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Mercedes MBE 900 Diesel Engine Repair

This repair has a long history of scenarios which ended up as a major engine repair. The head gasket on this 2008 Mercedes MBE 900 diesel engine has failed. That’s the least of possibilities. Another option would be a cracked head but I’m staying optimistic. One thing for sure the top end of these diesels are very tough and I’m betting my money on the head gasket being the main problem.

The air compressor has been replaced already due to it pumping coolant out the air side through the air dryer and leaking oil into the diesel engine cooling system. The fill tank is black so once the compressor was changed out I thought that was the end of our problems. But not to be …the first run I got a call from the driver telling me he was losing coolant out of the surge/coolant fill tank.

I started wondering how the compressor would effect the engine side. The fact that the air inlet supply to the compressor had failed causing the compressor to give up on us made me instantly think about the air filtration to the engine. The one inch hose to the compressor was cracked so when the compressor was not pumping the engine would draw in the contaminated air as well but on a very small scale.

The compressor being a lot smaller in piston and cylinder area would be effected much more harshly than the engine. But we won’t know until the head is removed to see what the cylinders look like. The dealer told me Mercedes makes the cylinders much more stronger than the pistons because the block is a dry type. This means if the cylinders are damaged, scored etc the block has to be replaced.

Changing the pistons is much more economical so hats off to the engineers who thought of this idea. I’m hoping the pistons and cylinders are alright and we just have to clean everything up replace the head gasket and reassemble. This being the first Mercedes that we’ve pulled the head on I’m hoping for a good outcome.

The EGR cooler has already been replaced so that rules out any exhaust leakage into the cooling system. It’s hard to believe that a cracked core would allow enough exhaust to escape into the coolant and cause a pressure build up. Our Navistar DT MaxxForce diesel engines have proven that with coolant being blown out of the coolant surge tank due to a failed EGR cooler.

We test the EGR leakage by putting a gauge on the coolant fill tank and going for a road test. The system should run at 10 psi with a 15 psi radiator cap. The two failed EGR coolers caused the pressure to get as high as 20 psi overtaking the rad cap set pressure and blowing out of the surge tank. Failures happen on a 3 times minimum basis since we purchase multiple buses with the exact same specs. That’s the way when you’re working with a fleet of vehicles.