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The GMC engine is a Detroit Diesel 2-53 2-cyl, 2-cycle supercharged diesel. I haven't worked on one in years, but believe that parts are still available for them. They were real screamers - sort of like a vacuum cleaner on steroids. There were several issues regarding the various seals in the supercharger. The worst problem occurs if the oil seals leak. Then you get lube oil mixing with the intake air. In some cases, these engines would not stop running - or worse, run away completely - because they're burning the lube oil. Usually if one won't start at all you need to look at the fuel system, though. These engines don't have injector pumps, but have a third lobe on the camshaft that operates a plunger on the upper end of the injector. If you are getting fuel delivery, but no running, then you need to look at compression. Part of that comes from the supercharger. The bottom line is that they are more complicated than the alternative, gas powered, engines but are not impossible to understand or work on. There is a service manual available from JD for this series engine. If you don't have one, I'd suggest contacting JD publications and ordering one. The JD manual is excellent in covering everything you need to know when working on them.
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