I'm having an intermittent problem with my 2008 ML 320CDI. On really cold days, it appears the turbo wont kick in and the engine wont rev past 3G. Really frustrating on the highway. I guess this is called lim or limp mode?


After the car has warmed up and sits for a while it will work like normal.


Anyone come across this and had it resolved?

 

Have you tried shutting it off and restarting it as soon as it has warmed up (even a bit)?


Seems these engines have lots of different sensors which an out of spec signal will send the engine controller into a "safe" (or Limp Home) mode. Limiting boost, RPM and total power.


Restarting the car resets everything and often the same error is not there so all is well.


My GL320 (same engine) has done it on warm starts in wet weather also.


Some say it is the EGR, other the intake manifold "stirring" motor, others the exhaust filter differential pressure, others have had bad, wet or oily electrical connectors cause the problem.
I guess eventually the part fails, and the check engine light comes on and then the code stored points to the 'real' problem.

 

Thanks for the reply. Yes, that's what I do. After it has warmed up and I shut down, if I restart the car as long as it is still warm, it acts normal. The check engine does come on and scans have not shown anything conclusive. The mechanic did a "re-gen" of the exhaust? And that didn't help because the very next cold morning (-20 celcius) same thing.

 

What is the code shown when the check engine light comes on?


If the DPF is clogged you may need to have it cleaned or replaced.

 

If the CEL is on, then there is a code. I pulled my EGR valve out cleaned it and its been perfect ever since. Others have had to replace the pressure differential switch, an apparently inexpensive part and easy to replace. Do a search on here for EGR as well as on benzworld.org

 

Limp mode barely revs to 2100 RPM and won't accelerate faster than a VW Microbus.

 

Thanks for all the tips

 

One more thing. My drive way is sloped. I recently noticed that when I park the car on the incline (rather than the dcline when I back it in because of all the snow) the turbo does work the next day even if its cold. Does that point to something that was mentioned in one of the replies?

 

Is it consistent?


Any connection with wet as oppose to cold?

 

So far, yes including this morning

 

Angle of parking would indicate moisture collecting somewhere.
Maybe moisture entering a sensor where it should not be, or a pressure sensor that is supposed to be resistant to moisture that is no longer.
Or maybe a little water in a line blocking the flow of vapor and throwing off a pressure reading?

 

That makes sense.. Any sensor under the hood that you could suggest? I know I have had water come in behind the rear right tail light that flooded that board buried in that quarter panel. Could that be a possible area to look? Wondering of the turbo gets controlled through that board, as I think there is still moisture seeping in to that area?

 

I am just guessing, but I would suspect one of the sensors usually mentioned with 320 limp issues.


When I first started reading about it, everyone jumped right away to blaming the "intake tumbler flap motor" getting soaked with oil form leaking turbo intake gaskets. Now a year later a lot of people seem to be focused on the DPF pressure sensor, the Intake Temp sensor and various parts of the EGR system.


I would doubt the rear SAM has anything to do with engine control at all.


The lines to the DPF sensor are kind of long, so I wonder if that is to give the gas time to cool and protect the sensor. Being as they are long, they also offer lots of opportunity for fluid and condensation collection.


Does gas flow through the sensor or it measuring static pressure differential?
How great is the differential (If it is static could some fluid in the line affect the reading.
It is also under the car and receives water, salt and debris from the road, could it be the electrical connector going bad?


Lots of questions, no solid answers yet.

 

Funny you should mention the tumbler motor. Not knowing what this is, I always remember hearing (and feeling) a light thump close to my dead peddle under my left foot every morning when I entered the car before start up. I haven't heard it all winter and maybe this is a coincidence that the turbo works intermittantly. I have confirmed by trial and error though, that the turbo will definitely not work (the next day)if parked overnight facing downhill, but definitely does when parked facing up hill.

 

You may be on to something.


Water collecting in sensor or electrical connector.

 

Thanks again

 

Thank you!
Please keep us in the loop, as we are all dealing with these "quirks"

 

Update:
Problem solved-water got into DPF sensor for turbo. Piece was changed and working great now.

 

The DPF sensor (The one under the car and hooked with two lines to exhaust is not specifically "for the turbo". Just that when the DPF pressures are out of spec (or the sensor says they are due to its failing) the car limps without allowing any turbo boost.

I think I am going to put this one my list for spring maintenance (assuming I make it through the winter without too many limps.


Mine only happens when it is wet and cold so far.

 

PoolmanEd - Please give us the CEL code or what was the "fault" ? Im just curious if its the "differential pressure sensor" that goes to the DPF or was it something else. If its the diff press sensor - I think that can be cleaned first. Its not hard to get to.

 

Or at least the part number of the service ticket.


Not sure how you would clean the differential pressure sensor?

 

http://www.ebay.com/sch/i.html?_from...34928&_sacat=0

 

The lines to the sensor or the sensor itself could have water. If you clear that water - the problem might go away.

http://pmmonline.co.uk/technical/cau...m-regeneration

 

When this problem was first suggested for our recurring issues I wondered about water in the lines.
I guess since others had luck replacing the valve, I stopped thinking about it.


I would think any break or leakage would show as a constant error, not an intermittent.


However, if you look at the diagrams, the lines are fairly long and a low point could collect condensation, then a freeze would trip and error.
Although mine has not tripped in very cold weather, but only when cold and wet. I am wondering if it is just a bad electrical connection. (or cracks in the sensor allowing water to get in.)

 

N_jay - you are right, when it is cold electrical resistance increases. when you combine damp/wet conditions it gets even worse. All Im saying is that if you are a DIY person - you can start troubleshooting by making sure the lines are clear of any moisture and that the sensor and its electrical connectors are clean and dry. If it still fails - replace the sensor. Its really up to you if you want to buy a $99 sensor and replace it right away.