No-start hard start diagnostic on an MBE900

       Hello, everyone. I wanted to share a diagnostic solution I recently came up against. The vehicle was a 2008 uniform delivery van with a Mercedes MBE900 engine. The truck had already been to 4 other shops, at least one of which was a dealer, and all efforts had failed to fix a no start, hard start condition. According to the customer representative, the truck would intermittently fail to start, sometimes after sitting overnight, sometimes after sitting only a few minutes. At other times it would start, but only after a very long cranking time. The problem had been diagnosed and addressed as losing prime. The fuel tank and all filters had been replaced, as had the high pressure lines from the unit pumps to the nozzles. The leak line check valve between cylinders 3 and 4 was also new.
Of course, after it was towed in it started ok, albeit after a fairly long crank. It was due for a D.O.T. inspection, so that provided a good way to do a visual check of the fuel system. I also tried starting it a few times in the next hour, and it always ran after a cranking time of 10 seconds or so. This in itself indicated a problem, as a warm engine should fire almost immediately.
Although the check engine light was never on, I always scan for codes anyway. Stored codes can give a clue to a problem, and I wanted to look at the datastream. Unfortunately, the only program we had that would communicate with the engine was our generic DG Diagnostic. This is a good program, but doesn’t support all the datastream items an OEM program would.  All items looked normal while the engine ran, but there was a stored code for the “engine speed sensor”.  This sensor output was an item not shown on the DG datalist, so I had to deduce its location from Mitchell’s component locator and a wiring diagram. Trouble was, Mitchell called it a crank position sensor. If it was an engine speed sensor its failure would be unlikely to cause a no start, but a crank position sensor could. On the other hand, no MIL was set, so it was not a likely problem. However, a replacement is inexpensive and I have learned the hard way that diagnosing with a known sensor problem is a good way to get caught with your pants down.  After clearing the code, it came back after the next engine start, so I decided to replace the sensor.
     While waiting for the part, I hooked up a fuel pressure gauge to see where I was at with fuel supply. Even with the engine running, fuel pressure was less than 5psi, the lowest value my gauge will read. This was puzzling, but I know that a Cummins ISB will idle and even drive at low speed with a failed transfer pump.  It was obvious I needed to find some information.
Thankfully, about this time the engine quit running and failed to start. I am always relieved when a customer complaint can be reproduced. After that happens, it is just a matter of time and persistence until a fix is found. A fuel delivery problem was confirmed when I took the top of the prefilter off and found it dry. There was also only about an inch of fuel in the main fuel filter housing. These events corresponded with the end of my shift, so I put things away for the night. Often after getting away from work and sleeping on a problem, a clear course of action comes to mind. I also needed information and schematics to go any further.
      Getting repair and diagnostic information on this engine took me a couple of hours of online searching, but I finally found a site with a pretty good PDF document. I usually try to find what I need without calling a dealer for help. I am not likely to talk to someone there who has what I need without their needing to spend lots of time looking, and I don’t want to wear out my welcome unless my need is really critical. I guess I didn’t look too carefully when I tapped into the fuel system to check pressure; upon studying the schematics I found I was on the suction side of the system. I also studied the system in general in order to devise a plan. Fuel is pulled to the engine through the prefilter, and then it is pushed through the filter to the cylinders. Each cylinder has a unit pump running off a camshaft, and each unit pump feeds high pressure fuel to that cylinder’s nozzle. The transfer pump runs off the front of the unit pump camshaft. This particular application had an additional water separator plumbed between the tank and the prefilter.
Back at the shop the next day, I had to make a fitting to check fuel pressure. The fuel temperature sensor, near the filter, is removed and an adapter screwed into the vacant hole. The hole is metric straight thread with a washer seal, so I used an oil drain plug, drilled and tapped to take my gauge, as an adapter. After the filter housings were refilled, the engine still failed to start and again showed zero fuel pressure. Afterwards, the prefilter was again dry, but the separator was still full. If air was getting in the suction side, it was probably getting in between the separator and the prefilter. The line between the two was only about 15” long, so I took it off to check it. It was a nylon line that looked suspicious, so I made up a new one, put a piece of rubber hose on its end, and stuck it in a can of fuel. This would eliminate the whole fuel system up to the engine. If it ran now, the problem was between the tank and the separator. Of course, that would have been too easy. Still no start, no fuel pressure, and a dry prefilter. Interestingly, fuel remained in the main filter housing, even after repeated cranking episodes.
It was while checking this main filter the last time that I noticed metal glitter in the housing after the filter was taken out. I had seen some earlier, but had cleaned it out before refilling. There was no glitter in the prefilter and the only thing between the two filters is the pump, so I thought maybe something got into the pump and damaged it. The pump cover was pretty easily accessible, so it was easy to check out. Problem was that there was absolutely nothing on the inside of the cover to indicate a problem. No scoring or evident wear, no loose metal. I took one of the gears out and it too, as well as the back of the gear pocket, was fine. I was about to put the cover back on when something caught my eye. Part of the spring for the check valve in the cover was sticking through the valve. I thought about trying to pry the valve from the housing, but that would have ruined it, so I tested its function by putting a little fuel in a pop bottle and pouring a little on the valve. The fuel ran right through the valve and through the passage to the other side of the cover. The valve obviously didn’t seal properly, but I wasn’t sure it would cause the symptoms. Still, the pump had to be replaced, and the dealer had one on hand and at reasonable cost.

