GM EFI Tuning



So you want to know what is going on inside the brain of your OBD I GM vehicle?

We’ll start off with going over some of the terminology used in some of the more common scanning software and scanners.

BLM-Block Learn mode. This is often referred to as the “long term” fuel trims. Long term in computer speak is a few seconds.

INT-Integrator. Also called the “short term” fuel trims, as in a second or two.

IAC- Idle Air Control. This is usually a small stepper motor that moves a pintle in a seat to regulate airflow at idle and is expressed in steps, usually 0-255.

TPS Throttle Position Sensor. This is a potentiometer that tells the ECM how far open the throttle blades are. Its readings are in percentage and voltage (0-5 vdc)

MAF- Mass Air Flow (sensor). This sensor tells the ECM how much air is entering the throttle body. The most common MAF’s are the “heated wire” type.

MAP-Manifold Absolute Pressure (sensor). This sensor is used in place of a MAF on some systems; it measures pressure within the intake manifold and sends the readings to the ECM for fueling calculations.

ECM -Engine Control Module. This is the computer that runs the engine systems, gathers information and makes corrections to the systems as needed.

PCM- Powertrain Control Module. This serves the same function as the ECM, except that it also controls the transmissions in vehicles equipped with “electronic” transmissions.

MAT-Manifold Air Temperature (sensor). This device measures the temperature of the air in the inlet tract.

CTS-Coolant Temperature Sensor. Measures the temperature of the engine coolant and sends that information to the ECM for fuel and spark calculations.

CSI- Cold Start Injector.  This device was used mainly on the 1985-1988 GM TPI engines. It supplied extra fuel for cold starting of the engine.

EGR-Exhaust Gas Recirculation valve. Primarily used as an emission control it has a major benefit of cooling the combustion chambers, which helps control detonation.

O2-Commonly used to refer to the oxygen sensor. This sensor “reads” the oxygen content of the exhaust gasses and reports that to the ECM.

VSS-Vehicle Speed Sensor. This sensor tells the ECM how fast the vehicle is moving.








Some things that you’ll need to get started are; a scanner or scanning software and a laptop computer with communication cable. A PROM burner, blank PROM chips, software to edit the information file used in the ECM (referred to as a .bin file), a “mask” or ECU file for the editing software (this is used to tell the software what kind of ECM the .bin file is for and what information to display).








Auto X-Ray scanner by Actron.

The pros of a device like this are that they are pretty much “plug & play” and require little, if any configuration by the user and can be used on many different vehicles. The cons are; price (starting at around $300), and limited screen size for data display.


A simple ALDL cable to use with the software listed below (except Diacom)







This is a screen shot from Free Scan. As you can see there are several “tabs” of information to look at in this software. The first 20 scans are free, after that its $100.

This software supports several different OBD I vehicles









This is a screen shot of one of the pages of data from ECM851 by Craig Moates ( This software is a free down load from Craig and supports 1986-1992 GM F Body’s with TPI engines. It will also work on 1986-1991 Corvettes. We’ll be mentioning Crag, his products and contributions to the hobby later on.








This one is CarBytes, developed for the Holden vehicles from Australia.






One other program is Diacom+ from Rinda Technologies. This software will scan any GM OBD I vehicle… The cost is $500-$800 and comes with all cables and adaptors for the particular vehicle make.

All of the above programs will run on older, less expensive laptops, so there is no need to go out and buy the newest most expensive machine to tote around in your car and have lying around the garage.


Now that we’ve got our scanner, we’ll look at what some of the data they display means.


We’ll start out with the Integrator (Int) or “fine fuel correction” or “short term fuel trims”. The .bin in the RPOM is usually set to display a minimum number of 108 (rich) to 160 (lean). When scanning the vehicle the Int should be averaging around 128 and will usually be fluctuating from about 122-134. This is the ECM making minute corrections in fuel delivery based on several factors, such as; engine load, temperature, oxygen sensor readings, air flow through the engine, throttle position, to name a few. When the Int has been at a particular setting for a length of time the ECM “learns” this new fuel trim and applies it to the BLM.

The BLM (Block Learn Multiplier) or “course fuel correction” is the true indicator of weather an engine is running lean or rich. Like the Int, it reads from a low of 108 (rich) to 160 (lean), with 128 being the ideal reading.


Next we’ll move to knock counts. The ECM “listens” for engine knock (detonation) using its knock sensor and if detected will “count” the knocks. Once the knock count gets to a specific number the ECM will begin to retard ignition timing in an attempt to eliminate the knock. Once the knock is eliminated and a specified time has passed, the ECM will begin to add the pulled timing back in until it either detect knock again or until all of the pulled timing is added back in. Knock counts are very important to your engine and can mean life or death for it.


