Volvo :: 740 Engine - Rough Idle / Fair Amount Of Carbon At Exhaust Pipe

I've got an '88 Volvo 740 Turbo Wagon: 5-speed, 225K miles, that's been acting up over the last year. She doesn't burn oil (qt in 1500 typical) and appears to still be tight. The Turbo works and bearings make no noise. The engine service light does NOT glow (but I admit I haven't checked the bulb). I do have a Haynes manual and have been using it to assist with some results (see replaced items), but I'm not trained on this car and this manual is woefully lacking in good diags for some obvious things. (IMG: [URL] .....)

Symptoms: She cold starts OK, but try to restart warmed at all and she usually has to sit at least an hour or no go. Outside temp seems to make little difference. There's a gassy smell while cranking warm and it's very much like a flooded carburetor engine. Often, the engine will finally catch with the pedal floored after a long crank. Also, after warming and driving a few minutes, the idle erratically will not return to normal, sitting up at 1800-2700 RPM for a bit before finally calming down. It was also running a little rough. There's a fair amount of carbon at the exhaust pipe and MPG is only ~20 and I'm a light foot. The idle RPM varies a bit much moving around between 600-1100 RPM warm.

What I've done this year: It had a broken thermostat (replaced), failing alternator (replaced), failed starter (replaced), failing fuel pump (replaced with filter), recent oil change (has turned black after 800 miles). Most grounds around engine and firewall and 12 volt ignition contacts cleaned. The bat cables were cleaned and terminals coated. Ohm tested the injectors OK (all exactly the same, on spec) and they all "click" in operation. I replaced a bad coil (low ohms by 20% for both primary and 2nd) and changed the carbon fouled plugs (which were also ill-gapped by some hamhand), both of which worked for the roughness considerably. The plug wires all came up roses well under 10k ohms. The battery was a suspect for testing low so much when resting, but the alternator seems to have slowly fixed that (and problems come when system is charging OK). However, the main warm start and idle problem persists. (IMG: [URL] ....)

My next good guess: An obvious rich condition exists and, having eliminated quite a few other things, a sticking "Air Control Valve" seems the likely suspect (some call it an idle control valve but I'm going by the manual). My specific observations of this device: after a warm no start, I pulled the ACV electrical lead and she fired right up and idled well at 16-1800 RPM. I plugged it back in and she chugged and nearly died before idling ~800 RPM. As it idled normally, I pulled the rubber hose from the upstream air connection to the ACV and it chugged and died within 5 seconds. I noted the vacuum was very strong. Putting the hose on, it wouldn't restart till I removed the electrical lead again. The ACV circuit has continuity at the device but I haven't ohm tested it yet. My best diag is the mechanical part of the ACV is gummed up or worn out. The Haynes manual was almost no way in arriving at this conclusion. (IMG: [URL] ....)