all plugs.

5) 25 miles later pulled plugs and noted front 3 plug carboning up again, rear plugs are fine.

Am I having an injector problem, fuel regulation problem. How can I diagnose? I tried listening to injectors with stethoscope as recommended on the web. Can not tell difference between different injector sounds. Cannot get to fuel injectors as they are under fuel rail otherwise would check their resistance with an ohm meter.

Car runs poorly once warm, at highway speeds with low rpm (barely pushing on gas pedal) car stutters. At stop lights car nearly stalls. During cold idle car runs fine.

Detailed Story for those interested:

About 3 months ago car began throwing Check Engine Light. Code was P1170. Since I didn't know what the code was I pulled up the plug wires and found oil around them indicating bad valve cover gasket (which is a known problem on Millenias with 50,000 miles. I later found that the P1170 code was a bad upstream Oxygen Sensor. Of note, my car has 3 upstream Oxygen Sensors.

Took it to a dealer who replaced the valve cover gaskets and felt that changing plugs and wires was the answer for the Oxygen Sensor Code despite the fact I was not getting any misfire codes. I tended to agree with them at the time since it made sense to me that oil around the plug wires could be causing problems.

After getting the car back from the dealer the Check Engine Light came back on. Another P1170. Since Oxygen Sensors are not covered by my extended warranty and the dealer wanted $315 each for them I changed all 3 upstream sensors one at a time until the P1170 Code stopped coming up.

The car seemed to be running well for awhile (approximately 200 miles) then I started getting a multiple cylinder misfire code (P300) combined with P1170 and P1173 (which is downstream O2 Sensor Code). I figured that the last O2 sensor behind the converter needed to be changed So I changed it and cleared the codes.

During this whole oxygen sensor fiasco, the car was running poorly as if it was misfiring. Which could be explained by bad oxygen sensors triggering the engine to richen up. I did notice poor engine performance once the engine was warm. At cold idle the car ran fine at warm idle (at stop lights) the RPMs would drop from 800 to ~200 rpm and the engine would almost stall. On the highway during lower RPM driving (my foot barely on the gas pedal) the car would stutter as if misfiring. I ran through two bottles of injector cleaner and did change the fuel filter.

The car continued with check engine lights and poor performance, but now p300, with an occasional p1173. I figured that I was now getting regular cylinder misfires resulting in the Oxygen Sensor codes. The only part of the ignition system that had not really been addressed thus far was the Cap and Rotor.

I changed the Cap, which did appear bad with corrosion build up on all the contacts. The rotor appeared fine, but was also changed. I thought that I had found the magic bullet, car immediately began running better for about 24 hours. Then that damn p300 started in again.

I decided to pull the plugs and put some dielectric grease on them and the boot, since this was the magic bullet on one of my other cars. While pulling the plugs I noticed something interesting. The plugs in the front three cylinders (2, 4, 6) where all carboned up and the plugs in the rear three cylinders (1, 3, 5) where fine. I figured that maybe during the bad Oxygen Sensors days that the car was running rich on the front 3 cylinders. The plugs were pretty bad so I decided to change all 6 again. I have since put about 25 miles on the car (no check engine lights) and pulled the plugs. The front 3 plugs are starting to carbon up again. The rear 3 plugs look like brand new.

I am now at a loss. I think that I have a fuel injector problem on the front 3 cylinders or some type of fuel regulation problem. The car appears to be multiport fuel injected with the fuel line running to a regulator valve then to a fuel rail that supplies the front the cylinders and their injectors. A U-Shaped hose connects the front fuel rail to the rear fuel rail.

Hey Lefort

I am having a similar problem. I have the 2.5 motor. I went to autozone and have them hook a computer up to my milly. I got codes P0300, P0400, and P1170. As you can see P0300 is random misfire detected, P0400 is EGR Flow malfunction and P1170 is front oxygen sensor no inversion. I had also replace plugs (Bosch platinum +4) and wires (Bosch), it didn't do a dam thing. I also notice while I'm driving, If I am constantly at around 1500 RPM's, the engine kinda stutters like lack of fuel or something. I then put a new fuel filter on it and again, it didn't do a thing. Also when I come to a stop, then engine surges up a few 100 rpms before coming back down to normal idle speed. I don't know if you are experiencing that problem. The engine haven't stalled on me yet (knock on wood). Then I removed the cap and rotor and cleaned it out. It made a little difference. My idle was a little better. So I then removed the air cleaner and cleaned out the throttle body as much as I can to remove the carbon deposit. BIG difference. It ran much better. It ran great for a few weeks then the trouble began again. I assume this was cause by a rich mixture and not I am kinda lost as to what to do to solve this problem. I don't want to go and replace sensors for no reason as this can get VERY EXPENSIVE. I was hoping that we can maybe brainstorm this problem out. Feel free to email me. I am open to all suggestions. Can you check if your distributor coil terminal point a and c to see what the ohm reads? According to the book, mine read a little high bout I wanted to check hopefully with others to see if that would be fine before changing it. It cost $340 part along. I couldn't get the coil alone. I know that it can cause code p0300. It should read .58-.86 ohms. Thanks

