1991 Suburban 350 TBI Misfire

[Chantzzzzz]

I'm looking for a second opinion on an issue with my Suburban: it's got a nasty misfire/backfire when it's warming up. I've got suspicions as to the source of the problem, but I was hoping to get some other opinions as well, since I'm going to have a little bit of cash to fix it up soon, but not enough to do a lot of experimentation.

Basically, the problem starts once the truck has been running for a short time. No trouble starting or anything, but when it starts to warm up, it pops and splutters (and more recently, backfires) out of the exhaust. It's dual exhaust, so I know it's coming only from the driver side bank, and likely from the same cylinder because the time between misses seems consistent. Eventually, once the engine has warmed up a bit more, the popping/backfiring stops, but the misfire will continue to come and go.

This has been getting progressively worse over the past 16 months or so, and it's frankly pretty embarrassing. The engine only has about 250k KMs on it and seems strong otherwise (good oil pressure, strong starting, pulls well). At this point, the truck has had new plugs, cap, rotor, wires, coil, ICM, ECT sensor, O2 sensor, fuel filter, fuel pump, and alternator. Seems to me to point to something internally wrong, but hopefully someone can offer me up some opinions? Thanks!

[Keitha]

Do a compression test. Could be a bad valve.