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June 2, 2013

Play detective to find start problem

(Continued)

Checking for ignition spark is also a great clue to the no-start puzzle, but this is best left to a pro, or perhaps the process could be taught by a skillful friend. During a no-start episode, checking for a missing hum, click or zap can help a lot to narrow the diagnostic search.

A faulty crankshaft or camshaft sensor could extinguish spark or fuel injectors. An inoperative fuel pump could be caused by a faulty pump relay or the pump itself. An erratic electrical connection in perhaps a half-dozen possible places could also result in any of these systems performing poorly or not at all. Since the fault comes and goes with temperature and the truck apparently runs well between bouts, the fault is highly unlikely to be mechanical in nature.

If I were the technician checking this, I’d make a prearranged shopping list of test points, such as the crankshaft sensor and other terminals at the powertrain control module. I’d also take a close look at all scan-tool engine data during a normal start, and be poised to take a data snapshot during an episode for comparison.

A fuel pressure gauge, noid light (injector signal tester), and spark tester would also be ready for action. During an episode, it will be necessary to work quickly and decisively to zero in on the cause before things return to normal. Problems like these are challenging, yet fun to trounce.

Brad Bergholdt is an automotive technology instructor at Evergreen Valley College in San Jose, Calif. Readers may send him email at under-the-hood@earthlink.net; he cannot make personal replies.

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