Turbo turns off every time in 3th gear 5-6RPM , when i restart car it turns on. I get only this error P029921 , changed wastegate, solenoid, diverter valve same problem, no leaks, what could be problem?
scaryfast
Junior Member
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Oct 5, 2025
Hi Revi, Is the turbo original to the car? Over time, the wastegate linkage can develop play. With this excessive play, the internal wastegate flap does not close properly and allows exhaust gases to bypass the turbine wheel causing an under boost condition. With a hand vacuum pump tool connected to the wastegate actuator, manually add vacuum until the wastegate flap closes fully or the actuator rod reaches its maximum pull. Grab hold of the linkage and wiggle it to check how loose it is. You may need to disconnect the exhaust downpipe to visually inspect the condition of the wastegate flap.
It sounds like you replaced that wastegate actuator. On a M274 engine, the actuator rod is non-adjustable, right? Perhaps verify that the new vacuum actuator rod is the same length as the OE part. I don't think Mercedes sells that part separately as it is included with the complete factory turbocharger assembly. It's only offered as an aftermarket part.
If you do have excessive play in the linkage, and the flap is not closing fully, you may need a new turbocharger. However, there could be some companies that offer services to refurbish the bushing, pin, and arm of the linkage on your turbo. Not sure, but it would be worth pursuing.
One thing that you could try if you find that the linkage play is not too bad, add some washers on the two threaded studs of the actuator. Just 3 or 4mm in width at the most. The washers would go between the actuator and the mount, creating a gap. The two nuts would hold it in place as normal. This will physically mount the actuator farther from the linkage, which changes the pulling distance of the rod to be longer. The washers just give the actuator a little head start allowing farther pull travel of the rod. This longer pull could take up enough of the play in the wastegate linkage to firmly close the internal flap and stop the low boost condition. Worth a try.
It sounds like you replaced that wastegate actuator. On a M274 engine, the actuator rod is non-adjustable, right? Perhaps verify that the new vacuum actuator rod is the same length as the OE part. I don't think Mercedes sells that part separately as it is included with the complete factory turbocharger assembly. It's only offered as an aftermarket part.
If you do have excessive play in the linkage, and the flap is not closing fully, you may need a new turbocharger. However, there could be some companies that offer services to refurbish the bushing, pin, and arm of the linkage on your turbo. Not sure, but it would be worth pursuing.
One thing that you could try if you find that the linkage play is not too bad, add some washers on the two threaded studs of the actuator. Just 3 or 4mm in width at the most. The washers would go between the actuator and the mount, creating a gap. The two nuts would hold it in place as normal. This will physically mount the actuator farther from the linkage, which changes the pulling distance of the rod to be longer. The washers just give the actuator a little head start allowing farther pull travel of the rod. This longer pull could take up enough of the play in the wastegate linkage to firmly close the internal flap and stop the low boost condition. Worth a try.
