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You can't tune around a mechanical problem.

1) Ensure that brake pads and brake system are in good working order. Bleed brakes if necessary.


2) Check coolant level and flush/fill if needed. Properly purge all air from cooling system.


3) Source and repair any oil or coolant leaks.


4) Check oil level and change if due. Do not tune on old oil. Consult engine builder for correct oil type if using an aftermarket engine. Stock engines I like Castrol Syntec 5w40.


5) Check thoroughly for exhaust leaks. If there are exhaust leaks, the car cannot be tuned accurately.

6) Check and replace clogged catalytic converters, if present.

7) Check thoroughly for boost/vacuum leaks. These are the #1 issue of turbocharged Subarus and have the single greatest impact on performance. Here is a good guide on how to do a boost leak test. Use a pressure regulator when filling engine with air or damage can occur. Block off the proper fittings or damage can occur. If you are unsure how to perform a boost leak test, I can recommend a shop who can perform this service. If you have boost leaks and get the car tuned, then resolve them later, this will cause a dangerous lean condition and overboosting which can cause damage!

8) Check and replace fuel filter and fuel strainer sock if it has not been performed in the last 60K miles or unsure of condition. Ensure fuel pump is installed properly. If fuel pump, strainer, or hanger is configured incorrectly a lean condition can occur and cause damage.

9) Check and replace spark plugs if they are an unknown type/gap or have not been changed in 15K miles (iridium) or 5K miles (copper). When gapping iridium plugs DO NOT TOUCH the center electrode and do not ever use a coin-type gapper or electrode damage may occur. If you drop a spark plug on the floor immediately discard it and use a new one. If unsure how to install or properly gap spark plugs you may wish to consult with a shop that has the proper tools/experience. Spark plug recommendations are done on a per-car basis and may vary based on fuel, intended use, and power output. Contact me if unsure.


10) Check and ensure vacuum hose is connected to fuel pressure regulator nipple and vacuum/boost source. Zip-tie or super glue in place.



11) Check and ensure boost control system is configured properly. If you are using a 3-port EBCS do not drive the car in boost or more than 30% throttle until the tune is complete or damage can occur. Zip tie all vacuum connections. 3-port MAC Valve (Cobb/Grimmspeed) EBCS routing should be as follows: Port 1 returns back to turbo inlet pipe, or vent to atmosphere on full speed density tunes. Never VTA with hybrid SD tunes or MAF tunes. Port 2 goes to the internal wastegate nipple or the side port of EWG. Port 3 goes to the compressor cover of the turbo or a manifold pressure source, if your turbo doesn't have a pressure port.

12) Check and ensure battery is in good working condition and delivering over 13v. Low voltage can prevent ECU flashes or cause failed ECU writes, that in the worst case scenarios, will result in an inoperable ECU.

13) Know the type of fuel in your car and what you typically run. E10 blended fuel tends to be more consistent than "no ethanol" E0 gasoline. It is best to run one specific type of fuel, and for that reason, I typically recommend E10 for 91 octane cars as it is the most universally available fuel.

14) E85 users can test their fuel with an ethanol content analyzer installed to the vehicle, or by purchasing an $18 handheld tester. This should be performed before the initial tune and under most cases, should be tested every couple of months. Thanks to the wide lambda values in which E85 fuel produces maximum torque, minor blend shifts are no cause for concern.

15) E85 users should change engine oil and filter every 2000-2500 miles or every 3 months, whichever comes first. Change more frequently if you take short trips.

16) E85 users should use Lucas Ethanol Fuel Stablizer. This will add lubrication, stability, and resists water absorbtion. One bottle is less than $10 and treats 80 gallons... cheap insurance. My testing with bore scopes shows absolutely NO deposits on intake valves, pistons, or combustion chambers. Injectors are clean and no black gunk with a proper tune even on cars running E85 nonstop for 15K+ miles.


17) Methanol injection users check and ensure proper working condition of your kit. Replace all inline filter assemblies. Clean or replace individual nozzles and nozzle screens if corrosion or blockage is present. Ensure failsafe wiring connections are properly made.

18) Clean MAF sensor with CRC MAF Sensor cleaner and air dry for 1 hour. Can also use gentle compressed air to accelerate drying process. Do not spray with force or damage can occur. Check and ensure MAF sensor o-ring is not damaged and reinstall in MAF housing with a very light coating of silicone lubricant (fuel injector o-ring lubricant or similar). Do not get lubricant on or near MAF sensor element.


19) Check O2 sensor configuration:


O2 sensor table

Front AF Sensor: this sensor is installed in the header on EJ engines and directly after the turbo on FA20DIT engines, and is responsible for closed-loop fuel trimming operations. Replacement sensors are available on Amazon, same part as Subaru OEM at less cost. This sensor is REQUIRED.

Rear O2 sensor: this sensor is installed after the catalyst(s) and is responsible for catalyst monitoring and minor fuel trimming operations. These sensors can be ordered from auto parts stores. These sensors can be DELETED if catless, but most leave them connected.

