Troubleshooting a miss or rattle - 2004 Mazda 6s wagon

The car is my daily driver; a 2004 Mazda 6s wagon, 3.0 v6 manual. It started to have what I would call a misfire where it sounded a bit rattly and had a small power loss under throttle especially evident at low RPMs and under heavy load (say, going uphill). It was getting worse and started doing it on the freeway, but it did not throw a code. I decided it was likely to be ignition related so I replaced the plugs and coils - they needed it anyway as they were quite aged. At first I thought it made no difference but it turns out to have helped. The car is running better with less missing/rattling on the freeway, but it's still definitely doing it.

That's the extent of my diagnostic work. I'm pretty pressed for time so haven't been able to poke around much. Any suggestions on what to check? I'm really thrown by the fact that the coils and plugs improved it but didn't fix it entirely.

Maybe a clogged catalytic converter? Just an idea that came to mind with the symptoms of rattly and power loss.

If it had a bad coil and was missing because of that it might have damaged a pre-cat. When the rear bank pre-cat melts pieces of cat material get sucked back into the engine through the EGR and can ruin the engine.

Hmm that is concerning. I'll have to think about it and figure out how to check out the precat.

I owned one of these beautiful wagons for a long time and sold it last year (217k miles and it was running great.) I agree with the pre-cat hypothesis. You may not want to drive it much until you are sure. These cars have a reputation for sucking pre-cat material back through the exhaust valves during deceleration which can make bad things happen.

I had a pre-cat failure on the front manifold and caught mine in time. It did throw a CEL for "catalyst efficiency" but I understand that they sometimes do not throw a code.

pappatho wrote: If it had a bad coil and was missing because of that it might have damaged a pre-cat. When the rear bank pre-cat melts pieces of cat material get sucked back into the engine through the EGR and can ruin the engine.

Was going to say the same thing. Hope that's not the issue.

Well E36 M3. It's been throwing the catalyst efficiency code since I bought it, on and off, and got advice from my mechanic to ignore it if it goes away. Is there a way to inspect the precat without removing it? I do have a new borescope camera if that helps. O2 sensor hole access maybe?

I've always wanted one of these. Looked around for a manual transmission one, but didn't find anything locally.

If I was able to find one, the planned first step was to replace the stock cats and exhaust and install headers. Not for power or sound, but for longevity of the engine.

Well from looking at pics of the manifolds, if there's access I can take the pre-cat o2 sensors out and stick the borescope in the hole and see the cats. If they're breaking up, the next steps for me are unclear. I don't have time to work on it - I have two car projects already on top of kids and big house and work - but don't want to pay a mechanic what it would cost to fix. I'm tempted to put it on the back burner and buy something else, for now. But that's getting ahead of things; I'll try to get a view of the cats tonight.

In my head, I've already bought a new daily driver - a pretty blue Volt I've had my eye on - and have put headers and fender flares and 245 width tires and a supercharger on this wagon. Seems like the reasonable course of action, right?

dculberson wrote: ...Seems like the reasonable course of action, right?

Yep.

In reply to dculberson:

Mine is having similar issues, how would you describe your power loss?

Not accelerating as fast or more like a hiccup while accelerating?

In reply to 1kris06:

It's not a huge power loss, and it's smooth, so it transitions from accelerating well to kind of slacking off a bit, then back to accelerating well once the rattling stops. That's why I initially diagnosed it as an misfire problem - one cylinder misfiring on a v6 can be tough to spot.

I hope to put the borescope on it tonight, but I'm not sure if I'll scrounge up enough time.

OK, I stuck the borescope down the front o2 sensor hole.

I initially thought it didn't look that bad but then

That looks like a pretty decent sized chunk out of it. I put the borescope down one of the front cylinders and didn't see any damage that I could spot. I do recall when I changed plugs that the rear plugs looked a little funky. I ran out of time so didn't have time to pull any rear plugs (requires upper intake removal, which isn't difficult, but I need to get to bed) or pull the rear o2 sensor and scope the rear cat.

What do you guys think, is this a failing precat?

The chunk from the edge missing is concerning. I can't remember if you need to take intake manifold pieces apart to get to the rear precat O2. It would be good to get a look at that.

This is what one of my failed cats looked like.

An update on this situation ... I think it was BAD GAS!! We don't drive a whole lot so a tank of gas will last us a month or two on this car, our commuter. So a bad tank would throw things off for me for a long time. I read a suggestion online that a "rattle" could be due to bad gas and thought, well, I need gas soon, might as well give it a shot. I filled up with premium from a different station than usual and the car was 99% better on the way home. I changed the oil and drove it on a test loop and it was 99.9% better.

I would go complain to the station I filled it at, but it's literally a crater in the ground right now as they tore it down and are rebuilding. Thinking about it that might be related - maybe they were leaving the tanks unfilled and some nasty stuff got pumped up that was previously floating on top of their tanks?

Anyway, I'll keep monitoring it but it's promising so far. This is my first experience with bad gas so I had no idea it would do that.

Good to hear it's not the worst case scenario.  Might want to preemptively delete the pre-cat, just in case

In reply to eastsidemav :

Agreed!

Hooray for easy solutions!