On a MAF based injection system, the MAF flow or measurement rate, tells how much air is flowing through the sensor and into the engine. This measurement is in grams per second and will be between 0 and 255.


Throttle position is expressed in two ways, one is percent (0%-100%) and the other is in volts, .54v-5v. As the throttle opens the voltage reading at the TPS increases to a maximum of 5v


The O2 reading is expressed in millivolts and should be constantly swinging from below 500mv to over 500mv. This is how the ECM maintains a 14.7:1 air/fuel ratio.


Vehicle speed, read through the VSS as a square sine wave of either 2,000 or 4,000 pulses per mile, depending on the type of VSS used in the vehicle. The reading is displayed in MPH or KPH, depending on the program or scanner you are using.


Injector pulse width is the amount of time the injector is being commanded to be open for, expressed in milliseconds. Some scanners will also use the ms reading to calculate the injector “duty cycle”. The duty cycle is the percentage of time the injector is opened for out of 1 second.




PROM Editing


To edit our PROM we’ll need the information gathered using one of the previously mentioned scanners and some editing software.

For editing there are just a few options out there. First there is Winbin.  Free software with limited ECM support but it does cover the most popularly used ECM’s


Above is a shot of  one of Winbin’s many pages and editable options

Below is the injector constant being shown.




 Above is showing the #1 MAF table








And this is a 3D chart showing the spark tables. This program supports the 165 and 730 ECM’s using the $32, $32B, $6E and $8D masks Unfortunately it looks like Winbin may no longer be avialable. But if you are up to it you can write your own mask’s or definition files.






Next up is Tuner Pro RT by Mark Mansur. Another free offering ( a $30 donation will remove the register nag) that can edit and scan in real time, hence RT.

A very nice program that is about as full featured as you can get for free. The author has ECM definition files for download that supports 1986-1992 Camaro/Firebird, and 1986-1991 Corvettes, 1987-1991 Chevy trucks 5.0L,  1995 C/K Truck/Van 5.7L/7.4L 1988 Holden, 1993+ Holden and the 1993 LT1 Corvette.
Find this program at;





Next up is C.A.T.S., or Tuner Cat. This program isn’t free but the service and support are worth the price of admission and they have a couple of programs to choose from. There is the basic Tuner for $69.95, Tuner RT (real time) for $99.95. The program comes with 1 definition file, $42, each additional definition file costs $19.95. The do it yourselfer will probably only need 1 or 2 definition files. But for the professional or the man that has to have it all, they offer their program with all of the definition files, 67 of them.

Check out Tuner Cat here



Here is a shot of Tuner Cat with some of the many tables opened.









Now we move on to PROM burning hardware. The programmer is used to transfer the modified .bin file to a chip for use in the ECM. Depending on what type of chips you plan to use you may also need a U.V eraser to erase the chips.

There are several programmers available and each has its own software to operate it.







The SPEP plus is a very capable and inexpensive programmer. This one runs $80





Batronix offers 2 programmers, one already assembled and ready to go. The other is available as either a kit, an etched board or just schematic and board layout.






By far the most popular is  the pocket programmer 2 from Xtronics. This one runs around $150.



There are several more programmers out there, but for the do it yourselfer these are your best, least expensive choice. From the $150 PP2 they spiral up over $3,000, more than the hobbiest would ever want to spend.



Getting Started.


If you haven’t modified your engine already, take the car out for a 30 minute drive and log the ECM data. This will give you a good comparison between a “correctly” running system and your new setup and you’ll know what data results you are trying to achieve.

If you’ve already modified your engine and didn’t data log first, your job will be a little more difficult but not horrible.

Some things to keep in mind are anything that alters airflow through the engine will require a chip modification. These changes include but are not limited to, larger cid, larger throttle body, larger runners, higher flow intake manifold or just a swap to a different style manifold, installing a different cam, different heads, larger valves, forced air induction (ie; turbo or super chargers). Any change in fuel injector size will also require a change to the chip.


Chips and adaptors.


There are a couple different manufacturer’s chips you can use, and different sizes and configuration of chips as well.


There are the 27C series, which are erased using a UV eraser. These were commonly made by Atmel and are still available even though they are no longer made.

Atmel now makes “flash” chips that can be erased and rewritten a thousand times, no need for a UV eraser. These are the 29C series. AMD also makes a suitable flash chip for our purposes, the AM29F chips, this is also a flash chip. And there are also the SST27SF chips in various sizes we can use.

