Had poor compression on a 93 Bravada 4.3L, oil burner, quart every 500 miles, but only 70K miles on, poor valve seals. Fel-Pro took care of the valve seal problem. But for the low compression, poured in Seafoam in the TB, but not once, three times. Like the instructions say, kill the engine, let it sit 10 minutes, then start it up. Ha, did this at 3:00 AM in the morning on a dead county road, left a smoke trail ten miles long. but after this the compression was 155 across the board. Good guess with 195 K miles, could be a broken ring on that 35 psi cylinder, but could try the Seafoam and see if this helps. Been doing this for years, sometimes lucky, sometimes not. either time for complete engine overhaul or dump it. Also been adding a full can of Seafoam every 4K miles to a full tank of gas to prevent this carbon buildup, this is a 120 year old problem with HC's, one gallon of gas, 6 pound has 5.5 pounds of carbon in it. Recently got a new 2017 Hyundai Elantra, first time ever, read about adding a can of Seafoam or equivalent as part of the servicing. PCV was a welcomed addition when it came out in the late 50's, replaced that breather pipe whenever you slowed down, sucked road dust into an engine. Ha, first tool was a shovel to haul out all the sludge. But also is a chain reaction, more carbon on the rings, more crap going into the engine, wasn't too bad with carbs, returned to the base of the carb, but would build up like crazy under the intake valves, recall as much of 1/4" of carbon buildup. Even worse today, with AIC has to be returned to the TB input, builds up carbon there as well on the face of the fuel injector with affecting or even blocking there spray. Worse with TBW, jams the TB vane. Seafoam does help, no definitions on exactly what top tier gas is. SAE is finally talking about H2 either direct burning or fuel cells, but still having problems with air with 80% nitrogen in it.