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2008 ML320 CDI
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Discussion Starter #1
I’ve owned my 2008 ML-320 CDI for just about a year now. For the first six months everything was fine, then I started having issues with my Exhaust Pressure Sensor. The symptoms are that the car will start and then quickly sputter and die. Sometimes this behavior is preceded by a P0299 - Low Boost Pressure, but not always. All that needs to be done is replace the sensor and the car is happy as a clam for another 1000 miles. Then it fails again.

I’ve replaced the sensor 4 times now, and without fail, after approx. 1000 miles it will happen again. So far the dealer is stumped and at this point it feels like they are shooting int the dark trying to fix it. Here are the fixes that have been attempted:

1st time - Replaced the sensor myself
2nd time - Dealer replaced the sensor and claimed the other one was bad.
3rd time - Dealer re-pinned the electrical connectors for the sensor.
4th time - Claimed it was the PCV leaking oil, I replaced that myself.

I’m coming up on the half way point of replacing the sensor the 4th time, but I’m not confident its actually fixed. Has anyone run into this issue before or have any suggestions on what to try next?
 

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2008 ML320 CDI
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Discussion Starter #3
I was just about to edit the original post to include this: Usually when the car sputters and dies, there is no CEL and no code. It simply won't stay running. Unplugging the sensor appears to break the turbo health check logic and allows the vehicle to drive without actuating the turbo; a slightly more powerful "limp home mode".
 

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2007 E320 Bluetec, 2008 320ML CDI
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Yepper. That's limp mode. You feel like your ability to actually move has become geological.
Does a key off - key on cure that? May be a redundant question if the engine dies?
Vehicle is up to temperature when it occurs or cold?
How many miles on vehicle?
What kind of mileage do you get?
 

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2008 ML320 CDI
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Discussion Starter #5
Yepper. That's limp mode. You feel like your ability to actually move has become geological.
Does a key off - key on cure that? May be a redundant question if the engine dies?
Vehicle is up to temperature when it occurs or cold?
How many miles on vehicle?
What kind of mileage do you get?
I don't think this is limp mode. I can still push it hard to redline and it will shift normally, it just takes its sweet time getting there due to no turbo.

Key off - key on doesn't help anything.

Come to think of it, it has never died while warm, only cold after sitting overnight. In fact the first time it happened, we could unplug the sensor, drive it for a while then plug the sensor in and everything would work until the next cold start. But that hasn't happened the last 3 times its died.

It has 110,000 miles on it and we average 20 MPG, mostly all city. On a recent road trip of a 500 highway miles we were around 26 MPG.
 

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2007 E320 Bluetec, 2008 320ML CDI
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Things that come to mind are:
Fuel pressure - are you changing the filter every 20,000 or so?
Turbo actuator may be failing. (Had this on my W211 Bluetec. Produced over pressure as well as under pressure).
Port inlet motor.
Glow plug controller or failed glow plugs.

Your mpg sounds similar to mine.

Skippy
 

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2008 ML320 CDI
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Discussion Starter #7
The fuel filter had 10,000 miles on it, but replaced it 500 miles ago, so we'll see if that does the trick.

As for the other suggestions, I've considered those, but I'm having a hard time wrapping my head around how it could be something like that, yet run fine for 1,000 miles after only replacing the exhaust pressure sensor.
 

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2007 E320 Bluetec, 2008 320ML CDI
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Seeing that it's an exhaust pressure sensor, the obvious question would be clogged DPF or other restriction in the exhaust.
Doesn't seem likely given your statements on running conditions.
 

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Discussion Starter #10
I'm in Salt Lake City, so roughly 4,200 feet, but thats not terribly high altitude.

I've considered the clogged DPF or exhaust, but ultimately dismissed those as issues due to it running strong between sensor replacements.

At this point I'm starting to think it may be something electrical slowly killing the sensor, maybe some voltage spikes or a short somewhere. But everything else is operating normally.
 

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2007 E320 Bluetec, 2008 320ML CDI
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I looked online to see what an exhaust pressure sensor looked like and the majority of the returns showed a differential pressure sensor. What does your sensor look like or what is the part number?
 

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2008 ML320 CDI
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Discussion Starter #12
It looks like the part has a few names, Exhaust Backpressure sensor, Pressure Gauge Converter etc, but either way the part # is 009-153-50-28. Its the sensor right on the top of turbo, just to the driver's side of center.
 

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2007 E320 Bluetec, 2008 320ML CDI
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2008 ML 320 CDI
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2008 ML 320 CDI

Hi How did it go this time with EX Back pressure sensor replacement. Ive had a string of issues with our ML.
I replaced the same sensor and am wondering if my test drive triggered it to go again I was driving a bit rough. Sometimes oil from the turbo leaks on the wiring harness bellow the MAF assembly. In my case it caused a 15 amp fuse to blow which was giving me multiple CEL's. Im going to open everything up and give it a good clean underneath MAF. Maybe same issue causing your Sensor to short?

anyways now
Now it feels like my waste gate keeps exhausting and not providing any boost. I got my wife to hit the throttle and i saw what looked like the acuator motor moving up and down so I don't think its stuck. Anyways good luck brother
 

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2003 E500, 2008 ML320CDI
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I am helping a friend diagnose an identical issue. So far we have had the rear pressure sensor replaced once. The symptoms are back, same as before, pretty much identical to yours. We are in Centerville UT, so maybe there's something in the diesel.......:frown
 

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2003 E500, 2008 ML320CDI
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I looked up the description of that sensor on an online shop manual, here is what it says:

"The exhaust back pressure sensor determines the exhaust back pressure upstream of the turbocharger. The pressure value is used by the CDI control unit (N3/9) to monitor the diesel particulate filter (DPF) fill level and for protection of the turbocharger and the engine.

The exhaust gas back pressure deforms a membrane, which affects a potentiometer. This alters the resistance value of the potentiometer and influences the signal voltage, which is used as information regarding the exhaust gas back pressure for the CDI control unit."

I wonder if excessive back pressure could potentially rupture the membrane in the sensor.
 

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2008 ML320 CDI
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Discussion Starter #17
I looked up the description of that sensor on an online shop manual, here is what it says:

"The exhaust back pressure sensor determines the exhaust back pressure upstream of the turbocharger. The pressure value is used by the CDI control unit (N3/9) to monitor the diesel particulate filter (DPF) fill level and for protection of the turbocharger and the engine.

The exhaust gas back pressure deforms a membrane, which affects a potentiometer. This alters the resistance value of the potentiometer and influences the signal voltage, which is used as information regarding the exhaust gas back pressure for the CDI control unit."

I wonder if excessive back pressure could potentially rupture the membrane in the sensor.
That sounds plausible. Although it would be odd for the sensor to fail under the exact conditions its meant to detect.
 

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2003 E500, 2008 ML320CDI
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My scanner shows the miles since last DPF regen are just below 1000 miles. This seems to coincide with your failures of the sensor at 1000 miles. Perhaps the new sensor is triggering a DPF regen cycle, then failing when the back pressure gets to a high enough level.

Although, this seems like the intended operation, so like you say, it's odd that the failure is caused by the exact conditions it is intended to detect. My wife drives an identical 2008 ml320cdi, so I am going to compare some of the sensor readings from her car for reference. Her's hasn't ever had this issue that I am aware of.
 

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2003 E500, 2008 ML320CDI
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I have the iCarsoft MBII scanner. That is one of the menu options, but I haven't found much in the way of instructions on how to do this. I assume that the engine has to be running, and currently the ML that we are working on will only run for about 5 seconds before shutting down.
 
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