1996 Volvo 850 GLT Really Rough Idle and Starting Issues
 

I have a 1996 Volvo 850 GLT, with well over 230,000 miles (odometer stopped working around 2014). My car had started idling rough and not wanting to start. It would have RPMs all over the place, and would lose most RPMs and wanted to stall. And when I would be driving if I let off the gas and then pressed the gas again, it would lose a lot of RPMs and then quickly kick back in again. Especially when it was at a complete stop at a red light or stop sign. Now it's not wanting to start, and when it does, it starts with low RPMs and takes a little bit for the engine to get up to regular idle RPMs (about 1000 RPMs). And doesn't really seem to suffer from wanting to stall when stopped, or when giving gas at a complete stop.

I have replaced the oil cap as it had quite a bit of play in it. And have as well found a hose that was missing its clamps, which I replaced. And that at first seemed to fix it, but it started doing things again. I have trouble codes P0172 and P0116. I do not know what could be the problem. My main guesses are mass air flow sensor and/or a vacuum leak, but I can't really find a vacuum leak, if there even is one.

You can't necessarily see the vacuum leaks in your PCV system.

The P0116 is telling you your engine coolant temp sensor is out of range - possibly a bad sensor, wiring or stuck thermostat. The sensor is located under the thermostat housing - just follow the upper radiator hose back to the block and look under the hose. You can test the sensor by undoing the connector then measuring resistance (google for temp charts to determine cold vs warm values)

To replace, remove the thermostat cover (two torx screws, leave the hose attached) to give access to the sensor. you can pull it out without draining the radiator - you'll lose about a cup or two of coolant). Replace the sensor, the thermostat and the overflow tank cap.

With that done, then you can clear the codes to see if your P0172 code returns. If yes, then I'd try cleaning the MAF and check for vacuum leaks, particularly around the tree...