Overboost/Underboost problem solved
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Newbie
Joined: Oct 2019
Posts: 9
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From: North Bend, WA
2012 ML350 BlueTec
Overboost/Underboost problem solved
This post is only meant to help those in the future who might find the same problem related to an Exhaust Pressure Sensor and turbo overboost/underboost, as I found little on this specific problem for this specific engine.
Got a check engine light and limp mode suddenly while at highway speeds. My cheap bluetooth ODB scanner gave the codes P0234 (Turbo Overboost), P0299 (Turbo Underboost), and P0471 (Exhaust Pressure Sensor A range). I also have live data on scanner, so I saw that the Commmanded vs Actual boost pressure was kind of all over the place but within 30%; the EGR Valve Commanded vs Actual was off +/- 20%, and most importantly the Exhaust Back Pressure Sensor was reading over 200 kPa when it should be closer to 100 kPa at idle. I put in a new Exhaust Back Pressure Sensor (part number A0071534328, $30 on amazon), it fixed everything. Now looking at live date, the EGR error is less than 1% at all times, and the turbo boost is correct. The Exhaust Back Pressure Sensor is between the turbo and EGR, at the top back of the engine just to the right of the turbo; it comes off with a 24mm (or 15/16") deep socket. Normal depth socket will not work.
Side note 1: all of the pictures I find of genuine Exhaust Back Pressure Sensors online say "Made In Mexico", same as the new one I put in. But the bad one I took out says "Made in China". Not a comment of Chinese manufacturing, but there might be two OE versions of that part, or there may be knock-offs. Go for the Mexican version.
Side note 2: I saw a lot of posts about the turbo wastegate actuator potentially being the culprit. There are wires coming off the actuator's PCB that act as fuseable links which can sometimes break. I took the actuator controller off (the PCB part held on by 6 metal clips), and all wires looked good. There are forum posts and youtube videos showing how to remove that part and re-solder, should that be your problem.
Got a check engine light and limp mode suddenly while at highway speeds. My cheap bluetooth ODB scanner gave the codes P0234 (Turbo Overboost), P0299 (Turbo Underboost), and P0471 (Exhaust Pressure Sensor A range). I also have live data on scanner, so I saw that the Commmanded vs Actual boost pressure was kind of all over the place but within 30%; the EGR Valve Commanded vs Actual was off +/- 20%, and most importantly the Exhaust Back Pressure Sensor was reading over 200 kPa when it should be closer to 100 kPa at idle. I put in a new Exhaust Back Pressure Sensor (part number A0071534328, $30 on amazon), it fixed everything. Now looking at live date, the EGR error is less than 1% at all times, and the turbo boost is correct. The Exhaust Back Pressure Sensor is between the turbo and EGR, at the top back of the engine just to the right of the turbo; it comes off with a 24mm (or 15/16") deep socket. Normal depth socket will not work.
Side note 1: all of the pictures I find of genuine Exhaust Back Pressure Sensors online say "Made In Mexico", same as the new one I put in. But the bad one I took out says "Made in China". Not a comment of Chinese manufacturing, but there might be two OE versions of that part, or there may be knock-offs. Go for the Mexican version.
Side note 2: I saw a lot of posts about the turbo wastegate actuator potentially being the culprit. There are wires coming off the actuator's PCB that act as fuseable links which can sometimes break. I took the actuator controller off (the PCB part held on by 6 metal clips), and all wires looked good. There are forum posts and youtube videos showing how to remove that part and re-solder, should that be your problem.
