Well this thread has gone quiet , needs a re-gen excuse the pun!
OK go back to Renault Freres and their bulletin I posted where they admitted/stated the system is ,limited to about
160,000 kms of life.
They also seemed to agree that the SCR Cat arrangement with urea injection was the most effective method of taming diesel exhaust emissions.
There is an interesting statement that indicates the system life is limited by the,materials used and their limitations.
OK , now staying with the Bluetec Sprinter for a moment we all know that the system has a much longer duration in most cases of more than their stated
160K Kms threshold, or say 100,000 miles. That's a given!
In fact we have seen original system with much higher figures than those quoted with the MB Sprinter product
That stated we start to see these systems plugged up from about 135,00 to 175,000 miles.
Interestingly we change out SCR (Bazooka tube Cats) three time more than the exhaust gas converting Cat/DPF's which hangs off the turbo.
So what are the symptoms to determine that you might ask?
In short the answer is NOx efficiency issues, when the gas conversion rate across the unit dips to below a 50% rate max when the recognized standard of gas conversion should be
75% or more .
At point critical you will get the Adblue light on, maybe the "count down to going nowhere", and for many the anxiety of making teeth marks on the steering wheel, and a tow to the dreaded dealer for a wallet flush.
Now we can go over the multitude of NOx sensor issues & upgrades etc which have been beaten to death by posts on this forum, but few mention the internal upgrades on the SCR Cat itself. I know of four such changes where the internal vortex fixed vane has changes to promote DEF fluid mixing, the last one issued recently without fanfare has the rear NOx sensor orientation changed and moved up closer to the rearmost internal conversion "brick".
So if you have followed this narrative so far, and the SCR Cat's provision being there to merely convert (as the manufacturers say in their press blurb) an inert hot gas laden with nitrogenous oxides from activities created on the other side (upstream) of the exhaust system.
So why does it get plugged up?
After all its supposed in theory to have just hot gas for NOx conversion & some fine urea mist blown through it! Nothing more nothing less.!
Now I consider it naive to take the manufacturers statement as gospel, since we know the upstream provision the Cat & DPF to burn up soot to an ash substance and clean the exhaust gases of the most harmfully emissions produced of soot and carbon monoxide products .
So their statement falls short of the real fact that soot particles and ash will not stay in the CAT/DPF under hurricane like gas flow but rather drift down into the SCR cat and plug the bloody thing up!
Any wonder then when you exchange one old SCR Cat, it weighs about three times heavier than the new one.
So the unit is really acting as a filter, not just cleaning NOx gases as often stated.
Once the thing gets plugged up its curtains, its wallet flush time often resulting in changing the whole Kahuna.
But that's not all , with the system trying to get NOX parameters into programmed compliance, the DEF system is commanded to inject MORE DEF fluid in the SCR Cat which dries internally due to poor flow creates a stink and plugs it up some more..
Often the precursor of all this is a coated/ encrusted DEF dosing injector.
Its really like a word in your ear :-
"Hey Mr Jones take care of this crusted up dosing valve issue because the system is on the road to the undertaker".
I will add some more once this has sunk in.
Dennis