Scannerroid

You do not need a "Code Reader." Use a small wire connected between the A and B terminals in the ALDL connector. The check/service engine light will flash out the codes for you. You don't need any of those 20-30$ readers you see all over the place. Read ALDL and/or More on Tools.

ECM scanning for the rest of us.

Many references to using an ECM scanner show up in the forums... But just what is a scanner and what the hell do all those bloody number mean?

An ECM scanner is a computer in its own right and plugs into the Assembly Line Diagnostic/Communication Link connector. Depending on the scanner model and ECM involved it can do several things but the main ones are retrieving stored Diagnostic Trouble Codes (DTC is the formal name for the error codes.) and reading the Serial Data Stream. It is that second part that sets the scanner apart from code grabbers or a simple paper clip.

Many people make out like a scanner will instantly tell you what is wrong with the car. Very simply, it won't. You are going to have to practice reading and learning the numbers you see on a good engine so you know what to look for with a problem. Making the best use of an ECM scanner is as much art as science. If you have a poor grasp of how computer controlled engines work, the scanner may not help you much.

The main complication is that there's all sorts of things that can make the ECM feed you bogus data. You can't count on any one thing to tell you what is wrong. Another problem is that the OBD1 ECM data stream is far too slow to track fast moving items like O2 voltage and ignition timing. Even the more modern OBD2 systems have trouble keeping up with some items.

First you need a scanner...

If you have a laptop, you can use WinALDL and a special cable... There are many threads in the forums about it. The people using it like it. This works only with the older C3 ECMs used with the Fiero V6 and the distributor L4. The software is a free download far as I know. Someone in the forums sells the assembled cable if you don't want to make one. (WinALDL or The Fiero Store's USB tool will not work on P4 ECM, like 87-88 L4 Fiero.)

One of the least expensive standalone units is the AutoXray OBD1 GM Only scanner. (It's been discontinued.) autoxray.com. And try Actron, you can get their tools at any auto parts store. Commercial ones will work on C3 and P4 ECMs, including 87 and up Fiero L4 DIS motors.

I have the AutoXray. It is nice enough for what I need but if you need to look at several parameters at once it can be a pain. The screen is setup to do one item at a time. It will track everything but you have to scroll back and forth to see things.

Scanning while driving

Warning: If you notice that your engine runs significantly different with the scanner in operation, don't drive the car!

The ECM isn't made to be multitasking and the scanner can mess with the ignition timing, idle RPM, and more, in many configurations of ECM.

What can I do with it?

You can learn certain things about how the engine is running and if a change you made may be causing trouble. This isn't a complete guide but it should give you some starting points depending what is wrong with the car. What you can learn from the scanner depends greatly on what ECM is in use. The newer the ECM, the more it will tell you.

System Voltage

The voltage the ECM sees is important. Besides more obvious problems like weak spark with low voltage, you'll also have injection problems. If the voltage to the ECM is too low, the ECM will go into a low voltage mode that changes the entire fuel pulse setup. This is one reason why I keep harping about checking the charging system and grounds whenever you have poor idle and other engine problems.

Idle Setting

The scanner will tell you where the Idle Air Control motor is positioned. This can help you nail down small vacuum leaks or incorrect throttle stop settings. With the car idling in park/neutral, I believe most Port fuel injected engines should show about 30-35 steps on a hot engine. 700 series TBI units will show about 10-12 steps on the IAC. I don't have the number for the older 300 series TBI. (TBI units can't shut the throttle butterfly too much or the fuel won't be able to get by properly at idle.)

As an example, an Idle count of 3 at idle on a 700 series TBI likely means there is a significant vacuum leak or some twit has messed with the throttle stop. A miss adjusted throttle stop can also mess with BLM at idle.

The IAC should stay pretty stable at idle. An IAC that is dancing around indicates problems. Could be vacuum problems or any number of other things.

Integrator and BLM

If you think the car is running rich or far worse lean, these numbers can help you find out how bad the problem is.

Ideally both numbers should be close to 128. So you know what they do... On newer cars they are called short term and long term fuel trim respectively. These numbers tell you how the ECM is setting fuel mix.

Integrator, the short-term fuel trim, will dance around as the car runs. In many cases it changes too fast to be accurately read on the scanner. If it strays too far from 128 for too long, the ECM will change the BLM number to try to get Integrator to stay closer to 128. A car that is running rich will usually show a low, even pegged, BLM number. A high BLM indicates a lean problem that could be real bad for the engine.

