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Diagnosing a Failed or Clogged Fuel Injector on 1982-’95 Corvettes With OBD-I

Technically Speaking: Rough Running Part 3

James Berry May 26, 2016
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In this installment we will be finishing our series on diagnosing fuel injector problems. We will be covering some of the differences between the diagnostic procedures for the 1982-’95 Corvettes (OBD-I) and 1996-and-newer Corvettes (OBD-II). Most of the testing procedures will remain the same but the ways we read the information is just a little different.

For example, last time we covered Short Term Fuel Trim and Long Term Fuel Trim on 1996-and-newer Corvettes. On the 1982-’95 Corvettes we will refer to the fuel trims as Integrator and Block Learn.

Integrator And Block Learn

Integrator and Block Learn, like fuel trims, represent the percentage of change in fuel over time for the engine to operate properly. This information can be used to obtain how much the computer is adjusting the fuel injector pulse width so the engine has the correct air/fuel mixture. These values are read using a scanner.

How Integrator And Block Learn Works

I need to explain how Block Learn works. Don’t let this confuse you; it is simple once you start monitoring Block Learn on the scanner. So, if the theory seems puzzling, just go to the Using Block Learn as a Diagnostic Tool section and just start watching how it works.

The Integrator is normally updated frequently looking for the perfect 14.7:1 air/fuel ratio.

When the Integrator is less than 128, the computer is taking away fuel from the base calculation. For example, with an Integrator value of 0 this would represent almost 1 percent less fuel.

Basically, a value of 255 represents just under 1 percent of added fuel. If the Integrator hits 0 or 255, the computer resets the Integrator back to 128. It will then move up or down to the appropriate cell in a four-by-four Block Learn table of 16 cells representing four different engine-speed ranges and four levels of load.

Each increase or decrease of one in a Block Learn cell indicates approximately a 1 percent addition or subtraction of fuel for that speed and load range.

A restriction of only 8-10 percent on a single fuel injector can be enough to cause a misfire. When this occurs, unburned oxygen enters the exhaust and makes the O2 sensor read lean.


Integrator and Block Learn

Using Block Learn As A Diagnostic Tool

On the 1982 -’95 Corvette using OBD-I you will need to monitor the Integrator and Block Learn Multiplier data.

In simple terms, the Block Learn Multiplier is the long term fuel adjustment that the Engine Control Module (ECM) uses to try and keep the air/fuel ratio within acceptable parameters. The Integrator is the same idea, only for short term adjustments.

The Block Learn Multiplier value can range from 0 to 255. A value of 128 is ideal because it is the center point between 0 and 255. To maintain the correct air/fuel mixture (14.7:1) the ECM continually adjusts the Integrator or Block Learn Multiplier.

Since the ECM is continually adjusting to try and obtain the perfect air/fuel ratio on a good running vehicle, the Block Learn Multiplier will typically range from 105-150. The Integrator values will also typically range from 105-150.

If the numbers are 128 or higher for the Block Learn Multiplier the engine is running lean (the engine is not getting enough fuel).

If the values for the Block Learn Multiplier are lower than 128 the engine is running rich (the engine is getting too much fuel).

As the injectors get dirty, the fuel injector flow becomes restricted and the computer will attempt to adjust the fuel flow by opening the injector longer through the use of its Block Learn Multiplier function. Fuel injectors that are dirty will need an increase in pulse width (electronically holding the injector open longer) than injectors that are flowing the correct volume of fuel for the engine’s needs.

Remember, all of this is based on what the oxygen sensors are telling the engine computer. Other problems must be eliminated before cleaning or replacing an injector. Other fuel related problems can include vacuum leaks, weak fuel pump, restricted fuel filter, leaky fuel pressure regulator and so on, that can cause the Block Learn Multiplier to adjust pulse width.

To Test For A Vacuum Leak

A vacuum leak can create a lean running engine that requires extra fuel to balance out the fuel mixture. So the Integrator numbers will be higher than 128. Depending on the size of the vacuum leak the Integrator numbers will probably be 150 or higher.

To test for a vacuum leak, raise the engine rpms to 1,500-2,000 and hold it for approximately a minute. If the Integrator and Block Learn value drops back down to a more normal reading, it confirms the engine has a vacuum leak.

If the Integrator and Block Learn value does not change, the lean fuel condition is more likely a fuel delivery problem such as a weak fuel pump, restricted fuel filter, dirty fuel injectors or a leaky fuel pressure regulator.

After any repair, the Integrator and Block Learn memory should be reset so the engine is not trying to adjust for a problem that is no longer there. If you do not reset the Integrator and Block Learn memories over time they will adjust themselves back to where they belong.

I hope this helps you understand Integrator and Block Learn somewhat better and will help you use this as a diagnostic tool. I have been getting a lot of questions so I better get to work on the next segment.



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