Diagnose that engine trouble!

So, I'm out and about with the kids today, and what started off as a barely perceptible miss coming off idle developed into a frequent miss/stumble under load. Before I got back home, the check engine light was flashing periodically.

Vehicle is a 2005 Mazda MPV with the 3.0 Duratec. Mileage is just south of 90k. Everything is pretty much original except the oil and oil filter.

So, what say ye? Coil pack? I'm about to put a code reader on it now.

Fuel filter?

Coil pack is my first guess but that means nothing without actual diagnosis.

did it feel uneven as though only one or 2 cyls were missing, or did it feel like every cyl was missing evenly? Generally the first is ignition or fuel and the 2nd is fuel only and would be accompanied by low power before the missing starts.

Does it get worse as the rpms go up? Will it start to do it at higher rpms with no load in park?

Code reader should help in this case because it's definitely bad enough to set misfire codes.

Yeah, based on hearsay, I was leaning toward coil pack. Code reader shows one code: P0303 - cylinder 3 misfire. So that pretty much supports the diagnosis.

It felt very uneven and unpredictable, like a single cylinder misfiring sporadically.

I'm leaning toward coil packs too. Seems to be a common problem on these, especially on the rear bank (which is just about impossible to get to.)

Sounds like a miss-fire. Based on my recent research of that engine in the Mazda 6 coil pack or plugs would be my guess. You should get that fixed, the 6 melts cats if you drive it much with a miss.

So.... Should I pay 50% more for the dealer part, or go aftermarket? For time-related reasons, I can't order them online. Is one aftermarket brand better than another?

I think I'd start wiht the spark plugs but thats me

Also could be a leaky vacuum hose.

evildky wrote: I think I'd start wiht the spark plugs but thats me

Me too. Plugs are throwaway items, well before needing coil packs. It already has 90k on it, they are due for replacement. Now if the boot is burned or the coil looks exploded, it warrants replacing TOO.

Well, these two facts are influencing my thinking:

1) Coil packs are a notoriously well-known issue with these vehicles. 90k miles is an extraordinarily long life, well beyond the norm.

2) It is necessary to pull the intake manifold in order to get to the plugs and coils on the rear bank. I don't plan on doing it again soon.

Well if you were in a shop and you didnt want to screw up the diagnosis and have the customer mad at you you'd probably ohm the injector and coil and check their supply voltage at the closest accessible point in the harness, do a pressure drop test with a timed injector opening compared to the other injectors, use a low-amp clamp hooked to a scope to compare that coil and injector's 'signature' to the others on the engine (and make sure the signal isnt dropping out), and check the condition of the accessible plugs because they probably all look the same. You might even be able to get a stethoscope lead onto the injector at some funny angle and compare it's sound to the others.

And if that didnt lead you anywhere else, by process of elimination you would recommend a coil and all the plugs just because, and once you were there might do a compression check on that one cylinder just as a CYA.

If shops just replaced the coil because they were notorious, mechanics would end up being notoriously mistrusted or something. Oh wait! lol

Yeah, but one of the joys of doing your own work is that there's no one to blame but yourself. I won't brag, but I've found that in the 30 or so years I've been messing with cars, I've done pretty well just jumping in and doing stuff. I've seen enough different problems and worked on enough different cars to have a pretty good sense of what goes wrong.

Right now, I'm leaning toward changing 6 plugs and the coil on #3. I've got a credit that's fixing to expire at RockAuto, so I'll probably spend it on coils and keep them for spares for when the rest of them go bad. Not planning on buying a new car for at least another 100k miles.

Change 6 plugs and the 3 back coils. Taking that intake off in 6 months to fix number 1 or 2 is no fun. At least you'll have 2 known good spare coils.

If you're going to buy at least 3 coils anyway i agree with this idea of replacing them all and keeping the known good as spares.

I dont disagree with your approach. In all likelihood your idea will work since it's your own car and the stakes are low. That's how murphy's law works.. lol I just wanted to throw out some more testing possibilities and the idea that it COULD be something else, regardless of how likely it is to be the coil.

Sorry to keep everyone on the edge of their seats for so long!

The patient is doing well. Went ahead and did all six plugs (duh!) and the three coils that live in the rear bank under the intake manifold. The write-up on MPVclub.com was a fabulous reference. Not the easiest spark plug change I've ever done, but all in all, not a terribly difficult job. The cylinder #3 misfire is gone, and it's probably just my imagination, but the van pulls like a freight train now, and I'm pretty sure it would beat a Z-06 in a drag race.

Lol! Congrats! Yeah those 3.0 mpvs are no slouch. I personally like them a lot although i would slightly prefer to have that body style with the mazda 2.5. That's just being overly picky/wistful though.