First things first- What exactly is a tune-up? As a non-mechanic do-it-yourselfer, I define a tune-up as routine maintenance that is done infrequently. There's nothing actually wrong with the car, you just need to do all those things listed in the manual that are done rather infrequently. Things like changing the spark plugs, changing the transmission and differential oil, changing the air filter, flushing the radiator, etc. Check your manual for how often these things have to be done. On many cars, it's a very infrequent occurrence. However, failing to do these things will lead to premature failure and expensive repairs. Properly maintaining a car will maintain power, efficiency and performance and will prevent premature wear. If you want your car to last a long time, be sure to perform routine maintenance. It's easy and will save you a lot of money in the long run.
In this post, I'll talk about a tune-up I recently did on my Jeep Cherokee. Doing these things yourself will save a lot of money over a mechanic but paying a mechanic to do these things will still save a lot of money over ignoring these tasks and paying a mechanic a lot more later for expensive repairs.First- the transmission! If you want the specifics on Jeeps-
check this out.
This is a manual transmission and changing the oil is a straightforward job, though in reality it can be a painstaking and slow process. There are two plugs on the transmission, a drain plug and a fill plug. The drain plug is at the bottom of the transmission. Unscrew and drain into a drain plan.

The plug has a magnet which collects the metal particles from the transmission. If it's fine metal, you're all good. If you have chunks on the magnet, there is a serious problem. My Jeep has 160,000 miles on the original clutch and transmission- and I'm not really sure when the transmission oil was changed last- but the metal particles on the magnet were still like a very fine clay. The recommended replacement period for transmission fluid is 30,000 miles on my Jeep, which I'm pretty sure I missed!

The other plug is the fill plug. With the drain plug cleaned and reinstalled, fill the transmission with new transmission oil. But herein lies a problem. Because you're under the car, there isn't an easy way to pour in the new oil. (Note that automatic transmissions are different and most need to be filled from the engine compartment- check your manual).

You can pump the oil in or try siphoning it. I did this project on a cold day and the oil was like molasses. I aimed a hair dryer at it which warmed it up and decreased the viscosity of the oil. It still took two hours to fill the transmission, but it worked! When the transmission is full, no additional oil can be added the transmission without it draining out of the fill hole. Then simply reinstall the fill plug.

Filling the transfer case is even easier. The transfer case is where the power of the transmission is transfered to the front and rear wheels. If you have a regular two wheel drive car, you will not have a transfer case, because the power from the transmission goes directly to the front or rear wheels, not both. The transfer case has two plugs- a fill plug and a drain plug.

Check your manual, but my Jeep calls for automatic transmission fluid for the transfer case. Remove the drain plug and drain the old oil and fill with new oil, in the same manner as with the transmission. This will be easier because the automatic transmission fluid is less viscous than gear oil and there is less to add to the transfer case.

Next lets change the differential axle oil. The differential is the part of the car that takes the power from the drive shaft and turns the axle, and hence the wheels. Not all cars have these. Front wheel drive cars for example are typically driven by drive shafts directly from the transmission. To drain my Jeep's differential, you need to remove the differential plate and let the oil drain out, clean the surfaces, apply a gasket sealing material, reinstall the plate and fill the differential up to the fill hole. If you're lazy you could always siphon or pump out the old fluid. On my Jeep, this takes the same gear oil as the transmission. The fill hole in the front was a 1/2" socket hole and the rear was only a small plastic stopper which plugged the hole.

Now let's change the air filter! You can't really tell by looking at an air filter whether it needs to be changed (but if it looks terrible, it probably is), so check your manual to determine the recommended replacement periods. A clogged air filter will rob your engine of power and efficiency and if it gets bad enough, cause your engine to run rich (too much gas in the gas/air mixture) and harm the engine.
To replace the filter (at least on my Jeep), unsnap the clasps on the side of the filter box and the top will slide off the bottom. Remove the old filter and vacuum out any crud in the bottom of the filter box. Replace with a new filter and reinstall the filter box lid. Easy!

Next- spark plugs! Spark plugs do exactly what the name implies- they create a spark which ignites the fuel/air mixture at the exact right time to push the piston down, turn the crankshaft and send power to the transmission. But over time, the spark plugs wear out and become "fouled". They don't do their job as well and the efficiency and power of the engine is compromised. You might have a rough idle or even some of the cylinders not working properly. Remove the spark plug wires and then remove the plugs with a special spark plug wrench on your socket wrench. Do this on a cold engine only!

Reinstall the new spark plugs, be suring to torque them (measured with a torque wrench) to the proper level. Also be sure to check the gap- which is the distance between the center diode and the arm that reaches over the spark plug. This is where the spark jumps when the engine is running and the distance needs to be checked with a feeler guage (which checks width) to match manufacturer's specifications. Otherwise the spark will jump too soon or too late and the engine will not run properly. This may also be a good time to change your spark plug wires. These can also wear out, become less efficient and harm the performance of your engine. Check your manual for recommended replacement intervals.
Take a look at the difference between new and old! My engine ran much smoother after this job. These spark plugs were overdue for a change.

Be sure to check your car's manual, which will give you information on how often to do routine maintenance and what types of fluids, filters and spark plugs you should use. It is important to do maintenance as frequently as it is recommended by the manufacturer. This will ensure that your car will run smoothly for a very long time. We don't keep our cars around for a long time these days, because they wear out and we buy new ones. But a properly maintained car will last for a very long time and will save you a lot of money. Having a properly maintained car will run better, burn less gas and will not need to be replaced as often, which is thrifty and good for the environment.
Even if you do not do the maintenance on your car yourself, understanding exactly what your mechanic is doing will make you a more educated consumer. Most mechanics are honest and hard working craftsmen but there are always horror stories about unscrupulous mechanics defrauding customers. If you have a general idea how cars work and what mechanics are talking about, you won't be taken advantage of. Plus you will get a much better deal because you can compare the usually expensive car dealer with a more reasonably priced neighborhood shop.
I don't fix everything on my car, somethings are beyond my ability, but I always know what is wrong with my car before I pay someone to fix it and I always shop around for the best price and the highest quality service so the problem will be fixed correctly the first time. But many things can be fixed yourself- you'll save money, have fun, and have the satisfaction of knowing you are taking care of your car yourself.