"I bought used downpipes thru muffler...He didn't use the downpipes but cleaned out the existing ones." Do you mean the muffler shop gutted the pre-cats, and told you it would be the same. Or you already had downpipes installed, the muffler shop cleaned them, and then reinstalled them? If the muffler shop told you that gutting the pre-cats would be the same as installing aftermarket downipes, they're full of sh*t. It sounds like you have an exhaust leak either way.
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