So far, since my post I have had the car in to another Mazda Dealer. The first thing they tried was a fuel injection, thottle body, intake manifold cleaning with some kind of high power cleaner thing that they hooked right into the fuel rail and let the car run on for two hours. Interestingly, those nasty looking plugs looked like brand new when this was done. The reason why I point this out is that I am sometimes skeptical of what seems to be "voodoo magic" especially when it cost a total of $278.00.

Anyway as to the results. My car runs much better, but still getting P300 and P1170.

I am also suspicious of the distributor. Primarily becuase of this thread in another forum. On the second page 3rd message from the bottom.

http://forums.mazdaworld.org/index.php?showtopic=2982&hl=distributor

Interestingly, I changed my cap (to an 3rd party cap) and cleaned my rotor, as mentioned above and the car ran better for a couple days. The shop things my problem is from that 3rd party cap as they have had several cars be fixed by replacing the 3rd party cap, but to me it doesn't seem logical that that cap is the problem.

Putting the new cap on did not make the problem any worse or any different, in fact it got better for a few days. I could understand them going after that cap if it had changed something in the way my car was running.

Additionally, they ran all kinds of diagnostics on the car, all my 02 sensors are fine, all my catalytic convertors are fine (I was hoping for these to be bad so I could get them covered on mazda's 8yr/80,000 mile warranty)

So the car is going back in on Monday (am am still under an extended warranty that will cover most things) as I was also having a whine come from the front that was attributed to bad wheel bearings, which also apppears to be a millenia 50,000 mile thing. I have already told them that the car seems improved, but still having problem, I am going to request that they check the distributor to see if it could be the problem. I am sure they will push me into one of those $295.00 MAZDA distributor caps made of the PURIST PLASTIC and whose connectors and contacts are PURE PLATINUM.

Let me know if you get anywhere with your car. To me sounds as if we might be barking up the right tree

Wizkidd007 can you send me a scan of the manual as to how you want me to test the distributor?

Can you send me your email address so I can send you a scan of the distributor for testing?

Another option that I am thinking of is to eliminate the distributor (if that's the problem) and put a coil pack in it instead, this way I don't have to deal with the cap and rotor. The ignition system would then be way better than the stock.

I have 2002 Millenia V6 2.5 liter engine. My car misfire randomly and more on 3000 rpm and i chaged the plugs distributer cap and router to after market. Still same problem, i took my car to dealer and they said i have put factory spark plugs, distributer and router. I did that and its still the same problem. The code the pulled was P0300 Random misfire.

Does any know what's the problem. My car has 30K milesPlease let me know Thanks

Hey bayarea

Email me your email address. I don't use this post too much.

Later

Is that the only code your having?

did you replace distributor or just the cap and rotor?

did you replace distributor or just the cap and rotor?


I am ordering O2 sensors and thinking about getting the ICE PAK from jacobs. Let me know how you do with your distributor.

My wifes 2000 Millenia S was just serviced at 60K. Replaced timing belt, spark plugs, flushed transmission, and replaced leaking engine gasket. Sum total of $1400.

Now a couple of months later, the car is having acceleration problems from stop. Also notice some subtle sputtering while driving at a constant medium to high speed.

Took it to the dealer and they got a "Random Misfire Code". They tell me it'll be $100/hr to diagnose the problem. They tried to act like they had no idea about what the problem is - "it's random". I told them that they must know what sub-system is having the problem (i.e. electrical/ignition).

Anyway, I can't afford to drop unlimited funds into this POS. Can anyone pass along their Mazda Millenia wisdom on how to fix this problem???

I would be very appreciative. We're trying to close on a house and really can't afford to blow a wad of cash on this car right now.

This sounds really weird. I had the same problem with the DTC as mention here. I was also totally loss at the cause of the DTCs.

About 2 weeks ago, I changed my timing belt and tensioners and waterpump and all the other belts. After putting it back together, my car ran much better. I ran it for about 2 weeks so far and no problems at all (Knock on wood). It's not misfiring at all, also engine light has not come on. (Yet). My engine has about 70000 miles on it. Kind of hard to believe, but it seems like the timing belt solved everything so far. I figure that the timing belt might of skip a tooth due to probably weak tensioner or something and cause the computer to get false timing signals. Also maybe the old belt was stretched out a little? Does anyone know if this is possible at all? All I know is that my milly runs very good right now. I hope this info helps out others cause it sure is a bitch to figure out.

