Can you take a screw out of a Cadillac converter and blow the corrosion out corrosion out?

What is your question?

My car was not driving over 30 mph, and was backfiring. The guy who sold it to me said it was the spark plugs, and that he would fix it for me. So I took it back to him and he did some work under the front wheel. The car now drives great, other than the noise. When started, it's really loud like a race car then slowly quietens. But each time I step on the gas its loud again then gets quieter after rushing the gas for a minute. The exhaust is terrible. Enough to knock the block out. The guy who's fixing it said he took a screw out of the Cadillac converter and poured some kind of thinner into the gas tank, then said hopefully it will blow out the stuff that's clogging it. Is this possible or is he not telling truth?

What makes this problem better or worse?

Pressing on the gas does both makes the noise eventually goes quieter but the exhaust is terrible

How long has your 2003 Ford Explorer had this problem?

Since I purchased the vehicle from an independent owner