Diagnosing a 3 month old flowmaster cat that melted during street driving

[Peertwelve]

I bought a 1990 325i in July 2021, have been slowly fixing / restoring it since then. Car came with a rattling cat, which was replaced on 4/2/2022. A flow master cat for the m20 was installed. All was well until 2 weeks ago.. when the new cat started rattling like the old one. Took car back to shop today, where they cut it off and said that it was melted. Asked if I tuned the car. I said no. Asked if I tracked the car, I said “not yet”. Could a tune cause this? Any advice on how to diagnose the air / fuel ratio at the cat?

[moatilliatta]

Seems like a lot of them burn up.

If the runs well I wonder if oil burn of high zinc oil could do it.

Maybe look into another brand?

[Peertwelve]

Seems like a lot of them burn up.

If the runs well I wonder if oil burn of high zinc oil could do it.

Maybe look into another brand?

Thanks for the reply. I haven’t noticed it eating / leaking oil, or any exhaust smoke. Will keep this in mind.

[jbontke]

I had two cats do this on an LS 5.3 engine. They were cheap cats. Are you buying a quality cat (will pass smog test) or a cheap one?

[Peertwelve]

I had two cats do this on an LS 5.3 engine. They were cheap cats. Are you buying a quality cat (will pass smog test) or a cheap one?

The shop recommended the dual pipe magnaflow cat. They replaced the melted one ( new in April 2022) with the same model, hence this thread.. Could an aftermarket performance chip boost the temp that hot? Any way to monitor air fuel ratio via the obd1 connector? I do notice a bit of burble when coming off the gas, clutch out and engine braking. Not sure how to monitor this antique!

[jbontke]

Cats are made to run hot. I don't think the temp ruining the ceramic is the issue, it would be the glue possibly? I have never had OEM cat rattle and they will last a long time with a decent running engine.

[moatilliatta]

If you scour the internet regarding that style of cat youll find the same results as you. Maybe try the Carb compliant one as mentioned above.

[reelizmpro]

Running rich or too lean will kill an aftermarket cat. I've blown through several carsound (magnaflow) cats in my day. Assuming your car is stock, check the DME to see what chip is installed. If an aftermarket chip is installed, make sure you're running premium fuel. Then make sure your oxygen sensor is installed properly and in good shape. Many forget that they are service items and should be replaced every 60-80k miles. These will solve most issues but there's also fuel pressure, Air flow meter, etc Only way to know your exact AFR's is by using a Wideband oxygen sensor. Get your AFR's right before you spend money installing another cat or the same thing will keep happening. Goodluck.

[Peertwelve]

Running rich or too lean will kill an aftermarket cat. I've blown through several carsound (magnaflow) cats in my day. Assuming your car is stock, check the DME to see what chip is installed. If an aftermarket chip is installed, make sure you're running premium fuel. Then make sure your oxygen sensor is installed properly and in good shape. Many forget that they are service items and should be replaced every 60-80k miles. These will solve most issues but there's also fuel pressure, Air flow meter, etc Only way to know your exact AFR's is by using a Wideband oxygen sensor. Get your AFR's right before you spend money installing another cat or the same thing will keep happening. Goodluck.

Outstanding thank you!