Mercedes MBE 900 Low Power Troubleshooting is a step by step process and in this video I’m going through where to hook up to do a supply fuel pressure test.
When we change a bus on a run it’s up in the air how it’s going to peform. In one case we gave a 2008 pusher bus on a run out of town that has a steady 10 to 15 minute grade to climb and it’s lacking a lot of power. Originally it was on a highway route with a short grade heading home.
So eventually the new driver phoned up and complained about the power problem saying it barely did over 35 kph up the main grade on his run. Of course his bus before was much better with 20 kph more speed on the same haul. The video explains the first test that is required after replacing the fuel filter and air filter.
Checking the source is always how we start troubleshooting…. in this case the fuel system. The supply fuel pressure has to be up to specs or the horsepower will be compromised. The pressure has to be at least 72 psi when the engine is hot. To the test the fuel pressure a gauge has to installed into the fuel pressure sensor port.
The sensor has to be plugged back in to the harness and placed to one side while the bus is taken on a road test. Going up a grade and loading up the engine will give us an accurate supply fuel pressure. The source as I said before must be at the right specifications. After our road test the pressure was in the 70 to 80 psi range which is great.
We have also performed an “idle speed balance test” which tests the injectors. What this does is analyze how much fuel is going to each injector. The MCM (motor control module) reads piston speed and feeds more fuel if the piston speed is too low and less fuel if piston speed is too fast. Once the test readings hit +100 or -100 the injectors require replacement because they either restricting fuel injection or over fuelling.
According to the specs the injectors are within the parameters allowed. There are no misses among the injectors and the turbo boost is adequate as well. This problem is turning into a “needle in a haystack” scenario. I will comment below if we come to a final conclusion on what is causing this lack of power.
Just thinking about the way the injection system works I’m going to test the unit pumps which supply each injector with the high pressure they need. The pumps are mechanically driven and electronically actuated by the MCM so you never know if one or two are lacking enough injection pressure.
Using the Detroit Diesel Diagnostic software has been very helpful with cancelling out certain areas of the engine including turbo boost and injector stability. There is a section in the software called “instrumentation” that has dozens of different readings you can choose to highlight while on a road test. Each time you log in to the software it logs the data that is produced along with the date, time and vehicle VIN.
I’ve got a hunch that the high pressure side is adequate but not 100% and that’s why there are no codes in that area. So that will be the next step in this troubleshooting process finding out what the high pressure injection system is doing while under load.
While on our last road test we noticed that all of a sudden the power picked up substantially and then there was a lagging feeling shortly after. This only happens under load when maximum fuel injection is required. Onward we go….you will be seeing the solution in the comments below when we find the problem.