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Discussion Starter #1
Hi there, This is my first post on here. I have owned my 300c for 2 years now and covered many miles. I am an active member on the facebook forum but I am in dire need of help! My car is well looked after and cared for and since owning her I have cover 36,000 miles. now to the problem. 2006 CRD Estate.

P0299: underboost.

Symptoms: Jerking/kangooring under light acceleration(but only In drive/ under load). Car will boost fine with a heavy foot. When on the motorway and in cruise control at 65MPH the car will rev and boost every 20 seconds or so and make a very strange Turbo flutter.

Things done to it prior to fault: The car was taken in to have oil cooler seals done and swirl flaps removed. Upon collection a stationary DPF Regen was ran on a snapon tool (under mercedes) which read 300% full on dpf burnt down to 1%. I had taken the car for a short drive before the DPF regen and hadn't noticed the issue but I was quite "spririted" so may have avoided the bucking threshold.

Tests so far:
Air hose (and pretty much entire engine) sprayed with soapy water but no bubbles found. O rings checked on intercooler pipes and turbo hard pipe.
Kideny delete pipe checked.
Intercooler sprayed with soapy water and inspected.
Tube checked for play (none).
Turbo actuator changed.
Exhaust back pressure sensor momentarily swapped with a donor car.
02 Sensor changed (thought the DPF may have burnt it out)
Maf cleaned (but not changed)

The strange bit: With the Maf unplugged the car performs more like how it used to (though burns through more fuel)

I can get live data graphs in the form of .CSV files if this will help anyone? I just want Mathilda to work again, I have put so much time effort and money into making this car perfect for me :(
 

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You seem to have checked most of the likely causes. For what it's worth, this is what the Service Manual says about P0299:

Possible Causes:
INTERMITTENT DTC
CLOGGED AIR FILTER
INTAKE AIR RESTRICTION
INTAKE AIR LEAK
BOOST PRESSURE SERVO MOTOR
TURBOCHARGER
 

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Discussion Starter #4
Hey There!

Thanks for replying (Nice coloured tourer:p ),

Under the kidney delete pipe there is a slight bend in the intercooler pipe but not enough to cause a restriction I wouldnt have thought, I have a K&N panel filter fitted that is still nice and red (cleaned around 4000 miles ago).
I shall have another look at the intercooler pipes but with the maf unplugged you would assum the problem would still persist wouldn't you?
 

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Sorry, can't shed any more light on this problem. I've never experienced P0299 myself and I can't recall this coming up before, although I would try a search on the forum. There are loads of posts on the web about this fault, with many of them pointing to the turbo actuator, which you have already changed. It does sometimes happen that a replacement part can also prove to be at fault. Why did the original actuator fail?
 

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Discussion Starter #6
Hi again,

The actuator didn't fail, I changed it incase of the fault stemming to it (as so many previous cases do!)

Today I changed out the MAF, the MAP sensor, and theboost sensor (by the"Throttle body") and the issue still persisted. I managed to get some live MAP and MAF data from a completely stock 300c to compare to mine. We did the same drive and the readings are VERY different!

For clarity I have had my car remapped (long ago), have modified the airbox (no resonator and holes on cold sides), and I have a K&N panel filter.

This is the standard car running fine, no faults:
175069



Now this is my car. Notice that the Manifold pressure drops when accelerating whereas the standard fault free car does not. My MAF readings are almost double also! My pressure sits slightly higher on idle but its the drops and spike that confuse me. I would get this translates into a leak somewhere but would love some other opinions on this, is there anyone else that you think can weigh in?
175070
 

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I wasn't aware that you'd had a remap. I've no experience with remaps and/or issues that may result. I would assume that turbo activation could be controlled by the remap. Perhaps you need to restore the stock map before continuing the diagnostics? Maybe whoever provided the remap would be able to perform a detailed test and diagnosis?
 

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Discussion Starter #8
Hi there,

the remap was done around 2 years ago and has been flawless however you are correct, it would be good to rule it out! Unfortunately the tuner I used lives quite a distance away, so it may be some time before I can get to him to try this. I have contacted a garage about trying a pressurised boost leak smoke test so that is one of my steps, I also have an empty dpf to chuck on and see if that helps solve the issue.
 

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I've had another look at your MAP/MAF data and noticed a significant difference in MAF between cars. The standard car MAF peaks at 85 g/sec, whereas your car is peaking at 130 g/sec, so where is all this air going? It does look as if it's leaking out somewhere, or maybe the MAF sensor is faulty? If the MAF sensor is over-reading then this might result in turbo boost being removed. This could explain why unplugging the MAF sensor appears to offer some improvement.
 

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You say you have modified the airbox and in particular the resonator. Removing or altering the intake resonator will for sure impact the MAF readings. Chasing after cold air intake effects is great but suggest you restore the original resonator to see if this evens out the MAP and MAF fluctuations that you are seeing....that is after all what intake resonators are supposed to do. Good luck .
There are some interesting papers on intake resonators and most agree that playing with the rersonator more often than not actually reduces power.
 
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