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Refine your game plan with a logical process to diagnosis

Saturday, June 1, 2019 - 06:00
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Franklin called me back over to get me up to speed and brain storm our next move. Thinking how there was a possibility of hydrocarbons being in the brake booster diaphragm if the filter diaphragm was leaking, there was a good possibility of the smoke being consumed and not being visible. We switched from shop air to CO2 on the smoke machine and tested for leaks using the Bullyseye leak tester. The testing along with using CO2 uncovered the leak in the power booster.

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As we depressed the brake pedal, we noticed that the leak would be worse or nonexistent at times. I assume the reason for the different reading was that the power booster diaphragm was flexing causing the leak to be worse or better depending how it flexed. We blocked off the power booster to confirm our finding then ordered a new booster. Franklin removed the old power booster and install the new one that resulted in a normal engine idle.

Figure 10

With the engine now running normal, Franklin connected the GM Tech 2 and reset the Adaptive Fuel Trim. The Adaptive Fuel Trim resets (Figure 10) the fuel trim so the engine will not continue to add a high commanded rate of fuel. Remember after any Fuel Trim repair an Adaptive Fuel Trim reset is needed to get the engine fuel delivery commands back in a normal operating range. This is an important step that is commonly overlooked that can cause other problems such as a P0420 to pop up. The rich condition caused by a command that has not been reset can take a border line converter and push it over the edge. After the repair, the fuel trim reset, along with a good test drive it was time to recheck the vehicle for DTCs and fuel trim readings. Since the Tech 2 was left connected for the fuel trim reset it was taken along for the test drive and used to recheck the vehicle when it was returned to the shop. The LTFT readings were now back to normal along with a stable idle and good brakes.

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