P0303 (misfire cylinder 3) at 38K miles on 2.3L engine - Mazda Forum
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post #1 of 1 (permalink) Old 03-18-2017, 02:22 PM Thread Starter
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P0303 (misfire cylinder 3) at 38K miles on 2.3L engine

Looking for some advice. Basically, my Millenia S 2.3L engine began having a misfire on cylinder #3 two weeks ago. I wasn't expecting this type of problem because my engine was rebuilt 38K miles ago. Total mileage is nearing 155K.

More detail (some of it irrelevant, I suppose, but it's an edited version of what I gave the local mechanics that looked it over): For many months the car has had no DTC codes, but it sat unused during much of the winter, which was one with lots of snow but average winter temperatures but then warmer temperatures recently that appear to be an early Spring. Perhaps about 200 miles after being used again this problem happened. The first time was on an afternoon when the temperature was quite warm (about 65 F). That morning it had been colder (about 35 F) and I drove the car for about 5 miles up to about 60 mph which was a bit faster than I had driven it up until then. It was parked until that afternoon and the MIL came on during the drive home. While the car was idling I noticed that the engine was missing a little bit - running a bit rough. I read DTC codes when I got home and found two: P0303 - Cylinder 3 misfire (middle one closest to firewall) and P1601 - Transmission control module (TCM) - engine control unit (ECU) communication error. A few days later, after doing some research using mazdaworld.org and milleniatech.net, I attempted to check the wiring harness connections behind the rear intercooler. I didn’t completely remove any connector, but wiggled them all a bit, which may have improved contact for some wires. When I attempted to start the vehicle, I was surprised that I was only able to crank it once, and that not long enough to start the engine. It was surprising because the battery was new a few months ago. After charging the battery on 6-amp setting for an hour or so I started the car and made a 7-mile shopping trip. Initially I didn’t notice any problem, but about halfway to the store, the MIL came on. Pulling into the parking space I heard and felt a few misfires. On the way back, particularly after accelerating onto Main Street and then for a block or so afterward, the MIL started flashing. I noticed a little jerkiness during this time. A few moments later, as I was still trying to remember what flashing MIL means and whether I should pull over, the MIL stopped flashing and then engine ran smoother most of the way home. While waiting at traffic lights I discovered that if put it in neutral and kept the engine speed at about 1500-2000 RPM the engine would run smooth, but when I let engine speed drop down to 500-600 RPM where it idled naturally, I would hear an occasional misfire. Upon arriving home I found that the P0303 was the only diagnostic trouble code, and none others were pending.

I took it to the local mechanics that I usually use and provided the above account. I also printed out the DTC info at milleniatech.net - P0301 / P0302 / P0303 / P0304 / P0305 / P0306 and added a note about TSB 0102702, which was the only TSB that looked applicable. I couldn't find the PDF file of the TSB, which apparently used to be on milleniatech.net. But calling the dealership (about 100 miles away) the guy at their service desk found it. He read through it and said that it recommends to “use a suitable top-engine cleaner” to “decarb” the engine and to ask the customer to use 87 or 89 octane fuel in the future. I have been using 91 octane so far.

The mechanics I go to are friendly and not hostile towards my attempts to understand problems with my vehicles. They are quite experienced and I trust them. But I also know from experience that this engine is a challenge to understand and that some problems can defy the best of them. Some DTCs can mislead in weird ways if vacuum hoses are leaking. Shortly after the engine rebuild I hunted down all the vacuum line problems that my vehicle had and changed all hoses to silicone hoses and brass tees, except for the very large vacuum hoses. The reason I was able to get into the location under the supercharger and change hoses there was due to a warrantee-covered service on the rebuilt engine (it was leaking oil from a location above the serpentine belt) and before putting it back together they let me come over and change the hoses. Probably a gasket could have been changed at that time, but being quite new gasket-maker was used instead. You can find other posts I'm made to this forum if you desire to be entertained by reading further about the problems I used to have that were related to my misrouted hoses (engine stall right after every gas-up was the weirdest - could make a good April fools joke).

The report back from my local mechanics was that after doing the engine decarb that I'd asked them to do, they took a look at the spark plug holes in front (cylinders 2-4-6 I believe). They didn't look at the spark plug holes in the rear because they are much harder to access. In the front spark plug holes, however, there was about 1.5 inches of oil. The rubber boots/coils didn't appear to have broken down because of the oil, but they suspected that the oil had gotten in there because of a leak in a gasket (I think they said it was likely to be a valve cover gasket). They gave it a 5-mile test drive and the same thing basically happened as on my shopping trip - there was some rough-running due to misfires and P0303 came back on, with no others. They suspect that the electrical pulse in cylinder 3 is sometimes finding an alternate path to ground because of oil surrounding spark plug #3 . They said that oil in spark plug holes is not an uncommon problem for many engines. Over time oil will cause the rubber to break down that protects the spark plug connections. In my case it seems likely that the cylinder #3 spark plug connection is probably the worst of them. Possibly it has an intermittently-bad coil-on-plug also, and they advised changing all the plugs and that one coil. They recommend replacing coil on #3 because the labor to move it and see if the problem moves with it will cost more than the price of a new coil. Extra labor is required because the intercoolers and other stuff need to be removed to get to the coils, and afterward put back on before the car can be driven and tested for a misfire.

I do not think the mechanics removed a spark plug to check its condition. They are NGK brand and were new 38K miles ago. As for the oil in spark plug holes, I didn't ask whether that could also happen from spilling the oil while doing oil changes. I change my own oil and often don't bother to use a funnel. I have a pretty steady hand but sometimes I miss and a little oil doesn't go where it should. Normally I put some paper towel around oil filler neck and I've assumed I catch almost all spilled oil that way. Also, the gas tank is full of 91-octane fuel so until I put quite a few miles on it I cannot try using lower-octane fuel as recommended by TSB 0102702 (actually, I'm not absolutely sure about applicability of that recommendation to 2.3L and/or 2.5L engine). Only about 1/4 of the fuel is now a few months old. There could be some water in the gas I suppose from the tank being low and condensation from significant variations in outdoor temperature and high humidity conditions during rain and snow storms.

For now, I've decided to long-term park the car rather than put money into it. I'm interested to know what Millenia-experienced forum members think about this problem and what options I have to deal with this misfire problem. The engine does start normally and run. If I drive the car just right it seems like I can keep the engine from misfiring (such as putting the transmission in neutral when at stop lights and driving no faster than 45 mph or so). None of the other emissions-related codes have come on (like P0421) to indicate that the engine is not running clean/correctly. I don't want catalytic converters to be ruined because of this problem. But if this problem is bound to spread to other cylinders and get worse quite soon, then perhaps I should bite the bullet and take care of it now if I can find out with high confidence what the problem is (or problems are, as the case may be).

If you've read this far, then thanks. This forum has been a huge help in the past. Finding this site and getting help here was largely responsible for preventing the purchase/rebuild of this car from being a total financial loss. It has caused me to spend extra time compared to the other cars I have owned and the result hasn't been great, but I've learned a lot and it hasn't been a financial disaster. Plus I've met some nice people who are smart on mechanics .

Last edited by OldMazzies; 03-18-2017 at 02:38 PM.
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