“Under normal conditions, the turbo should produce a pressure between 2 to 8 psi, but can peak as high as 10 psi.”

I still put in some hours at a repair shop and recently dealt with a problem on a 1996 Chevy Truck K20 with a 6.5L diesel turbo engine. Whenever the owner drove along the interstate, the malfunction indicator light (MIL) would come on. If he pulled off to the side of the road and shut the engine off for a few seconds, he could restart it and there would be no further problems until his speed reached 72 mph. The truck's performance did not seem to be affected when the MIL came on.

Possible causes of this problem:

1. The waste gate solenoid is incapable of stopping the flow of vacuum;
2. the circuit is grounded externally of the computer or by the computer, allowing vacuum to continue to be applied to the waste gate actuator; or
3. the waste gate is defective or stuck in closed position.

Under normal conditions, the turbo should produce a pressure between 2 to 8 psi, but can peak as high as 10 psi. With no vacuum applied to the waste gate actuator, exhaust gas forces the waste gate open to the bypass position. When the computer grounds the waste gate solenoid, vacuum passes through the solenoid and acts upon the diaphragm of the actuator, forcing the waste gate to the full boost position. The computer monitors boost pressure by a special manifold absolute pressure (MAP) sensor located on the intake manifold. When the computer senses pressure in the intake greater than the desired amount determined by the various sensor inputs, it will start to pulse the waste gate solenoid off and on to decrease the vacuum being applied to the waste gate actuator. With the vacuum now reduced, exhaust gas forces the waste gate open to bypass some - or all - of the exhaust gas from the turbocharger turbine, which results in a reduction in turbocharger performance.

The scan tool showed a DTC P1656, indicating a possible problem with the waste gate. I road tested the truck to verify the complaint and to determine if there were any other performance or driveability problems. And just as the owner said, the MIL came on at 72 mph with no change in the truck's performance.

From previous experience with this DTC, I knew that:

1. When the conditions are correct for setting DTC P1656, the computer should release the ground from the waste gate solenoid.
2. When the ground is released from the solenoid, vacuum should no longer be allowed to pass to the waste gate actuator.
3. With no vacuum applied to the actuator, exhaust gas should then force the waste gate to open, which should reduce the amount of boost.
4. With the turbo unable to apply any boost pressure to the intake manifold there should be a reduction in power. But this is not what happened. Instead, the same amount of power was produced as was present before the DTC set.

The next step is to connect a pressure gauge on the turbocharger. Tee a vacuum gauge on the hose leading from the waste gate solenoid to the actuator, and monitor the computer's control of the solenoid with a lab scope.

When I road tested, system operation appeared to be operating normally. At 65 mph there were 8 pounds of boost, and the computer was still allowing full system vacuum to be applied to the actuator. But nearing 72 mph, the boost gauge showed 10 psi or better and when I accelerated to 75, there was a full 11 pounds of boost. By this time the MIL came on, resetting the DTC. When I reduced vehicle speed to 65 mph, I used the scan tool to clear the code. As I increased vehicle speed to 72 mph, I monitored the vacuum gauge and the lab scope. As boost pressure neared 10 pounds, the vacuum applied to the actuator started to decrease. The lab scope showed that the waste gate solenoid was being pulse-width modulated, but boost pressure was not being reduced. Accelerating slightly, boost pressure increased to the point where the MIL came on. When the light was activated by the computer, the waste gate solenoid was shut off by the computer and the vacuum applied to the actuator was now at zero.

This indicated that the computer, the solenoid and the vacuum supply were all working correctly — which only left the waste gate inside the turbo as the cause of the problem. The waste gate had rusted into the full boost position. No matter how hard exhaust gas tried to move the waste gate into the bypass position, it just would not move. Why didn't I just reach over and try to move the waste gate actuator to determine if that was the problem? I could have — but I was trying to determine if there was more than one problem causing this DTC to set.

How embarrassing to tell the customer, “You need a turbocharger,” only to find out later that there is a second problem with a waste gate solenoid that won't shut off the supply of vacuum to the actuator. If I had found a defective solenoid, I would have also checked the waste gate to see whether it moved freely.

Lester Bentley is an IDENTIFIX GM specialist. He is an ASE master, Buick service master, Chevrolet service master and is certified with the Pontiac Service Guild. In addition, he has his L1 certification and has 24 years of diagnostic and repair experience.

This information is provided by IDENTIFIX®. IDENTIFIX® resources cut diagnostic time and provide repair solutions that increase the shop's bottom line. From Repair-Trac pattern failure quick fixes, to Diagram-Online wiring diagrams by fax, to the Repair Hotline staffed by 32 master techs who specialize in diagnosing complex problems by phone or fax, IDENTIFIX® helps techicians fix more cars in less time. For more information on IDENTIFIX, call (800) 288-6210, 8 a.m. - 6 p.m. Central Time. www.identifix.com. © 2008 IDENTIFIX. All Rights Reserved. © 2000 IDENTIFIX. All Rights Reserved.

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