Wideband O2 sensors: these are not required for custom tuning but make an excellent addition to any tuned car. Will allow you to view health and running condition of the car at a glance at any time. These should be installed in the bellmouth area of the downpipe. Preferred wideband kit is the AEM UEGO.


20) Wideband sensor installation location should be in the bell mouth of the downpipe, if possible. Wideband sensors cannot be installed pre-turbo and they cannot be installed after a catalytic converter. Getting the wideband signal into your AccessPort to ease tuning and data logging involves connecting the 0-5v analog output wire of the wideband gauge into the factory Subaru ECU. I offer a plug and play wiring harness for this purpose or you can manually splice into the ECU harness, using either the LH or RH TGV, or the rear O2 sensor signal wire (preferred). See Products page.

21) EWG users make sure that any unused air ports are BLOCKED OFF or vacuum leaks and poor running will result. If using the traditional side port connection method, ensure the top port of the wastegate is open to atmosphere.


Contact me with any questions. Failure to complete this checklist in full will result in poor results from tuning... we cannot "tune out" a mechanical condition!


Looking for general information? Here is a good knowledge base if you have questions on the above: Unabomber's Manifesto (NASIOC)

Vehicle Specific Pre-Tune Notes

2008-2014 WRX owners (or any vehicles with the plastic intake manifold), please read the following:


1) If running top mount intercooler, ensure throttle body rubber coupler and rubber hose ends are intact and installed correctly. This coupler must also be clocked to the proper orientation or fitment issues will occur. It is a good idea to replace this hose preventively, you can find one-piece silicone versions that are more flexible. Hair spray (anything cheap, in the aerosol can, like aqua net) can be lightly misted inside silicone or rubber couplers to help them stick to metal surfaces like the throttle body. Warning, this can make them tough to remove in the future... but hey, at least you won't have a boost leak!


2) The vacuum reference line that goes from the back of the intake manifold to the bypass valve (BPV) easily pops off on these cars. Where it connects to the intake manifold, on the nipple there is no flared end to grab the hose. I recommend installing a clamp on this line, not a zip tie, with a shot of hair spray onto the vacuum nipple to ensure a good seal. You will need to remove the intercooler, and remove the two bolts holding the wiring harness bundle to the intake manifold. Pull the harness bundle back towards the firewall and you will have enough access to find the bypass valve reference line and perform the above steps.

Special Notes for Old/High Mileage Cars

As any car gets older, mechanical parts begin to wear out and eventually fail. The Subaru Impreza WRX was the most popular model sold in the US and since it has gone on sale in 2001, it has become one of the most beloved japanese turbocharged import. Subarus are known for their reliability, but they still suffer from the typical wear and tear, general degredation, and fatigue just like any other car. Prices for used WRXs continue to drop, which means it can be very affordable to buy one used, and most owners can't wait to begin modifying their new car once they get their hands on one. But the reality is, they're inheriting a car that has probably had 3-5 owners by this point, and numerous repairs or modifications with little to no service history. Many of these people send me messages and I hear the same questions over and over, multiple times per week. So this is going to be an attempt to centralize the information I have learned when it comes to tuning and owning a high mileage, older turbo Subaru.


First step: ascertain the health of your engine and chassis


This is the most crucial, and should be done before you buy the car. However, 90% will skip this phase and move directly to ownership. Still, it's a good idea to find out what you're working with. You should perform a compression test, and if the readings across all four cylinders are not 120psi or more, or not within 10% of each other, then you have an issue. Old factory engines could have a broken piston (ring land) which would cause oil consumption, misfires, and lack of performance. Built motors with high miles or improperly assembled tolerances tend to lose compression overall, but somewhat evenly. Neither are good.


Chassis health is something different. All it takes is a lift or jack stands to get under the car and see what sort of damage it has suffered. Body shops can be good at this too. Cars with frame damage should be avoided at all costs, as it is easy to find a replacement with an intact structure. These were commuter cars produced in massive numbers, don't be afraid to keep looking.


Second step: general repairs (bring the car up to par)


Of all the advice I give people, this is the one that falls on deaf ears the most. It's also the most important for the overall running of the car, let alone its modification potential. I'm talking about general state of condition of the engine, and its various components. Again, these were reasonably priced consumer cars when they were new. They came with a 5 year, 60K powertrain warranty that could be extended to 10/100K. This means that Subaru built and designed most of its components to last roughly 100K miles, and we see that as the years go by and the cars get older. For example, most people know that the timing belt should be changed every 100-125K miles, but we're finding many other components have similar life spans, and should be changed out too. According to my work and my client base and what I see, here's the real world, AVERAGE expected life span of each OEM component found in your turbo Subaru (and don't tell me that your friend's neighbor's dog drives a 300K mile WRX with no issues because he's a liar, or just living in denial):


Electrical components


  • Coil packs: 75K miles on stock cars, 40-60K on modified vehicles. These do not always fail in an obvious manner and will gradually drop off in performance, eventually causing misfires under heavy load where the mixture is hardest to ignite.

  • Spark plugs: 60K on stock cars running up to stage 2 power levels. The higher the HP the faster plugs will wear. Cut this number in half for every 100 HP you add to stock. Copper plugs for heavily modified cars should be changed every 10K miles. Misfires, poor economy, and poor performance are indicators of worn plugs.

