Black smoke under acceleration that comes and goes. Have changed mass air flow sensor and did not help. Model is 2005 six speed turbo diesel

black smoke = excessive fuel = bad injector tips , low injector pressure , turbo not working properly
take the car to an accredited diesel specialist professional for a pump pressure and injector pressure test, injector calibration and tip replacement

if turbo charged diesel , have the turbo inspected and check the hoses for tight at connections

Check and replace if necessary the Mass Air Flow (MAF) sensor.

I assume its the 6.0L diesel...If so sigh......double sigh.. I had one of these...triple sigh. I need a bit more clarification. How many km on the engine/vehicle. Also, you say that it blows black smoke while running at a constant speed at 2600 rpm, or while you are accelerating to 2600 rpm. Also, when you say the boost goes down, does it decrease by itself, or when the throttle is let off. The boost naturally goes down when the throttle is released (not trying to be a ****, i dont know how much you know). However, if the boost is going down by itself, while the engine is under load (its not supposed to, it should stay constant) it could be a couple of things, which "can" contribute to black smoke. If there is a leak anywhere from the turbo to the intake of the engine, it will cause the engine to get less boost, which means less air into the cylinders, which means black smoke. Check all of the connectors/tubing coming from the turbo to the intercooler, and from the intercooler to the intake manifold and see if there are any problems. The more costly problems which results in boost going down, and black smoke, deals with a problem with the wastegate on the turbo (controls how much boost is produced, and when too much is being made, it will bleed off excess pressure). So if your wastegate is malfunctioning, your boost will go down and you will get black smoke cause the engine is not getting enough air. also, around 160km...80mi, the injectors "can" start to go on the 6.0L diesels....I would check the turbo for leaks, the tubing for leaks, and the intercooler for leaks (a big rock can punch a hole in radiators and intercoolers...intercooler will be in front of the radiator). I hope this helps

Changed the timing belt and knocked loose the wires or connections to the sensors on the mass airflow device.

Turbo failure is never normally intermittant so I doubt it is a turbo fault.

The wastegate relys on a signal from the ECU via the MAFS.

Mass Air Flow Sensors are very common to fail on VW TDi and this also fits in with your symptoms.

Kind regards

Lloyd

sounds like your air flow moniter sensor is faulty should be on the presure side of the intake system some where not sure where tho and this might turn the light off if it solves the problem when replaced hope this helps

i had this problem and thought the fuel filter oil filter needed replacing so i have just had a service done but problem is still there but not as bad. been told it could be the AGR valve or a split pipe going to it which recuirculates unused fuel back to burn again. therefore unspent fuel is coming out of the exhaust causing the black smoke. i am still looking for the answer aswell, hope this helps.

I had a similar issue to this after I got in a VERY small fender bender (literally) in an 87 Turbo Supra. In my case, my pasenger fender got pushed in JUST enough to crack a charge pipe and cause a boost leak.

Basically, what was happening was the turbo would spool and build boost, the mass fair flow sensor would read all this air coming in and tell the ECU to dial up the the appropriate amount of fuel. When the air reached the cracks in the charge pipe, it would escape out of them. Thus there was a much lower air-to-fuel ratio, causing less fuel to combust, which lead to power loss and black smoke out of the exhaust.

My guess is either an oxygen sensor, or more likely a boost leak.

Good luck mate.

I'm sorry to say this, but it sounds as if your turbo is shot.