I own a 98 millenia (not the miller cycle engine) with 90K miles. A few weeks ago I had some work done on it, including diagnosing the infernal CEL which has come on too many times to count in the five years I have owned the car. The codes at that time were PO325 and PO300, knock sensor and random misfire. I had the knock sensor replaced and was told that the PO300 code was due to "spark plug wires arcing", so I paid for that repair as well. Three weeks later the CEL is back on, different dealership diagnosed code PO300 once again. They say it is due to carbon build-up and that I need a "top engine clean" and an "air induction clean" (total cost $200). I'm trying to figure out, well, whether or not I'm being taken for a ride (why did the previous dealership service dept three weeks ago not notice/mention carbon build-up?) and whether or not these repairs are important and necessary. Don't know jack about cars. Thanks in advance for any advice.

 

It is fairly unlikely that they know for a fact that the random missfire is due to carbon build up. More likely they are going off of an older TSB released from Mazda concerning missfire and check engine light coming on after a cold start. Did the CEL come on shortly after a cold start up? Do you ever feel the engine missfiring or running rough? Also what grade or octane fuel do you use? The bulletin deals with carbon accumulation on the exhaust valve seat from using high octane fuel. This carbon does not allow the valve to close fully, insufficient compression and missfire is the result. Below is the first part of the bulletin so you can read for yourself. There is another TSB for 97-98 that talks about random missfire caused by an overly sensitive PCM but this is far less likely due to the age of vehcile. Would have showed up long ago if this were the case.
IF the carbon accumulation is the problem then you might be able to clean with a good fuel additive that is designed for that very purpose and making sure that you are only using 87 octane fuel. It would take longer as additives for fuel tank are not nearly as strong as one such as top engine cleaner.

TSB: 01-027/02
1995 - 2002 Millenia (KL engine) - MIL ILLUMINATION WITH DTC P0300 TO P0306
APPLICABLE MODEL(S)/VINS
All 1995 - 2002 Millenia models with 2.5L V6 (KL)

DESCRIPTION
After a cold start, MIL comes on with DTC P0300 to P0306 stored in the memory. In most cases, this concern may be caused by misfire occurring in one cylinder during idle. The use of premium fuel (high-octane fuel) may be allowing carbon deposits to accumulate on the exhaust valve seat, causing momentary insufficient compression.

To correct this concern, remove carbon deposits using a suitable top engine cleaner.

NOTE: After removing the carbon deposits, be sure to advise the customer to use lower-octane fuel (87 or 89 octane) to help prevent future recurrences.

 

Thanks so much for the information. Yes the CEL came on right after a cold start. The engine idled rough for a few seconds and then evened out and I have not noticed any roughness or misfiring since.
I have always used premium gas, highest octane available, as that is what my manual instructs. Apparently I could have been saving myself a lot of money and preventing carbon build-up if I'd known about this issue with high octane fuel! When you say fuel additive, I think of Techron. Is that what you're talking about? Is there something better available? Do you think fuel additive is sufficient to do the job, just that it takes longer, or do you think I should fork over the 200 bucks and get it done quickly and thoroughly? Thanks again!

 

I would try the Techron first after switching to 87 octane myself. Lot of things I could do with an extra $200 plus the money savings from getting the cheaper fuel. If it continues at 3 or 4 tankfulls and you need to do the professional clean then you are still not out anything except a few bottles of Techron. You could actually do the recomended procedure by yourself. There are no special tools you need outside of a bottle of top engine cleaner and a small restrictor which is available from mazda for $2.10. The procedure is quite simple to do.

 

Thanks again. Do you mean to use techron in 3 or 4 consecutive tankfuls of gas? Also, I should mention that the CEL went off shortly after it came on and has stayed off. It was actually off when I took the car to the dealership. I don't notice any symptoms so I don't know how I'll know if it's "fixed" or not, just as I don't know if there's really carbon build-up or not. I'll definitely switch to 87 octane now. I wonder why the manual recommends premium?? [:@]

 

Add a bottle in 3-4 consecutive tankfulls. Directions for amount should be on the bottle. It will probably take at least that many to effectively clean the exhaust valve seats as much of the cleaner is burned up in the combustion chamber. They cleain it will though. The only symptom you will be looking for is that intermittent rough running on startup and no CEL with P0300-P0306 stored. Premium was recomended in the manual because the 2.3 engine really needs the higher octane fuel or it develops problems on its own. Was not specified in the owners manual and Mazda did not realize the problems that were developing for quite a few years.

 

Thanks babyhuey. Just to clarify...if I switch to 87 octane, is that going to cause other problems with the engine?

 

No. Higher octane fuel really only needs to be used if you begin to experience engine pinging on accel or under load. If the engine doesn't ping or pre-detonate then stick with the cheaper stuff.