I was recently involved in a repair of a 2006 Mercedes MBE 4000 with a hard start problem.
The customer said it wouldn't start without ether, and when it did run it didn't idle very well. The check engine light was not on until I got the truck into the shop.
I went out on the lot and tried to start it, and couldn't get it going by myself. It took one of the other guys to feed it ether while I cranked it, and finally got it to start and run well enough to get it into the shop.
I hooked up our Bosch scanner and pulled codes for #1 and #2 injectors, that became active while the engine was running. I then hooked up the Detroit program to verify the results and run any tests that were needed. I usually hook up the Bosch first because it is often less messing around to get running and check for codes. Detroit verified the injector codes, then I did a cylinder balance test showing a higher torque deviation on those cylinders.
I wasn't convinced two faulty cylinders could cause the hard start I was experiencing, especially seeing as the Mercedes has a unit pump injection system. Each cylinder has its own unit pump running off the camshaft and as far as I understand the system, the injectors are like old-school nozzles that just open at a given pressure. Granted they are trimmed electronically, but I needed more proof that they were the source of the issue.
Just to cover all bases I checked the fuel pressure. This is easy on this application, as there is a compu check fitting on the filter canister. A special connector like a hydraulic coupler goes on the fitting for a quick, no mess pressure check. The fuel pressure was approximately to spec at idle and at high idle. Note here that high idle on a diesel engine is no load, pedal to the floor.
Another possibility was that an injector was stuck open and cylinder compression was getting into the fuel circuit. Luckily under each valve cover is a banjo connection for the return fuel from each injector, feeding a line that routes back to suction fuel. It was an easy matter to remove the valve covers and remove the banjo bolt at each injector, one after the other, while someone cranked the engine. A stuck injector should emit air from its return, but again this test proved negative on all cylinders.
A quick scan showed that the injector codes had gone inactive, and stayed away after being cleared. Evidently the codes were due to the air in the system. Fuel enters the head at the front, so the air affected the front two cylinders the most. The engine started immediately hot and cold, so I pronounced it fixed.
Thanks for reading.
Bruce
The customer said it wouldn't start without ether, and when it did run it didn't idle very well. The check engine light was not on until I got the truck into the shop.
I went out on the lot and tried to start it, and couldn't get it going by myself. It took one of the other guys to feed it ether while I cranked it, and finally got it to start and run well enough to get it into the shop.
I hooked up our Bosch scanner and pulled codes for #1 and #2 injectors, that became active while the engine was running. I then hooked up the Detroit program to verify the results and run any tests that were needed. I usually hook up the Bosch first because it is often less messing around to get running and check for codes. Detroit verified the injector codes, then I did a cylinder balance test showing a higher torque deviation on those cylinders.
I wasn't convinced two faulty cylinders could cause the hard start I was experiencing, especially seeing as the Mercedes has a unit pump injection system. Each cylinder has its own unit pump running off the camshaft and as far as I understand the system, the injectors are like old-school nozzles that just open at a given pressure. Granted they are trimmed electronically, but I needed more proof that they were the source of the issue.
Fuel aeration is indicated by air bubbles in the fuel returning from the engine to the tank.
Next I looked in the fuel tank. The fuel looked good and there was plenty of it, but while the engine was running a problem was obvious. The fuel return could be clearly seen near the bottom of the tank, and it was emitting a steady stream of air bubbles. it looked like when you were a kid and you blew bubbles in your milk with a straw. That much air coming back from the fuel system was doubtless the cause of the hard start.Just to cover all bases I checked the fuel pressure. This is easy on this application, as there is a compu check fitting on the filter canister. A special connector like a hydraulic coupler goes on the fitting for a quick, no mess pressure check. The fuel pressure was approximately to spec at idle and at high idle. Note here that high idle on a diesel engine is no load, pedal to the floor.
Find the source of fuel aeration start by eliminating the fuel tank and pickup line and running the engine off a can of fuel.
On this engine that is an easy matter. The fuel lines to and from the separator are regular nylon airline, so I just disconnected the one to the tank, stuck a tight fitting piece of rubber hose to it, and stuck it in a bucket of fuel. after a few minutes of running the fuel return was still producing air, so I took the line off the other side of the filter housing and ran the engine with the filter bypassed. It still made air, so I knew the problem was neither in the tank, the pickup line to the separator, nor the separator itself. The rest of the fuel system has hard plastic lines with banjo connections, so it would be harder to check.
Check the lines and fittings between the filter housing and the transfer pump for leaks that could be sucking air.
We have an electric pump we use for transferring fuel and pumping out tanks, so I used that to pressurize that part of the system. This would pressurize the whole system up to the regulator at the rear of the engine, but the only thing I was interested in was the suction portion from the filter to the pump. This test again showed no leaks.Another possibility was that an injector was stuck open and cylinder compression was getting into the fuel circuit. Luckily under each valve cover is a banjo connection for the return fuel from each injector, feeding a line that routes back to suction fuel. It was an easy matter to remove the valve covers and remove the banjo bolt at each injector, one after the other, while someone cranked the engine. A stuck injector should emit air from its return, but again this test proved negative on all cylinders.
The only other possibility left for air entry, or fuel aeration, was the fuel pump.
It is a mechanical unit mounted on the front cover, so if the pump's front seal was bad it could suck air there. Unfortunately there was no easy was to test the pump, other than just changing it out. I did so, which took about an hour, and started the engine. It seemed to start fairly well. but there was still quite a bit of air coming from the return even after a few minutes of running, and the engine still ran rough. During a short test drive, however, it smoothed out and when I got back to the shop there was no longer and air coming from the return.A quick scan showed that the injector codes had gone inactive, and stayed away after being cleared. Evidently the codes were due to the air in the system. Fuel enters the head at the front, so the air affected the front two cylinders the most. The engine started immediately hot and cold, so I pronounced it fixed.
Thanks for reading.
Bruce
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