CRITICAL part of the tuning process. Start here, really. If you fail here, you will never succeed.
Several areas of the vehicle should always be analyzed before you begin the effort.
Smoking – learn to identify fuel (black) vs. oil (grey-blue) vs. coolant (white/sweet smelling). You cannot fix oil smoke or coolant smoke with a tune.
Compression – you should have all cylinders within 10% compression of each other. If smoking, damage to old spark plugs or general appearances make you suspicious of the motor’s health, check it before you start. It’s a lot easier to deal with a motor with poor compression BEFORE you beat the snot out of it in the course of tuning it. Many people skip this but it is something to think about because a motor that is already hurt is very likely to blow up or experience a catastrophic failure during tuning.
Look at MAF intake routing, make sure there are no obvious vacuum / intake leaks between the MAF and the intake valves. Think cracked/split/loose hoses, bad gaskets, open ports, dry rotted couplers, hoses connected both before and after the MAF, …
O2 sensors should be operational without any exhaust leaks before the sensors. For some reason, cut and soldered “extensions” for long tube headers often cause problems. Plug and play extenders are *highly* recommended.
If applicable, pay attention to which bank the wideband is installed in – bank-bank differences can be a powerful diagnostic tool. Pay attention to how far the wideband is from the engine’s exhaust ports – there is always some lag between combustion events and measurement. When things are changing quickly, this is critical.
Make sure you have enough fuel pump and injectors for the power level you are looking for. For a V8, “Injector size in #/hr * 14 = max hp” is a crude rule of thumb. There are tons of injector calculators to be found if you want a better idea.
Ensure fuel pressure is correct. 60 psi with no vacuum reference is generally about where most OEM regulators are set and returnless systems.
You need a functioning alternator and battery. Battery voltage plays a role in crucial things like injector opening time and coil charge duration. If your charging system is not functioning correctly, your tune may drastically change if/when you fix it. Rule of thumb: if your battery voltage ever drops below 13 volts with the motor running, you will run into trouble.
Basic maintenance should not be overlooked. If it is important for a “normal” car it is twice as important in a performance application.
Spark plugs: correct heat range, appropriate gap, not fouled. Consider power level, fuel and ignition system. AVOID PLATINUM PLUGS FOR PERFORMANCE APPLICATIONS!!! Copper or iridium will serve you much better.
Plug wires: no cracks/arcing, properly crimped ends, appropriate length so there isn’t too much tension
Coil packs: Coil-per-cylinder OEM coils best. Aftermarket coils tend to be extremely unreliable.
Oil and coolant: always check fluids before starting. Quick check, potentially horrible consequences if low/out.
Fans / overheating: it is always a good idea to check that radiator fans work. A car that overheats cannot be tuned.
Belts and Idlers: All serpentine belts must be in good shape. Cracks, missing ribs, etc. will all cause problems. Any idler pulleys must spin freely.
Tension: Belt Tensioner should not be extended fully with the engine off. Adjust belt length so that tensioner is in the lower third of its adjustment range with the motor off. (i.e. it can move 2/3 through its range to increase belt tension – it should be mostly compressed when motor idle) This is particularly important for supercharged applications.
Fuel filter: Always a good idea to change the fuel filter if you have unknown milage or time on the current one.
Fuel age and type: Gasoline degrades with time. Do not expect fuel that is more than a month or two old to be of the same quality as fresh gas. Be particularly careful with heavily oxygenated fuels (i.e. VP Q16) and alcohols (ethanol, methanol, E85, etc.) in contact with fuel system components for large periods of time.
Clean air filter and MAF. Oiled filters generally cause MAFs to get dirty. Clean MAFs only after they have had a long time to cool – hot MAF+liquid=death.
Remember, you can’t fix mechanical or electrical issues by reprogramming the PCM!!! The results you achieve with tuning will only be as good as the material you start working with.