Burns lots of gas?

Park on a slight incline, shift to neutral, then see if the car creeps downhill on its own. If it does not, suspect a sticking brake. A common cause is rust buildup inside where the metal bracket is crimped around the middle of the four rubber flex hoses. That wheel will get very hot too after driving a few miles. The brackets can be opened up a little with a large pliers to remove the hose constriction. That will let brake fluid pass freely and allow the brake to release.

The only other thing is to check for a leak.

Your mechanic can connect a scanner to view the operation of the oxygen sensor. It should be switching between "rich" and "lean" a couple of times per second. If it is, the fuel is being burned properly.

The MAP sensor has the biggest say in how much fuel enters the engine. If there is a vacuum leak or the sensor is failing, it can report that the engine is under load and more fuel is needed, but you would notice poor performance too. Based on the oxygen sensor's readings, the Engine Computer can only modify the amount of fuel going into the engine by plus or minus around ten percent.

Other than those things, you might suspect it's overdue for a tune-up, but even then there isn't much to look at besides spark plugs.

Black soot is not uncommon. Your mechanic's scanner will also show short-term and long-term fuel trims. If those numbers are high positive, that means the Engine Computer is requesting more fuel than what was pre-programmed at the factory. If the numbers are high negative, it means the computer knows the mixture is too rich and is trying to resolve that and apparently not having any success. Your mechanic will try various tricks while watching those numbers to see what affects them in a good way or makes them worse. That will give him some clues as to the cause of the rich condition.

Then you'll need to borrow or rent a scanner that can display live data, but learning to use them efficiently is part of a two-year Automotive training class. That isn't really something that can be learned in just a few paragraphs. Often the various fuel trim numbers and sensor readings don't point to a specific problem on their own. Experienced mechanics look at all of the information and use it to draw conclusions and to give them an idea on what to look for.

Then you're working blind. Gotta have the tools to do the diagnostics. If dollars are an issue, you might consider looking for a nearby community college with an Automotive program. We were always looking for live work to give the students real-world learning experiences. They are well-supervised but you will have to give them your car while they are studying Engine Performance. They won't accept it during Brakes or Suspension and Alignment classes, for example. That means your car might fit what they're teaching only once or twice per year. You'll also have to leave it with them until they have the time to fit it into their schedule. The trade-off is they typically charge for parts and very little for labor.