Hello, all. Recently I had a chance to do a repair on an old 6V 92 Detroit Diesel. (Actually, "had a chance" means I was forced into it). I have never worked on one of these before except to change oil and such, and that not for quite a few years.
For younger readers or those who have been in the business less than about 20 years or so, a little background information is in order. Detroit 2 cycle diesels were in production since before World War 2. Several versions and different cylinder configurations from 1 to 24 cylinders provided power for tanks, landing craft, shipboard generators, railroad locomotives, trucks, and other applications. 6V and 8V versions were very popular in road diesels until the series 60 electronic engine came out. Since each cylinder in a two stroke engine fires on every revolution, they make good power for their size; the 6V 92 I worked on makes 300 horsepower from only 552 cu.in. They were phased out in favor of more modern, 4 cycle designs in order to meet future emissions and fuel economy requirements.
The 2 cycle Detroit is a simple engine, but it operates differently from engines now being produced. Check out this video to see how it operates: detroit 2 cycle animation
The engine I worked on was one of the last iterations of the type, installed in a local fire truck built in 1991, and has electronic fuel injector control. It was smoking profusely from the exhaust and oil was running out of the right hand airbox drain at idle. The airboxes supply air movement to the cylinders through the cylinder ports. Removable covers on the outside of the block allow airbox and liner port inspection with the engine in place and while running. Drains allow oil to run out of the airbox at idle, preventing the engine from running away if lots of oil gets in there. It seemed like the most logical way to proceed was to remove the right exhaust manifold to see where the problem was. When the engine was started, the right rear cylinder was throwing smoke and lots of oil. With the airbox covers on the right side removed it was obvious the problem originated at the rear cylinder, as the oil ran down from around that liner, while the other cylinders were (relatively) dry. My plan was to remove that piston and liner, expecting to see broken oil rings or some such defect. I thought maybe something even got into the cylinder and damaged the piston and injector, because the exhaust smoke had a strong fuel flavor and smell. Before I did that I looked online for postings with similar issues, and talked to a couple of mechanics from other shops who had some (albeit limited) experience on that engine. It didn’t seem as if I was missing anything, so went ahead with teardown.
For younger readers or those who have been in the business less than about 20 years or so, a little background information is in order. Detroit 2 cycle diesels were in production since before World War 2. Several versions and different cylinder configurations from 1 to 24 cylinders provided power for tanks, landing craft, shipboard generators, railroad locomotives, trucks, and other applications. 6V and 8V versions were very popular in road diesels until the series 60 electronic engine came out. Since each cylinder in a two stroke engine fires on every revolution, they make good power for their size; the 6V 92 I worked on makes 300 horsepower from only 552 cu.in. They were phased out in favor of more modern, 4 cycle designs in order to meet future emissions and fuel economy requirements.
The 2 cycle Detroit is a simple engine, but it operates differently from engines now being produced. Check out this video to see how it operates: detroit 2 cycle animation
The engine I worked on was one of the last iterations of the type, installed in a local fire truck built in 1991, and has electronic fuel injector control. It was smoking profusely from the exhaust and oil was running out of the right hand airbox drain at idle. The airboxes supply air movement to the cylinders through the cylinder ports. Removable covers on the outside of the block allow airbox and liner port inspection with the engine in place and while running. Drains allow oil to run out of the airbox at idle, preventing the engine from running away if lots of oil gets in there. It seemed like the most logical way to proceed was to remove the right exhaust manifold to see where the problem was. When the engine was started, the right rear cylinder was throwing smoke and lots of oil. With the airbox covers on the right side removed it was obvious the problem originated at the rear cylinder, as the oil ran down from around that liner, while the other cylinders were (relatively) dry. My plan was to remove that piston and liner, expecting to see broken oil rings or some such defect. I thought maybe something even got into the cylinder and damaged the piston and injector, because the exhaust smoke had a strong fuel flavor and smell. Before I did that I looked online for postings with similar issues, and talked to a couple of mechanics from other shops who had some (albeit limited) experience on that engine. It didn’t seem as if I was missing anything, so went ahead with teardown.
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| 6V 92 airbox covers (rear cover removed) |
Much to my surprise, everything looked fine! There was some minor scoring on all 3 cylinders, particularly on the rear one, where I could feel one scored spot with a fingernail. Other than that, the oil and compression rings were intact, the piston and liner undamaged, and head sealing seemed ok. With the liner out I was able to see up inside the airbox a little, but didn’t see anything wrong. The oil passage that feeds the rocker groups is drilled up beside that cylinder, but I didn’t see any cracks or any problem there. Once again I went online then talked to a mechanic who my boss contacted, again without any new insight. Against my better judgment and being under pressure to get it running again, I put the engine back together with a new liner and piston. Of course, the same problem was there when I restarted it, minus the exhaust smoke. At least the bad injector was taken care of, and I now knew the liner and piston was not an issue. In retrospect, I guess I should have put my foot down and insisted on more time to find the source of the oil, because now I was faced with tearing down the engine again.
It occurred to me that the leak could probably be found by pressurizing the oil system with a dye oil mix. With the liner out again hopefully the leak could be seen. After blocking off the passage on the block deck that feeds the rocker groups I used the oil line to the air compressor to feed dyed ATF to the system with an A/C flush bottle, thinking thin oil would leak more quickly and the color would help. On the third bottle of oil, I finally had oil dribbling out of the air box. It took some searching with a blacklight and mirror, but I finally saw dye from between the block and the aftercooler under the blower. There are oil feed and return passages in the end caps of the blower to feed its bearings and it looked like the rear of the gasket was leaking internally, past the aftercooler and into the airbox. Indeed, when I took the blower off the rear of the gasket was in pieces and was not adhered to either surface. It looked like the gasket had shifted when the blower was last installed and that may have contributed to the problem.
Everything was put back together, the engine started, and all was now well. A test drive and a period of idling confirmed the repair.
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