The consistency of your vehicle's performance will depend greatly on how well your intake/turbo system is sealed.
Just as much as a dirty air filter will affect performance, a leak in the intake system will promote poor performance. The mitsubishi engineers employed a Mass Airflow Sensor with the ECU (Engine Control Unit). This sensor measures the intake air so as to determine proper air/fuel ratios, as well as a multitude of other control parameters. Since this system relies so heavily on the accuracy of the measured intake air, it is critical to have an 'air tight system' in order for it to properly perform.
If the leak is consistent you can easily tune the problem out, but the day that the leak seals (fixes) itself - then you will be running too lean, and the day the leak grows bigger - you will be running too rich. I think I would rather fix all leaks than hoping the problem doesn't grow worse, or in some cases it might fix itself, rendering your perfect tune not so perfect anymore.
When the intake manifold is under vacuum, EVERYTHING in the intake tract (or connected to it) is under vacuum. Once you go into boost, the only portion of the intake tract still under vacuum is from the air filter to the inlet on the turbo. Everything past that is under boost. Therefore, if you perform a boost leak test and you have a leak in this first portion, it is a vacuum leak (suck, if you will) and you are introducing unmetered air into the system. If you have a leak anywhere beyond the turbo you are loosing boost pressure (air) and you will end up running rich (and lose power).
In order to ensure that the intake system has no leaks, we can use Sir. Bernoulli's principle: PRESSURE
Build yourself an Induction Piping Pressure tester
The test is simple and requires simple equipment. You just need to build a large tube plug/bung with a way of connecting to the end of it an air pump such as a compressor, or car/bike tyre pump.
Some people have used a speaker magnet with a hole drilled through the center of and a wheel valvestem installed. You can get creative here as well and build your own.
Connect your pressure tester to the input side of the intercooler. i.e just after the turbo. (some people remove the Air Flow Sensor and attach it to pipe that goes to the intake side of the turbo.)
To start with it only takes a few psi of pressure to find a leak, start with up to 7psi to find any basic leaks to begin with. Then work you way up to the same PSI that you run your turbo at, this is very low for some vehicles, but as we all know Mitsubishi Evos run some very high boost. Your Evo most likely will be boosting 16-22psi stock, or 22-30psi with aftermaket turbos and race applications.
Common Places to check for boost leaks:
All leaks will be easily heard, and will definately feel them with your hand. If you want to get really padantic, you can spray soapy water on these joints to see if any bubbles are generated by the air leak.
• Leaking Seals around the throttle body shaft - Open and close the throttle plate where the throttle cable attachs itself to the throttle body assembly. You should not be able to hear any tonal differences with a properly sealed throttle body. If you can hear air leaking from this area, if you can feel air leak, or if you can hear pitch changes as the air leaks when you gently wiggle the throttle plate shaft then you will need to remove and replace the throttle shaft seals with stronger ones. Also check the throttle body gaskets/joins.
• Leaking fuel injector insulators - You can gently twist the fuel injectors to see if any leaks are apparent. you will need to remove and replace the factory insulators with new ones. New stock EVO fuel injector insulators work well. The part # for the stock EVO fuel injector insulator is MD087060, and these items can be purchased from your local Mitsubishi dealer.
• A common leak occurs on the hoses off the turbo and/or intercooler. The clamps can work themselves loose. The intercooler pipe couplers can leak air at any joint in the system, but these leaks are usually found at the bottom of the charge pipe (intercooler) system. When the system is pressurized to ~22-24psi you would be able to notice any leaks, you can gently wiggle the fittings/pipes to see if any leaks are present. Also there is the possibility of Holes in the intercooler itself.
• Holes in the intercooler piping, sometimes the rubber pipe inside the stock woven mesh pipes cracks, and you can't see it from the outside.
• Leaking BOV (blow off valve), you will need to replace the BOV with a stronger unit. You can replace the stock plastic BOV with an aftermarket BOV, or the stock EVO 9 BOV is an excellent replacement as well. The part # for the stock EVO BOV is 1545A001, and this item can be purchased from your local Mitsubishi dealer.
• Leaking Boost Gauge pipes, don't forget to check all the small pipes going to and from the intake manifold. also check inside your car and around the pipe that connects to the back of your boost gauge on your dash.