Monday, March 25, 2013

Mercedes 900 Diesel Engine Runs poorly after fuel filter change

The Mercedes MBE 900 Diesel Engine. What I have experienced since we acquired these engines in 2005 is they are built well and stand up to the rigorous start and stop routines involved in a school bus fleet. The pre 2007 engines are not emission controlled like they are today and have given us very little grief. However I would like to share a tip on a troublesome problem we had with a 2005 freightliner conventional school bus.




The Video I made above explains how to properly bleed these engines. Strangely enough we were told to crank over the engine continuously after a fuel filter service until the engine fuel pressure built up. At first we were scratching our heads thinking we're living in the new millenium and this diesel engine has no priming pump and only one starter...that was destined for failure eventually from the constant load it would have to endure turning the engine over and over just to prime the fuel filters.

We tried to innovate and pressurize the fuel tank and crack a line at the secondary fuel filter to help the system prime up faster. Unfortunately some of our new buses had fuel guage sender problems and there were buses running out of fuel out on the road. There was no choice but to load the hell out of the starter to get it going again.

By the way if you loosen the fuel line from the unit pump to the head the line must be replaced. Recommended by the dealer / engineers. We were forced to crack the lines only once and re-tightened them after start up and did not have any leaks. So I imagine the odd time the fuel line will not seal properly after it has been disturbed?  

Engine Runs Poorly After Changing Fuel Filters

We recently experienced a problem with one of our MBE 900 diesel engines after a fuel filter change. It ran poorly and was not picking up fuel. It eventually kicked in and ran fine for a few days. However it died on it's bus run and had to be towed. It ran like it was starving for fuel like air in the system. The engine would idle fine but crap out under a load.

One thing noticeable was when priming the fuel system with the pump kit the pump just kept filling the lines and filter without the typical solid feeling at the pump handle indicating the system was primed and ready to go. Eventually the Mercedes diagnostics instructed us to check the return fuel volume from the head and the unit pumps.

There should only be 1.7 Liters of fuel returning to the tank every minute at an idle. We had double that...so it ended up being the overflow valve (similar to a check valve) which is in the return fuel plumbing to maintain around 60 psi. It was not doing this and caused a loss of back pressure in the return system which caused the hard starting, rough running condition. Now our shop will recognize the problem after these symptoms pop up again.

1 comment :

SaiKeerthi sudha said...

Enjoyed reading the post about the Mercedes diesel engine! Though it was pure tough luck, an experience which is cautioning!