I want to share some observations and solutions with emissions and engine codes we have been experiencing with the Mercedes MBE 900 diesel engines in our bus fleet. We have a dozen of these engines 2007 - 2008 emission controlled and they are all off warranty so this has allowed us to learn by "doing" with the various emission and engine codes that normally occur. The time we have spent troubleshooting these various problems has taught us a lot and we have adapted to the typical problems that happen with this model engine.
The EGR Cooler above had to be replaced due to a leakage problem. With a coolant leak they will eventually plug up with a sludge build up if it does not get attended to. If the EGR cooler is not leaking and has a soot build up it will require removal and a hot tank cleaning at your local radiator repair shop.
They are not cheap to replace however if they leak you'll experience coolant loss and engine code problems because of the soot wet mess that will develop inside the cooler exhaust core. This will cause poor exhaust gas flow and reduce engine power. The EGR cooler exhaust gases flow to the EGR valve which mixes the cooled gases with the charge air intake reducing NOx (oxides of nitrogen) creating reduced emissions out of the tail pipe.
The charge air cooler to the left split open right at the core on the engine side. This problem was hard to detect because of the noise going on when the engine is running.
The reason we found it was because of a pressure check on the cooler itself. There was a 2631 Mercedes engine code which is a "turbo boost performance" issue.
Mercedes has a procedure for troubleshooting this code and it's quite involved. You have to check the boost pressure sensor for soot build up then using the software check the EGR operation along with the intake throttle valve etc. Any part of the engine that isn't allowing full flow including the air intake and boost side will produce the 2631 code.
In this case we have blown charge air cooler causing a big drop in boost pressure. The good part is we found a problem the down side is it's located 10 days away in Memphis. This bus is down until then. NOTE: We used a regulator to pressure test this cooler at 30 psi and made plugs that can be clamped at both ends.
The doser code activated because the fuel pressure supply in the system was too low and this caused the regeneration process to fail.
The "doser" system shoots a mist of diesel fuel down stream to cause the regeneration operation to activate at the DPF (diesel particulate filter).
No regen operation meant the DPF was going to build up soot and ash to the point where the engine would eventually shut down. This is serious since the engine control unit is programmed to protect the engine and will not allow it to run when the emission parameters are out of the ball park.
The Mercedes power service literature is very good for troubleshooting information. This test shows how to measure the return fuel going back to the tank. At idle the lines are hooked directly to the secondary fuel filter return and a tee fitting is hooked into the overflow valve.
There are 2 fuel regulators one for the doser fuel side and one for the high pressure injection side (unit pumps). If there is too much fuel returning back to the tank the fuel pressure reading drops.
The fuel pump is a positive flow gear pump that is engine driven. Fuel pressure must get up to at least 65 psi to supply the doser system (does the exhaust regen operation) and the unit pumps which provides the high pressure fuel to the injectors.
Once we changed the 2 valves in the fuel return lines "overflow valve" and "doser fuel pressure regulator valve" the exhaust regen started working again and the engine power improved drastically. So it was a win - win in this case.
If you're interested in viewing more blog posts photos and videos check out mechanicshub.com and look for "blogs". I'm posting under the name "schoolbusmechanic"
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