A friend recently dealt with a 6.8-liter diesel farm tractor engine that was hard to start. The
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Diagnosing a problem often takes more time than expected.Photo: iStock-PointImages
customer said he had to hand-pump the fuel primer to get it to start, which often suggests fuel is leaking back and draining the fuel injection system when the engine isn’t running.
My friend did all the usual tests to diagnose fuel leak-back, and replaced various fuel lines, filter housings and other components that often cause that problem. The engine would re-start without hand-priming as long as it wasn’t worked hard. But if the engine was worked hard and then shut off, it would take 15 minutes of hand-priming to re-start.
He finally used a diagnostic procedure that included replacing one of the diesel fuel return lines with a clear plastic hose and then running the engine on a dynamometer. At both idle and full, no-load throttle, the fuel in the line looked good. But if he pulled that engine hard on the dyno, bubbles appeared, and the harder he pulled it, the more bubbles showed up in the return line.
His conclusion? The diesel fuel injectors and especially their seats/seals were worn and allowing exhaust gases to force their way into the injector, where they were introduced into the return line, which recirculates through the injection pump. When the tractor was shut off with bubbles in the fuel line, those bubbles consolidated into air pockets when the tractor sat for a couple hours or overnight. And air pockets in a fuel injection line nearly guarantee that a diesel engine won’t start.
It wasn’t cheap to replace the injectors, but what really constipated the customer was the cost of diagnosing the problem. Justifying to him the time spent pressure testing, leak testing and dyno-ing the tractor was the hardest part of the job. And it had nothing to do with actually turning wrenches.
— Dan Anderson
