Originally Posted by extralite
Once warm at idle it began back firing again. But after about 30 seconds the backfiring stopped and the EGT and CHT of number 4 cylinder dropped away and the backfire stopped. So at least i think i can isolate the backfiring to number 4 cylinder because it stopped when the cylinder stopped.
Any thoughts on the most likely problem? Blocked injector? Stuck valves?
Look for an ignition problem on #4. The backfire is likely not a backfire, but an afterfire. When a plug intermittently doesn't fire, the raw air fuel mixture gets dumped into the exh pipe during the exhaust stroke. When that cylinder fires again, the hot exh gas lights off that raw mixture sitting in the pipe, creating the explosion you are calling a backfire (when the explosion is in the intake system and uncommon without valve timing or ignition timing issues as well as a variety of issues during the start phase).
Many ignition issues are exacerbated by heat and sometimes only appear once warm. As you get warm, the intermittent ignition failure / misfire starts and you get the afterfires. Eventually the issue becomes persistent and the afterfires stop and the temps fall like a rock because the plug isn't firing at all.
can't have a blocked injector that only causes issues when hot. Also, you would see real issues at higher power levels. Symptoms don't mimic typical stuck valve, if only noticeable at idle. Symptoms are typically reversed - problem appears when cold and goes away when warmed up. Hence the term Morning sickness.