This may sound like Greek to a lot of people but believe me we have been studying and troubleshooting this Mercedes fuel system for many hours. The video explains how to do a flow test according to the “Power Service Literature” section supplied by Detroit Diesel. You can get there from the “accessfreightliner website without requiring a password. The link to PSL is in the sidebar.
The whole idea behind testing the flow rate is to see if there is an excessive amount of fuel returning to the fuel tank during engine operation. The 2 fuel return lines from the secondary fuel filter and the overflow valve are hooked directly into drain containers so the amount of flow can be captured. As you can see in the photo above the flow into the upper container is around 3 liters. This only took about 20 seconds so there is excessive loss of fuel to the return side of the fuel system.
The Flow Rate Test Specs
With the engine idling for one minute the rate of fuel from the secondary fuel filter return must not exceed 300ml and the overflow return must be between .9 and 1.7 liters. Both tests failed and we got well over those specs in less than a minute. So just for kicks we took apart the “doser fuel regulator valve” and stretched out the spring that seats the tapered seat and the results were the opposite with just under 300ml in less than a minute.
This was telling us that we would have to replace both the doser fuel pressure regulator valve and overflow valve. The high return fuel flow directly effects fuel pressure from the fuel pump that feeds the unit pumps for high pressure injection and the doser injector that supplies fuel to the doser injector during regeneration.
The fuel pressure which Mercedes calls “compensation pressure” was 45 psi which is low. The proper pressure should be over 65-70 psi. When we experimented with the spring tension on the “doser fuel pressure regulator valve” the pressure increased to 70 psi. More tension on the regulator seat restricted the high return fuel flow back to the fuel tank which directly effected fuel pressure increasing by 20 psi.
The diagram above (sorry it’s fuzzy) shows the components in the Mercedes MBE 900 fuel system. You can see where the overflow and doser fuel pressure regulator valves are located. The overflow valve controls return fuel on the unit pumps side while the doser regulator valve controls return fuel from the doser block that initiates the passive and parked regenerations.