I bought a 1998 2.5L Millenia back in March 2013 in GA. I had no problems with it, started routine maintenance and enjoyed the ride for a few months.

Here's a run down of what I've done to it since then. Scroll down for the current problem which may or may not be unrelated.

Fast forward a bit, I moved back to Indiana, and hauled my Millenia back with me. I had my oil changed in GA and they told me I had a small oil leak. I had it changed when I got to Indiana in June again and they said it was a larger oil leak. I knew I was losing oil somewhere pretty quickly.

Mid-June my serpentine belt gave out on a back Hwy. I was towed to Firestone, had them put in a new serpentine belt and timing belt kit. They found the source of my leak and replaced a few gaskets. I assumed all of this was regular maintenance that had been neglected. The car only had 115K miles in it. I figured it sat in a garage most of its life.

I went ahead and had new tires put on, changed out the upper and lower control arms on the front and back and replaced some link pins. Had my oil changed again in September and didn't have as much loss as before.

Here's where the current problem starts.

So my starter goes out about 4 days before Christmas. I'm stuck in a lot that has an Autozone so I replace the battery, it still won't start. Have plenty of gas. No click click. No nothing when the key is turned. I assume it's a starter so I had it taken out and refurbed for about $80. I picked it up after the car sat for a week, it started up fine and away I went. I fill up w/ gas, park the car at home and change to go to work. When I get to the end of the street after restarting the car, my CEL comes on. By the next stop sign it's blinking and I'm clearly misfiring and idling around 500 or lower. I stop the car, start again and the CEL remains by the car idles smooth at about 750-ish RPMS.

I get the code read as a P0304. Misfire on Cylinder 4. The guys says take it for a tune up.

Well, I changed out the spark plugs. Had a little oil in Cylinder 4 but not a ton. Spark plugs were WAY bad. Acceleration is better but misfire is still intermittent. Usually after what I've seen referred to as a heat soak. Never on a cold start. I could start it up, have it misfire, turn it off, restart and it'd be fine. Never took more than one or two restarts to get it going smooth. Drove it to work and around town without a problem other than the restart misfiring.

My gas mileage is WAY down. It may have been this bad before my starter was refurbed, but I don't remember it eating this much gas. My exhaust smells a little rich to me too. Someone said to swap my Cyl 6 and Cyl 4 plug wires, but would that have such an issue on gas mileage? Someone else said clean my MAF sensor and check it and my O2 sensor which it is currently too cold for me to do this week.

Today it's 10 degrees, I go to start the car and for the first time since this began, it misfired and low idled on a cold start and no matter how much I turned it off and back on again, the misfire wouldn't go away. I had to drive it the 3 miles home and it smoothed out and stopped misfiring(CEL stopped blinking) but whenever I would break to come to a stop, it would idle low and misfire again.

I'm frustrated now. I'm learning more about cars than I ever wanted to have to know. But now I'm afraid I'm going to burn out my catalytic converter and I want to find the source of this issue before I have to spend $2000 replacing that.

Thoughts on what I should check next? Cheaper fixes/tests I can use to weed out potential problems? I'm more in favor of shotgunning different possible solutions provided they aren't horrendously expensive. If I can't get this solved by the time my tax money comes in, I'll be taking it to someone who can just fix the damn thing for me but I'm hoping not to burn out my cat before then.

Best Regards,
Vada

 

you checked the spark plug in cylinder 4. was wet. are you sure it was wet with oil or was it wet with fuel? that is important. fuel, it could be a faulty injector or a bad spark plug or coil. oil, it could be a bad valve seal or bad piston rings. get a compression test, record the numbers and let us know. the starters job is over as soon as the engine starts running, so it has nothing to do with your engines performance. just make sure that #4 cylinder is firing and that it is not wet with fuel. how to know? feel, smell. it could be only a cable or ignition issue.

 

Thursday I am cleaning and testing the MAF and swapping my 6 and 4 plug wires and clearing and rereading the code.

Assuming it still throws a cylinder 4 misfire, I'll pull the plug on cylinder 4 on Saturday and see if it's oil or fuel for sure.

Is there a place I could get a compression test done or is this something I could easily do myself with the right tools? I can do that Saturday as well and bring back some numbers.

I just figured while they were in there trying to get the starter out since they access it top-down they may have moved or damaged a wire (that was likely damaged to being with if that's the case..they look pretty old).

 

The car being that old and that many miles it could be any number of things. New plug wires might be in order, especially if they are the original ones installed in 1998. After you swap wires between 4 and 6, have them scan the code again and if it's #6 missing now, you know it's the wires. I THINK a place like autozone can test the #4 wire if you pull it off and take it inside; they tested a coil for me a few months ago. Belts and hoses and even gaskets will simply go bad with age; it's not a defect.

The fact that re-starting usually corrects the problem makes me wonder if it's an ignition module.

If someone is quoting you $2,000 for a CAT replacement, keep shopping around. I had one replaced on a 99 Corolla for $280 installed.

 

I swapped wires 4 and 6 and now the rough idling is definitely MORE apparent. Not sure if that means anything..

Autozone is less than a mile away. Going to see if I can get a read on these wires and see what happens next.

My PCV valve was cracked when I got to looking at it so I replaced that and the gasket. Could I have had an issue with a vacuum leak?

 

YES

Damn Mazda! They never change the PCV valve! This valve should be changed every 3 years as far as i am concerned. When they fail you let extra oil go by and that's not good for the intake or exhaust!

If you have a Mazda, change your pcv valve! They don't always last as long as they say! This is my pet peeve!

Gonna copy and paste this again for the umpf time:

Missfires Can Be Caused By a Million and One Things,

Vac Leaks
Wiring Issues
Spark Plugs(Even Fresh Spark Plugs That Are Crappy Quality.... E3 And Such)
Ignition Coils
Alternators (Search EMI)
MAFS
Injectors
Injector O-Rings
Intake Gaskets
TB Gaskets
PCV Hoses
PCV’s
Malfunctioning EGR
Malfunctioning CAS
O2 Sensors
Aftermarket Intakes
Intake Hoses
Vac Lines
Timing Chains
Timing Chain Tensioner
Broken Cam Gears
Malfunctioning Cam Sensors
Incorrect Crank Gears(The I4 Has Like 40 Different Configurations For Whatever Reason)
Incorrect PCM Flash
Issues Within The PCM
PCM Wiring
Bad Diaphram Within The FPR
Issues Within The Fuel Pump
Dirty Fuel Filter
Dirty Fuel Lines
Pre-Cats/Poor Exhaust Flow

If the above gaskets/seals have not been changed, change them. Clean your MAF sensor, throttle body.
Change your PCV valve and probably that short hose as well.
Inspect the intake accordian tube for cracks. Inspect for vaccuum leaks.
Put in a good Fuel system cleaner: Red Line Synthetic Oil - Gasoline Fuel Additives - SI-1 Complete Fuel System Cleaner
Inspect wires. Re-set ECU after doing above.