94 Toyota pickup still running strong at 210K+ miles, started noticing gurgling noises from the upper radiator hose and an empty reservoir. I filled the reservoir and continued to drive, noticing the temperature gauge has not shown the engine more hot than usual

Today the coolant was visibly sprayed (most likely) from the radiator cap with more audible gurgling noises from the upper hose. When setting off toward the evening, I noticed the idle was about an octave higher (no tach .. probably toward 2K rpm). Luckily I had my bike, so I parked and rode home.

Seems like possible symptoms of a clogged overflow hose .. no change in reservoir level, pressure release from radiator cap, intake of air as the system cools. Are there many other explanations? Needless to say, I would be out there looking at the overflow hose now, but the truck is around the corner from work.

What could be causing the increased idle? Also, once the system is able to regain access to the reservoir, I imagine it being able to bleed air free of the system (with the necessary refills of the reservoir). I haven't exchanged the coolant in this vehicle for 90K miles, maybe it's about time?

 

Sounds like a bad radiator cap. It's allowing air into the radiator, instead of drawing from the recovery tank. Easy enough to check the hose. Pull it off both ends, and blow through it. Easy enough to replace, if in fact it's bad, but it sure sounds like the radiator cap to me. Make certain it's not the filler neck, too. If it's cracked where it meets the radiator it will act like this, too, and if it's cracked, replace the radiator. It's actually less expensive than getting the radiator repaired at a shop. I just checked around here, that's how I know.
If it's not drawing fluid from the recovery tank, no it won't burp the air out of the radiator. If you're hearing air burbling in your system, you've got trouble. Refill the RADIATOR, not just the recovery tank!
Remember, the gauge will only indicate what the water on it is, and if there's no water touching the sensor, the gauge will not read anything resembling reality.

Once you get your trouble cleared, though, YES. Flush and clean the system, and add new coolant/distilled water mix. Don't reuse the old, although it sounds like you're doing the "rolling water change". Use the Toyota red coolant from the dealership, not the general purpose green stuff. It's made specifically for our iron/aluminum engines, and reduces the differential metal corrosion, unlike the green stuff. And always, ALWAYS use distilled water, not tap water, or you'll get corrosion through your entire system. Promise! NOT fun trying to get it out, either.

You can use regular, garden hose type water for flushing, but make sure it's totally drained out before you do your final fill. Quick, easy way to drain it completely is to pull the bottom radiator hose off. Be careful, there WILL be water in it, and it WILL drain all over your face. I know, believe me. I swear (alot. Former Marine) I've drunk gallons of radiator water over the years, since I flush and refill my system every spring. And the hoses. Every spring.
Replace the radiator hoses, as well. Important! Remember that there are 3 main hoses. Top, bottom, and the jumper from the bypass pipe the bottom hose goes to, into the water pump, up under the power steering pump. Replace all 3. If there's nothing wrong with the old ones, throw them in the back, in case something does go wrong. Nice to have emergency spares on hand when you need them. Works for belts, too.

Finally, once you get all done with the whole flushing and filling of the system, burp it. Park on an uphill, or ramps, after filling the system, nose high to make the radiator cap the highest point in the system, put the cap on loosely, just the first 1/4 turn, and warm the truck all the way up. It will bubble the air out of the system into the recovery tank, and overboard from there. Let it run 20-30 minutes to make sure you get it all. Then, wearing a heavy glove, tighten the radiator cap down as normal, and let it cool. After it's cooled all the way down, check to see if the radiator, AND the recovery tank, need more water. Top them off, again, distilled water, or premixed water/coolant. Always try to keep a 50/50 mix. Don't just fill it with water OR coolant. If you can't find premixed, premix your own. Get a gallon of coolant, and a gallon of distilled water. Pour 1/2 of either one into an empty gallon jug, top it off with the other, and voila! Premixed at 50/50. If you put it back into either, or both, original containers, make sure to mark them as 50/50 mix, or premixed, or something so YOU know what's really in them.

Good luck, and replace that radiator cap asap.
Pat☺