Hard Start and White Smoke on a Cold Start With Suspect Glow System
A 2014 school bus with a 6.7L Power Stroke cranks 8–10 seconds before firing when cold (40°F), runs rough for 20 seconds with white smoke, then smooths out. Once warm it restarts instantly. The batteries are 12.6V at rest and crank at 10.8V. The shop finds no stored DTCs. The glow plug control module is commanded on at key-on. Resistance checks at the module connector show one circuit open. The fuel filter was changed 2,000 miles ago, fuel pressure at the rail meets spec when running, and the engine has 96,000 miles. Injector contribution tests are normal when warm. The block heater was not used on the morning of the complaint.
1. Differentiate between a glow system fault, an injector fault, and a compression fault using the symptoms given.
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The key discriminator is that the fault is temperature-dependent and disappears completely when warm. White smoke with rough running that clears as the engine warms, plus instant hot restart, is the classic signature of insufficient cold-start heat - glow plugs or cold-start fueling - rather than a mechanical compression failure (which would persist warm and usually show blowby or oil consumption) or a failed injector (which would misfire warm too and show up in the contribution test).
The open glow circuit found at the module is a concrete fault. However, one open glow plug on a 6-cylinder usually causes only a mild miss, so verify whether more than one circuit is open and whether the module is actually delivering voltage to each plug under command, not just receiving a command.
2. How would you confirm the glow system and rule out low compression before authorizing repairs?
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Confirm the glow system by commanding the module on and measuring voltage at each glow plug harness connector (should be battery voltage during the on-time), then measuring each plug's resistance and comparing to spec. An open plug or open harness is replaced/repaired. Verify the module's on-time is appropriate for 40°F.
Rule out compression by a relative compression test with the scan tool (cranking speed per cylinder) and, if any cylinder is low, a manual compression or cylinder leak-down test. A healthy relative compression test plus normal warm-running behavior makes a mechanical fault unlikely.
If glow and compression are both good, check cold-start fuel delivery - rail pressure during crank and injector pulse - because low rail pressure at 40°F can also cause long crank and white smoke. Repair the open glow circuit first, then re-test cold; only pursue injectors if the cold start remains poor.