The fuel pump cover for this MBE900 had
 a displaced spring in the inlet check valve.

Once the pump was changed I topped the filters off, but left the hose in the fuel can as the supply. The engine didn’t start after a couple of moderate cranking attempts, so I loosened the nozzle nuts to let the air bleed out. We used to start dry engines that way back when everything was mechanically injected. Soon I got a strong squirt from each nozzle, the fuel pressure started to come up, and the engine fired. After the nozzle nuts were tight, the engine ran well and fuel pressure was to spec. I let it idle for a few minutes, then took the hose out of my fuel can and hooked the line back up to the separator. Next I ran the engine at about 1000rpm for 15 minutes before a short test drive. Every restart was immediate and there were no problems at any speed or load. Still, I wanted it to be checked again after sitting overnight, so I left a note for our #1 tech to try it when he came in. Next day he reported that it started immediately the first time and several times after, as well as surviving another road test. The truck was pronounced fit to return to the customer.
        This experience illustrates several things. One of the most important diagnostic tools is the eyes. Mine aren’t nearly as good as they were when I was younger, and I nearly put that pump cover back on before I saw the defective valve. The light just happened to catch the copper color of the spring as I was picking it up for the last time.  It is important to take the time to research an assembly you are not familiar with. It would have taken me some time to even identify the fuel pump on this engine without information, as it has only one external fuel line going to it, and that is behind an engine mount bracket and is only visible from below.  It is also important to have a plan and a sequence of checks that takes progressive steps to efficiently solve a problem.
        Sometimes luck plays a role in getting to the root of the problem, and I may have had a little luck here. But that leads me to something strange that I have experienced before. It sometimes seems as if some problems want to be found and solved. Often a vehicle will give you clues that seem not to be relevant but still lead to an answer. In this case, I could have dismissed the glitter in the fuel filter as unimportant. It wasn’t as if there was an alarming quantity of it (and it turned out to be non-magnetic and not related to the pump), but it led me to the pump. I have seen this type of thing happen often enough that I now try to follow any clue I find, whether it fits my preconceived notion of the problem or not. In this engine I think the unit pumps move enough fuel to suck fuel from the filter and prefilter and this fooled other techs into assuming the engine was losing prime or sucking air. In fact, I would have run the return line to a can of fuel to check for air being sucked in if I hadn’t found the pump problem.

         Thank you for reading, Bruce