2011 Volkswagen Jetta - P0299 Underboost Condition

Question

My TDI keeps going into limp mode (flashing glow plug light) when I'm accelerating up an incline on the highway. If I cycle the key, the power comes back, but the P0299 Underboost code stays in the memory.

I've checked the vacuum lines and they seem okay. I'm worried the VNT (Variable Nozzle Turbine) vanes in the turbo are sticking. I've tried 'Italian tune-ups' to burn out the carbon, but it hasn't helped. The car has 130k miles.

BMW_Bjorn · Accepted Answer

✅ Confirmed fix: The issue was a pinhole leak in the Intercooler Outlet Hose. It wasn't visible under idle, but under load, the hose would expand and the crack would open up, bleeding off boost pressure.

I found it by performing a boost leak test using a soapy water spray and a localized pressure tester at the turbo inlet. Replaced the hose (Part #1K0145832E) and the car pulls hard again without any codes. Don't assume it's the turbo until you've pressurized the whole system!

FreightlinerFred · Answer

You need to check the vacuum actuator on the turbocharger itself. Use a vacuum hand pump and see if the rod moves smoothly. On these CR TDI engines, the actuator has a position sensor inside. If the diaphragm has a tiny leak, it won't hold the vanes open under load, leading to your underboost.

2011 Volkswagen Jetta - P0299 Underboost Condition

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