GM 3.1 Backfires into the Intake

Hey guys,
I've got a 1995 Chevy Lumina 3.1 that is backfiring into the intake. It make a loud poping noise and if you listen with a stethoscope you
can hear it really loud on the intake. I've checked compression on all the cylinders (good) and made sure that none of the wires were crossed. Everything looks good. Can't hear any vacuum leaks but I haven't checked real hard for that...
It only happens on the No. 3 cylinder. If I pull the spark plug on #3 the backfiring goes away. I thought maybe I had a leaking injector. From what I understand the coil packs fire both cylinders every time so I thought maybe the injector was leaking and igniting fuel into an open intake valve. I tried replacing the #3 injector but no change.
The pinging is consistent but not very rhythmic. There are no codes on the computer if I remember correctly. What are the chances that I poped a tooth on the timing chain? I can't see that affecting just the #3 cylinder. Any other ideas what is causing this?
Thanks!
Travis
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Pull the valve cover and confirm valve actuation, a broken rocker stud or bad cam lobe can cause this and the compression would still show good.

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I'll pull the valve cover tomorrow. Thanks for the ideas guys.
Travis
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Just an update. I pulled the valve cover and there was a broken rocker arm causing the problem. The bottom was broken out. Do you think that it was torqued too high when I had the intake gaskets replaced? (2 weeks before this happened)
Thanks again for your help!
Travis

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Could be a weak valve spring couple with a clogging Catylitic converter.
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