  • Boost control solenoid (OEM): 75K miles.

  • Fuel injectors: 100K miles. Typically will continue working but lose performance unless ultrasonically cleaned and serviced. Will usually throw a misfire code that moves with the injector when they fail.

  • Mass airflow meter (MAF): 100K miles. This one is tricky, because the common misconception "just clean your MAF" is so rampant. Many times, you can clean the sensor all you want, and it won't fix the underlying issue, which is the fact that the hot film element gradually breaks down and causes inaccurate readings. The only time I have seen a MAF be fixed by cleaning is when the amount of dirt or debris on the sensor element is so heavy, it's immediately obvious when you look at it. Using oiled K&N filters without blowing out the excess first, or driving without an air filter being the main culprits of that.

  • Knock sensor: 100K miles. Again, this is tricky. I have seen knock sensors that reported zero knock when there was audible knock coming the engine, indicating the sensor is not reporting anything at all, and it's only a matter of time until the engine suffers damage. Conversely I have seen them fail and report high and frequent knock. This robs the engine of power and optimal ignition advance. You can check for physical damage by inspecting the plastic insulator around the piezo element, but that's not a guarantee of whether or not the sensor is working properly.

  • Front O2 Sensor (aka Front A/F Sensor): 80K miles. This sensor is responsible for measuring A/F ratio during cruising, and is vital for proper economy and emissions. When it fails it generally will throw a code.

  • Rear O2 sensor: 60K miles. This sensor is a narrowband sensor designed to monitor the efficiency of the catalytic converter(s). When it fails it often throws a code. It can also be deleted from certain Subaru models entirely.

  • MAP sensor: 120K miles. This sensor typically throws codes when it fails, and the engine will run poorly as its basis for fueling is wrong (improper manifold pressure readings).

  • Throttle position sensor: 150K+ miles.

  • Cam position sensors and crank position sensor: 125K. Typically throws a code and can prevent the car from starting.

  • Tumble generator valve (TGV) motors and sensors: 80K miles. These typically throw a code. However, be careful on cold starts if the car runs very rough as this is a sign that the TGV might be seized (they start closed and then open up as the car warms up). If one side is more closed than the other, you will get a rich condition almost like a manual choke. Most people delete the TGVs to eliminate complexity, airflow restriction, and simplify the engine bay or make room for bigger turbo inlets or intake manifolds.

  • Coolant temp sensor: 150K+ miles.

  • Air temp sensor: located in the MAF sensor housing, the MAF itself will fail before the air temperature sensor ever does.

  • Barometric pressure sensor: 100K+ miles. Can be sneaky as sometimes it won't throw a code, and instead it will report an incorrect barometric pressure reading. This can rob the car of power, because at high altitudes the car runs less boost by design (thinner air). Close to sea level, you could lose up to 50% of your boost without actually having a mechanical issue. Easy to log with Accessport or scan tool.


Physical Components


  • Radiator: 100K miles.

  • Timing belt: 100K miles.

  • Water pump: 150K+ miles.

  • Oil pump: 150K+ miles.

  • Power steering pump: 150+K miles.

  • A/C Compressor: 125K miles.

  • Rubber boost hoses and couplers: 100K miles.

  • Turbo inlet pipe: 75K miles.

  • Turbocharger: 100K miles.

  • Early STI oil pickup tube: 60K miles. Critical... this is one of the most well-known issues that can cause oil starvation and bearing failure.

  • Ball joints, tie rod ends, control arm bushings, wheel bearings, strut mounts, struts: 80K miles

  • Engine oil: 5,000 miles up to stage 2 power levels. 2,500 miles on ethanol-based fuels. Heavily modified cars consult your engine builder for oil recommendation and interval.

  • Transmission and rear differential oil: 40K miles.


The conclusion


Not many people want to buy a new (to them) car and immediately sink hundreds of dollars into preventive maintenance, but that mindset is foolish and will only earn them pain and suffering. After all, just because your 2002 WRX with 175K miles only cost $5500, it's still a $20K car when it came out, and it's lived nearly twice as long as it was built to! So when it comes time to buy a used turbo Subaru, take the purchase price and add 15% and that is the TRUE price it will cost to buy the car and bring it up to spec, purely when it comes to OEM maintenance. This does not take into account any work required on the account of modifications. It also assumes you will be doing all the work yourself... if paying a shop, take that into account as well.


I'm lucky in that I get to work on a lot of Subarus, so I get to see what works and what doesn't. I get to see the various mindsets of the car's owner, too. The people who are patient, careful, detail-oriented, and fastidious in their approach to keeping the car in good mechanical condition always enjoy better results. As someone who prides themself on doing the best tuning I am capable of, each time, every time... it makes my job very difficult when I'm handed garbage and expected to turn it into gold. So make your life (and mine) much easier and happier... take care of your car. And if you can't afford to fix up your new (old) Subaru not if but WHEN something breaks? Then maybe you should look into another car entirely, or steer clear of modifications until it's up to spec. Just my humble opinion.




Last updated 3/29/2016

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