Exhaust Noise Only When Accelerating After Catalytic Converter Replacement

I have a 2005 Honda CRV LX. I replaced my catalytic converter with an aftermarket one I found on Amazon (https://goo.gl/GTBnjD). The check engine light came on for the converter and I took it to a shop that confirmed it needed to be replaced. I had also been smelling fumes for some time which was probably due to the corroded connection at the back of the catalytic converter. And it was a little noisy. It came with a flex gasket for the manifold, but it didn’t fit. So I bought a better one from Autozone (https://goo.gl/cL4iAP). It seemed to fit well. The spring bolts came with the converter and installed without a problem. I also replaced the spark plugs (same NGK Laser Iridium that it had in it) and air filter while I was working on it.

When I started the car after I was done, it was much quieter while idling. Even while revving the engine in park, the exhaust sounded great. However, when I actually drove it, it was quite noisy while accelerating. Quiet while coasting. Only during accelerating did it make noise. I also don’t notice any exhaust fumes.

I’m stumped as to what the problem is. It sounds like an exhaust leak, but there are no other indications of a leak and it’s quiet as can be when the car is not in motion. Any thoughts?

You might try checking for an exhaust leak the old fashioned way. Stick a potato in the exhaust pipe and start the car. Don’t rev the engine, just start it. If you hear a hissing sound and the engine is still running you do, indeed have an exhaust leak. I’d suggest the flex coupling between the engine and the cat as you may have finished it off when you replaced the cat or possibly the cat’s seal itself is a mis-match.

If the engine dies right away with no hiss, then there is no exhaust leak. Restart and rev engine and the potato should come shooting out the pipe. If that happens, the likely problem is that the cat you bought is not as good a muffler as the original equipment cat. You choices then are, ignore it, replace the muffler or add a additional bullet-style muffler between the cat and the muffler.

Thanks! I’ll give that a try. There is no flex coupling per say. There is a donut/flex gasket that fits over the manifold pipe and the flange of the cat is pressed against the gasket with two spring bolts. The only thing I can think is that the springs are not stiff enough to keep the joint closed under the exhaust pressure when accelerating. I assumed that revving the engine while in park would produce similar pressure to actually accelerating, but maybe not.

How much muffling does a cat provide? Like I mentioned it’s much quieter than before at idle. So I don’t think it’s the muffling ability of the cat itself.

I have been thinking about this and I think the reason this is happening while accelerating and not revving the engine is because the noise is not he result of increased pressure due to higher RPMs. Instead it’s caused by excessive rotation of the engine, leading to the flex joint (manifold to catalytic converter) opening more than it should. My first thought is that it could be due to insufficient spring force. The new springs may not have the necessary stiffness to hold the seal under load. I may try adding some washers to compress the springs and increase the force. I want to be careful not to increase it so much as to transfer the force further down the exhaust system where is it less able to tolerate the flex.

It also could be that the new bolts are a little longer, or the new gasket is not as deep. Either way it would be the same solution: remove the excess slack from the connection.

It may be that the engine mounts need to be replaced, preventing the excessive twisting of the engine during acceleration. I don’t know much about that, but imagine it’s more work than I’m willing to handle, or want to pay for.

A third possibility is that the donut gasket isn’t tight enough around the manifold down pipe, causing the flex in the connection to open more than it would otherwise. I am considering using a bit of Permatex 80335 Muffler and Tailpipe Sealer around the gasket to help seal it up around the downpipe.

What did you do, any updates? I have a similar scenario

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Also,. My replacement cat is noisy. All the pipes up to the actual muffler were replaced but now car is much noisier…I might try wrapping the cat,. No leaks that I see

Wrap cat with insulating tape I’m thinking…

There is a leak in the exhaust, or the muffler is bad.

It’s a pleasant surprise: how much good work an independent local muffler shop can do for a very reasonable price.

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Just to be clear: if I’m experiencing greater noise, then the noise itself is the only actual “problem”, right? There’s nothing else to be concerned with? I just noticed the added noise when coming home from getting government-mandated emissions testing (my '97 GMC Sonoma failed emissions a few days earlier which is why I got the new cat) and it passed, so would that indicate that there isn’t an exhaust leak? Or if there is, that the leak isn’t reducing efficiency?

If you have noise you might have a leak, or the replacement cat might just have less restriction so it is less of a muffler than factory. Unless you check for a leak, you won’t know which it is.

And exhaust leaks can make you sick or kill you. That is why the outlet is far away from the passengers.