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Catalytic meltdown

Catalytic meltdown

Catalytic meltdown

How lean is too lean for a modern (non-lean burn) car and how rich is too rich? Reason I'm asking is my new 4 cylinder Mazda 3 is going through catalytic converters like shit through goose. The dealer has been unable to solve this problem. Looking at the fuel mixture and its ECU, Its WOT fuel mixture is set at mid 11.5:1 air fuel ratio and part throttle is stoich 14.5:1. I want to do my best to keep the cat safe and prevent meltdowns. Do you think it's set to run too rich? Would 12.5:1 be a safer ratio given I retard spark to prevent knock? thanks...

RE: Catalytic meltdown


Which engine? Skyactiv 2.0 litre?


Unless you are driving like a madman, it should be running at stoichiometric the vast majority of the time and it should be designed to handle that, and it should not cause a problem.

Something else is wrong.

RE: Catalytic meltdown

If this is a vehicle within its manufacturer's warranty, this is not your problem to solve. Nothing you could do as a operator should result in such failures (aSSuming neither you nor anyone else has tampered in any way with the engine control).
If you are not able to use the vehicle as a trouble free black box appliance but rather are being inconvenienced with check engine lights, failure to pass emission tests required for registration, or any other issue related to engine malfunction, it is the manufacturer's responsibility to resolve such issues whether under emissions warranty, implied warranty and/or "fitness for purpose".

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

RE: Catalytic meltdown

Lemon law.

RE: Catalytic meltdown

It sounds like they have an issue with pre-heat or air injection control.
Is this a common problem with these engines?
Those ratios look perfect.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

RE: Catalytic meltdown

" Would 12.5:1 be a safer ratio given I retard spark to prevent knock? thanks..."

So, you modified/flashed the ECU? Far as I can recall, the Mazda 3 does not have a spark advance lever (like a Model T), nor any other way to tweak spark timing outside of warranty-voiding mods.

RE: Catalytic meltdown

what I have seen done is place a small copper coin shape in an strategic place in the air injection path and retighten the flange . nothing visible as a change and problem solved .

A tidy mind not intelligent as it ignors the random opportunities of total chaos. Thats my excuse anyway

RE: Catalytic meltdown

If you've messed with it, you own it. If not, there will come a point where you take advantage of lemon laws.

RE: Catalytic meltdown

My EFI engines run like crap when there is an air leak between the MAF and the throttle body; even a rubber elbow's edge folded under a hose clamp, and all but invisible.
Otherwise, they run great.

Mike Halloran
Pembroke Pines, FL, USA

RE: Catalytic meltdown

Original poster hasn't filled us in on what's going on here. As far as I know, the Mazda Skyactiv engines do not have a reputation for eating catalytic converters, and that's with most ordinary owners driving them with no knowledge whatsoever of ignition timing or air/fuel ratios and obviously without having messed around with them. A friend of mine used to work at Mazda Canada head office (until about a year or so ago) and would know about this if it were really an issue.

The Skyactiv engines don't have EGR or secondary air injection, and that's common nowadays. Where necessary, the EGR and secondary-air functions are accomplished by playing with the variable valve timing.

RE: Catalytic meltdown

Isn't the cause generally rich conditions that will overheat a catalytic converter?
Where those numbers from fuel trims? And how do you know they are accurate?
What things have been done to try to fix the problem? Are sensors reporting correctly?

RE: Catalytic meltdown

sorry for late response - did not realize this thread would gather this much responses. The car was unmodified with only rust proofing done. It's a 2017 2.0 skyactiv with 155bhp. The dealership at first voided my warranty for rust proofing but once I contacted Mazda Canada and they continue with supporting me with warranty work. The whole exhaust system was replaced. The failure of the catalytic was such that you could find pieces of it in the second catalytic, the resonator and rear mufflers. After the replacement the car felt a lot more powerful. However Mazda is not able to tell me what caused the issue and I have a coworker who experienced the same problem with an older skyactiv Mazda. I know the skyactiv goes through a very rough cold start which is cuts spark (it's like an induced misfire to warm up the catalytic). I have an OBD2 reader which I monitor my fuel economy and other stats with and I always see the car running slightly rich with -20 to -30* ignition BTDC or 30* ATDC timing on startup. Also during full throttle it logs fuel ratios as rich as 11:1. From what I gathered from my research, the front O2 sensor is actually a wideband as with most DI motor cars, so the readings should be accurate. I don't want this catalytic meltdown to happen again and since Mazda is unable to determine the cause I don't mind fixing it myself. I drive a lot of stop and go and the car cold starts 2-4 times a day in very cold temperatures (-20c tempratures).

RE: Catalytic meltdown

At that temperature, you could have cracks in hoses or other rubber parts. Even very small leaks can cause trouble. Worst is the kind that only open up with engine movement/vibration.
Very close visual/tactile inspection, the kind that dealer techs do not have time for, might be worthwhile.

Mike Halloran
Pembroke Pines, FL, USA

RE: Catalytic meltdown

I'd make sure the sensor is true. Then suspect the ecu mapping for startup etc. And that induced warm up sounds strange and is just the thing to cause problems. Like I said rich conditions are not good and what else could be happening to the converter by allowing raw fuel to accumulate every cold start? I wonder how prevalent the problem is with that car or engine?

RE: Catalytic meltdown

You've had one cat failure and you say the car is going through converters like shit through a goose?

Was the converter melted or did it just break apart?

RE: Catalytic meltdown

the first time i don’t know what happened to the catalyst but the car was only 1 month old from factory and i had check engine light. took it to dealer and they told me a catalyst was replaced. The second time (now car is 6 months old) the failure was so bad the car could not accelerate and you could hear rattling in exhaust. Dealership claimed it had broken apart inside. No the ECU is fine if you look it up skyactiv cold start retards spark to -20 btdc or less for 20 seconds to warm up cat. I believe also this is the cause of the issue and not the overly rich WOT condition. my car is always parked outside and i do a lot of cold starts here in canadian winter. I wish this catalyst warm up could be disabled. Also we don’t have lemon laws in Canada.

RE: Catalytic meltdown

The delayed ignition timing on cold start is built into the ECU which means ALL of those cars are like that. This is intentional in order to raise exhaust temperature, and that's intentional, in order to assist with catalyst light-up, to reduce cold-start emissions. Mazda is far from the only manufacturer to use this strategy, "they all do it" probably isn't far from the truth.

To my knowledge, there is not a pattern of catalyst failure on these cars. If your failure were really due to the delayed cold-start ignition timing then they would ALL be failing in this manner - but they're not. Something's wrong with your particular car; it just hasn't been found yet.

It's not the ECU programming, because that's the same on all of them.

How's your fuel consumption? Manual or automatic?

Do you drive it gently during warm-up for the first couple of minutes, or is the accelerator binary (0 or 1, off or on)?

Which part of Canada are you in? Mazda has a healthy market share in Quebec. It gets cold there. Still no pattern of catalyst failures there, to my knowledge.

If you're on facebook, I can put you in contact with someone who until a year or so ago worked in the technical department of Mazda Canada (he has certainly dealt with the Skyactiv powertrains). He's moved on, but still probably knows a thing or two about them. Failing that, I can be the intermediary here in the conversation. I will link him to this thread.

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