Technicians have long used scan tools to pull fault codes and system data from vehicle computers. But most scan tools do not have the ability to reprogram the operating instructions that are already in the computer. They can only read it. Consequently, the only way you could recalibrate the PCM was to replace the original equipment PROM (Program Read Only Memory) chip with an updated chip -- and then only on certain General Motors and Ford applications.
People who were interested in making performance modifications found that the PCM really limited what they could do. The solution here was to replace the stock PROM chip with an aftermarket performance PROM chip that provided more spark advance, fuel enrichment, bypasses the stock rev limiter and speed limiter. On applications where the stock chip could not be replaced because it was permanently soldered into the PCM main circuit board, the stock PCM could be fooled into delivering the desired ignition timing and fuel enrichment by piggybacking a second module into the vehicle's PCM wiring harness. This allowed the input and output signals to be modified to achieve the desired gains in performance.
With the introduction of EEPROM (Electronically Ereaseable Program Read Only Memory) chips in computers on many late model OBD II equipped cars and light trucks, reprogramming become possible -- but only with a "factory authorized" reprogramming tool. This was to discourage emissions tampering and to protect the OEMs against unwanted warranty problems caused by someone fooling around with the calibration of the PCM.
Tuner Scan Tool Suppliers
Various aftermarket suppliers sell specialized "tuner" scan tools for tweaking the PCM to improve vehicle performance. Some of these are AmericanMuscle's BAMA tuner tool for Mustangs, Superchip's "MAX Microtuner" for late model GM, Ford and Dodge car and truck applications, Hypertech's "Power Programmer" tool for Ford and GM applications, GM Performance's "Granatelli/Diablo Sport Predator" tool for GM, Ford and Dodge, and Crane Cams "PowerMax" performance power tuner for Chevy, GM and Ford trucks plus Hummer.
These products typically sell in the $400 range depending on the model, application and where you buy it.
So what can you do with a tuner scan tool? You can recalibrate various settings in your engine's computer to increase horsepower and torque. The suppliers of these products say you can dial in 10 to 30 or more additional horsepower on otherwise stock gasoline engines, and 50 to 100 or more horsepower or more on many turbo diesel engines.
Most of these tools can be used to:
* Change the fuel mixture
* Change ignition timing and spark advance
* Raise or lower shift points of automatic transmissions
* Increase the rpm setting of the rev limiter
* Recalibrate the speedometer to match different tire sizes
* Recalibrate the speedometer for changes in axle ratios
* Match the speed limiter to the speed rating of the tires
* Download custom tuning profiles from the internet that have been developed for specific engine applications.
* Plus read diagnostic codes and clear codes like a regular scan tool.
Features vary depending on the product and supplier, but most do give you the opportunity to play around with the stock settings of the PCM. Such modifications are usually necessary if you want to increase the boost level of a turbocharged or supercharged engine (which may also require installing higher flow fuel injectors or other modifications). Recalibrating the PCM is also needed if the stock cam, cylinder heads and/or pistons have been replaced with aftermarket performance parts. Ditto for adding an aftermarket cold air intake system or performance intake manifold. Anything that changes the breathing characteristics of the engine will usually require retuning the fuel and ignition curves to realize the full performance potential of the upgrades.
Performance scan tools that can modify PCM calibrations are a great product, but they are not without risk. The main one is that modifying the PCM calibration in your engine computer with a performance tune may void your factory powertrain warranty.
For example, General Motors will NOT pay for any engine, transmission, driveline or exhaust system failures that result from doing a performance tune on a Duramax diesel. See GM Duramax Diesel Warranty Issues for more information.
Another risk is that you can screw things up rather easily. For experienced engine tuners who know what they are doing, using a tuner tool makes performance tuning relatively easy. But for a novice user who may not fully understand the implications of playing around with the PCM's fuel and ignition timing settings, you may make your engine run worse if you are playing around and making your own custom tune adjustments. The best advice is to NOT attempt a do-it-yourself custom tune until you have gained some tuning experience.
The best and safest approach for most users is to install one of the pre-programmed tunes that the tool manufacturer gives you when you purchase their tool. The tune will be based on detailed information you provide them about any modifications you have made to your engine such as installing an aftermarket cold air intake, low restriction exhaust, performance cam, larger throttle body or other such modifications. Most scan tool suppliers have already developed tunes that should work with any of these common modifications. And if you are not happy with the results, they can usually tweak the tune to suit your vehicle. You can also go back to the stock tune if the new tune does not achieve the desired results.
Here are some changes to be careful about:
Fuel Mixture: A richer fuel mixture makes more power up to a point (about 12:1 max), but beyond that point it only wastes fuel and increases exhaust emissions. You might also foul the spark plugs if the engine spends much time idling. The greatest danger is getting the fuel mixture too lean as this can result in detonation under load and possibly burning a piston! The safest way to modify the fuel mixture is to do so while the vehicle is running on a dyno and the exhaust temperature or mixture is being monitored to make sure it doesn't go too lean.
Spark Advance/Retard: More spark advance helps low end torque, throttle response and off-the-line acceleration. But too much spark advance increases the risk of engine-damaging detonation and pre-ignition. The more advance an engine has, the higher octane fuel it requires. As with fuel modifications, ignition modifications are best done on a dyno.
Rev Limiter: The stock rev limiter kills the ignition if the engine is revved too high to prevent possible valve float that could result in piston-to-valve contact and valve damage. Raising the rev limiter beyond the capabilities of the stock valve springs is asking for trouble.
Speed Limiter: This is a built-in safety feature to prevent idiots from driving faster than the speed rating of their tires. Tires have speed ratings for a reason. They can explode if they are driven at sustained speeds beyond their speed rating. If you raise the stock speed limiter setting, make sure your tires have enough speed rating to safely handle any changes you have made. For more information about tire speed ratings and tire safety, Click Here.
Emission Issues: Changing the PCM calibrations on your vehicle may also create a problem is you live in an area that requires emissions testing. Some tunes are emissions-legal and won't cause any problems, but some performance tunes are really designed for off-road or racing only because they disable the downstream oxygen sensors. This prevents the OBD catalyst monitor from running, which will cause your vehicle to be rejected when you take it to the emissions test station. All of the OBD monitors must have run and completed before your vehicle can be tested, and there must be no faults found to pass the test. One option here is to temporarily reinstall the stock settings until the monitors run, then get the vehicle tested. Once it passes, reinstall the performance tune.
Don't lose the tuner scan tool! When you use a tuner scan tool to install a tune over the stock calibration in the PCM, the tool becomes married to your car. The tune in your car cannot be changed without the tuner scan tool that installed it. If you make additional modifications to your vehicle, or are having problems passing an emissions test because the tune disabled the OBD oxygen sensor monitors (which is a common modification with tuner programs), you will need the tool to make the necessary tuning changes. If the tool has been lost, your only option is to take your vehicle back to the car dealer and have the PCM reflashed back to its stock calibration. This will also require removing any add-on modifications such as a cold air intake, custom exhaust or other parts and replacing them with the stock components.