89 C1500 5.0L Backfire

My 89 C1500 (305) with 133K miles recently started backfiring upon startup. As it idles, it will 'pop' occasionally (not a loud backfire) until it warms up. Today I noticed the same sound as I let off the gas from 55mph. I burn about a quart of oil every 1500-1800 miles. I removed the catalytic converter this weekend, as I thought maybe it was plugged. Apparently, that wasn't the problem. Any thoughts or help would be greatly appreciated.

When was the last time you gave your truck a complete tune up? Have you checked your timing? Whe you check the timing on this truck you need to make sure that you unplug the esc wire (brown wire black stripe)under the relay cover on the right side fire wall in side the engine compartment. Your timing should be a 0 degress before top dead center. You will trip your check engine light, so when you are done timing turn off ignition, reconnect the esc, then disconnect the negative battery cable for 60 seconds, reconnect and check. If you are still having problems, look to see if you have a leaking injector. Hope this helps.

Horse - thanks for the info. I'm new to the forum and find it to be a great resource.
I guess it's about time for a tune-up. I'll find time this weekend to do it.