Before you buy your chips, make sure they are the correct type and size and that your programmer supports them.


Now we have to get our new chip onto the MEMCAL or CALPAC. As promised we’re going to cover the products made by Craig Moates. Craig makes a very nice variety of adaptors for just what we need. And he sells chips too, along with chip/adaptor combinations, at very good prices. Which ever of his adaptors you get, be sure to get a ZIF socket to go along with it. A ZIF (Zero Insertion Force) socket is a socket that you drop your chip into and flip the lever on it to lock the chip in place. This prevents bent pins on chips from constant removal and reinstallation during the tuning and burning process.

Visit Craig’s site here


Pictured below is Craig’s GP1 package, which consists of 1 adaptor, 1 ZIF socket and 2 chips.








Now that we have all of our equipment, software and chips, we’re ready to get into editing our .bin file.


First you’ll need to get a hold of a .bin file to start. If you are using scanning software most of them can do a “dump” of the information contained on your stock chip so you can use that as your starting .bin. Another place to get one is here  or here Just be aware that there is no guarantee that the downloaded .bins will be what they say or even be any good.


The first modification you’ll probably be making is changing the injector size. Start with whatever the injector mfg. says the size is. Example; if the injector is supposed to be 24 lb/hr, then set the injector size or “constant” to 24 lbs in the software. There will probably be a little “tweaking” of this constant later, but we need a good base to start from. After setting the injector constant its time to data log again. Have a look at your recorded BLM’s and what Cell they are in. Cells are based primarily on engine load (LV8). Now if your BLM’s are showing rich (below 128) or lean (above 128) across the board, you’ll have to play with your injector constant. If you are rich you’ll need to use a larger constant. For example; if your BLM’s were averaging 120, you’ll want to try a constant that is about 7% higher. The difference between 120 and 128 is 7%. So if we were using 24 lb injectors, our new constant would be 25.69 lbs. And if we were lean across the board, we’d need to find the percentage difference between 128 and our reading and then subtract that from 24 lbs. Data log again and repeat the process if needed until we get an average BLM of 128 across the board.




That one was simple, universally rich or lean is easy to correct. Now, what if just one or two cells were rich or lean? Correcting for this is a little more involved. First we need to get the BLM’s close to 128 with the injector constant. Then we’ll need to alter the tables that affect the Cells that are not 128. For a MAF system this would be the MAF tables. Within the tables are the grms/sec vs MAF voltage settings. Check your scan logs and find where the MAF voltage was when the BLM’s were not 128. In the table for the correct Cell find the MAF voltage that corresponds to the one in the log. Now adjust the grms/sec up or down by the percentage difference in the BLM’s, up to make it richer or down to make it leaner. Note: Only make one change at a time so you’ll be able to track the changes in the data logs. Keep making your changes until you are satisfied that you have the BLM’s in line. You’ll find that a MAF based system is much more tolerant to changes made to the engine than the MAP systems. This is because the MAF sensor measures the actual amount of air entering the engine. That allows the ECM to make corrections based on actual airflow.




Tuning the MAP system is slightly more involved than MAF and usually takes more changes to get it all right. The MAP system bases its fueling on vacuum readings from the MAP sensor that it then applies to a look up table pictured below.



There have been a couple of attempts to make the tuning of the VE tables easier with software. One is VE Master and the other is VE PHD. Neither one works perfectly, so we are left to do this manually. Well, notthat they don't work....But is has been joked about that you need a PHD to use VE PHD.

In getting started your changes are going to be done to the VE ( Volumetric Efficiency) tables as seen below.


Now is when you need to pay attention to your scans closely and match up the Kpa's to the RPM's  and weather you are rich or lean in each block. From there you will be making changes to the values in the blocks to try bringing your BLM's to 128. The 128 BLM is more critical in a MAP based system since the MAP sensor can't measure the actual amount of air flowing into the engine like a MAF can but instead uses these look up tables to add fuel for a given Kpa value, since the tables were programmed knowing how much air was entering the engine for a given Kpa value . Now you have modified your engine, a given Kpa value no longer represents the correct amount of air entering the engine, so the tables must be modified to either add or subtract fuel for the Kpa values that are not running at the optimal 128 BLM. This process has a steep learning curve and takes time and patience. As you get more experience reading your data logs, modifying the tables and tracking what your changes do, you'll become more adept at it.

A word of advice...again, change only one block at a time between data logs until you start to get the hang of it and know what your changes are doing to the engine.