BLM, Block Learn Mode, the long-term fuel trim, gets its name from blocks of numbers in the PROM program.

The Integrator will change at nearly every update. BLM should change very little or not at all as long as RPM is constant, like at idle or cruising down the highway. BLM will change based on RPM as the engine may need different fuel trim at 2500 RPM than it does at idle. This is normal. What you never want to see is BLM pegged at the low or high limit. Pegging BLM is usually a bad thing, but won't trip an error unless the Integrator also pegs for more than X seconds. 45 rich mix is the error you get with Integrator pegged low and 44 lean mix is pegged high.

The numbers are backwards

As I noted, low numbers mean the car is running rich. On the face of it this sounds to many people like the car is adding fuel. What the low Integrator or BLM number means is that the ECM is cutting fuel in response to a rich condition. A high Integrator or BLM number means the ECM is adding fuel in response to a lean condition.

When to worry

Check your fuel pressure? Fuel pressure can mess with BLM...

Pegging low will waste gas and cause drivability problems. Pegging the BLM high is especially bad as you will run the engine lean and likely burn stuff up in fairly short order.

The BLM not equaling 128 isn't automatically a problem. There are many things in the normal operation of a vehicle that will make the BLM move. If it is a couple notches either way then there usually isn't much problem. For example, 124-126 could simply be a response to the weather that day making it need a bit less fuel. On the other hand, a BLM at idle that is always 120 could mean something like fuel pressure is too high, you have the wrong injector(s) installed, or you have an injector with a slight leak.

As I mentioned you don't want to see the BLM pegged. The range of BLM can be fairly narrow. TK in the forums tells me the low BLM limit on all the V6 Fiero is 120. The low limit on the 87+ DIS L4 is 112. The high limit for most GM engines is 150. I don't have any more than that. If the BLM is more than 4 to 5 plus/minus from 128 then post in the forums so someone can help you figure it out. A BLM that is consistently low could be sign of leaking injectors or other problems even if it isn't pegged.

Temperature Sensors

Besides reading temperature sensor values, a scanner and thermometer can make testing the sensors very painless. Since Fiero allows easy access to the coolant via the thermostat housing, all you need is a cheap digital meat thermometer you can find in many grocery stores for about $15.

On a cold engine, key on and engine off... Scan the coolant temp from the ECM and read the coolant temp in the thermostat neck with a good thermometer. They should read within a few degrees of each other. Once you do that, run the engine up to temperature but DO NOT allow it to build pressure in the cooling system! Repeat the test again. (The second part of the test is much easier if you have an IR pyrometer. Then you can just cap the system and read the thermostat housing temperature without opening the system. You can get IR units for under $100 now.)

Same test can be used for the air and transmission temp sensors on cars that have them. Due to its location in the manifold, the MAT sensor used on DIS L4 will need to pulled and let it sit in free air a few minutes. A trans temp sensor will usually read the same temp you would get reading the trans oil pan with an IR pyrometer.

Coolant Fan and Cruise Control

On the later cars with P4 ECM, like 87+ DIS L4, coolant fan management and cruise control is integrated into the ECM and status of all controls is reported in the data stream.

AC control

In many cars the AC clutch is controlled at least partly by the ECM. The data stream will indicate something like "AC Requested" and "AC Clutch En/Disabled." AC Requested means all the switches in the AC system, Control Panel, High pressure, low pressure and clutch cycle if you have it, are closed. The final decision is up to the ECM and sets AC Clutch Enabled. Normally, ECM sets AC Clutch Enabled as soon as AC Requested signal is High/Active but most ECM programs have an AC cutout that activates above some throttle value. (Another reason to make certain your throttle cable is actually making the throttle body reach WOT.) This way the AC isn't sucking power and protects the Compressor from over rev when you mash the gas. Even the later V5 compressor based AC systems have this ECM control.

What you can't do with it

As noted, things that change rapidly won't be accurately reflected on the scanner screen. O2 sensor function and Ignition timing are just a couple examples. Others are less obvious. While you can use it to check the gross status of the Throttle Position Sensor, the scanner will not reliably tell you if there is a bad sensor. If the TPS has small dropouts, they may not last long enough for the scanner to register them. Items like these will need other testing equipment like a quality voltmeter or shop scope.