BTW in case if you want to know, the automechanic was going to charge me about $950 to change the timing belt and water pump. I can't understand why so much. So I decide to do it on my own, (I've done quite a few timing belts before, might want to consider having a experience person do it if you've never done this before). Cost me $160 for the timing belt kit and $60 for a water pump and $25 for both the Alternator and Power steering pump belts = $245. Everything was bought at Autozone. Definately have the water pump change when you do your timing belt cause it will save you money to do it now than later, you have to take off the timing belt to get to the water pump. As for difficulty from a scale of 1-10, I would rate this job a 7. Sorry for the long post, hope this info helps.

Hey wizkid007, how is your milly? Did doing the timing belt and all other belts still keeping your car from flashing the check engine light and from misfiring? Please let me know so I can have extra motivation and peace of mind to go get this done asap too. k, thanks


This sounds really weird. I had the same problem with the DTC as mention here. I was also totally loss at the cause of the DTCs.

About 2 weeks ago, I changed my timing belt and tensioners and waterpump and all the other belts. After putting it back together, my car ran much better. I ran it for about 2 weeks so far and no problems at all (Knock on wood). It's not misfiring at all, also engine light has not come on. (Yet). My engine has about 70000 miles on it. Kind of hard to believe, but it seems like the timing belt solved everything so far. I figure that the timing belt might of skip a tooth due to probably weak tensioner or something and cause the computer to get false timing signals. Also maybe the old belt was stretched out a little? Does anyone know if this is possible at all? All I know is that my milly runs very good right now. I hope this info helps out others cause it sure is a bitch to figure out.

BTW in case if you want to know, the automechanic was going to charge me about $950 to change the timing belt and water pump. I can't understand why so much. So I decide to do it on my own, (I've done quite a few timing belts before, might want to consider having a experience person do it if you've never done this before). Cost me $160 for the timing belt kit and $60 for a water pump and $25 for both the Alternator and Power steering pump belts = $245. Everything was bought at Autozone. Definately have the water pump change when you do your timing belt cause it will save you money to do it now than later, you have to take off the timing belt to get to the water pump. As for difficulty from a scale of 1-10, I would rate this job a 7. Sorry for the long post, hope this info helps.

Hey MV, so far since my timing belt and water pump change, I have not had any problems with my milly (Thank GOD). My engine light came on once, but that was due to a bad O2 sensor. I haven't had any more of the random misfire code (P300). FYI, I only use 87 octane gas, change oil religiously, I use fram filter and 5W-30 oil only. I use the 75000+ mile oils only. I am using BOSCH platinum+4 with NGK wires, I might go with NGK plugs too (Iridiums). Let me know what's up with your motor. Which DTC are you getting?

hey wizkid, thanks for such a prompt reply to my question. sounds like your milly is healthy and in good hands =)
as for mine though, the engine light has been coming on and off intermittently. when i go down a hill and at the bottom of that hill, there is a stoplight where i have to stop, as i approach a full stop, the engine starts to drop its RPM and start sorta "stuttering" in its RPM....then as i depress the accelorator pedal, everything goes back to normal and accelerates fine. i have already scanned the entire forum for possible problems that others have been having that fits my situation. sadly to say, the milly does seem to have an almost ubiquitous mechanical problem involving the check engine light and "misfiring of cylinders". btw, i am assuming by "DTC", you mean an error code? welp, i havent got around to doing that kind of computer assesment yet. but i need to change the timing belt soon anyways, so i will let you all know if wizkid's theory proves to be the cure-it-all to the many woes of millenia owners. as of now, i have about 80k miles on my car. i have a K&N filter and catback exhaust system as the only modifications done on the car(well, excluding the Mobil 1 i pamper it with every 3k miles). before all this though, the stock car seemed so sluggish(2.5L naturally aspirated). the catback exhaust seemed to really free up the engine's performance quite a bit. now, i am thinking of getting the airbox modified as well as timing concommitantly.
anyways, sorry for being so verbose. this forum has really been entertaining as it has been useful to me so far. thanks wizkid, and the rest of you guys. later

I have read over the other posts here but still have some questions. I own a 2000 Millineaum Edition Millena with the Supercharged Engine. My check engine light has basically been on since I have owned my millenia (year and a half). This was due to bad O2 sensors. However it just recently started blinking. This is why I took to Auto Zone to get checked and found that two O2 sensors were bad in bank 1 and bank 2 but also I had the PO300 or Cylindar One Misfire. The Auto Zone rep told me that it is most commonly an Ignition Coil problem. He showed me the part and it was listed at $260 for the part. He also said it could likely be a spark plug problem. I have been reading here on the boards and it looks like a timing belt change might be the fixer. What are your thoughts?

hey PF,
i don't think it's a sparkplug problem. i swapped out to a fresh set of iridium NGK's about a month ago, and the check engine light has been on ever since still.