6.7 CUMMINS: HARD START WHEN COLD, FINE ALL DAY AFTER

#1 Slow cranking when cold — tired batteries or a fading starter

For: The pattern fits cold-only perfectly: thickened oil drags cranking speed down exactly once a day, and the block heater helping a little is what a warm-engine speed test would show too. Two winters of slow growth matches batteries aging as a pair, and one new battery in a four-year-old set rarely restores cranking speed — the older one drags the pair down to its level.

Against: Nobody has measured it. Cranking speed is notoriously hard to judge by ear — a 20% slowdown is invisible to a listener but very visible to rail pressure build time. If the data shows brisk RPM and fast pressure build, this drops out.

#2 Cold-start aids not firing — glow plugs or intake grid heater out

For: A dead grid heater or a dead bank of glow plugs shows up exactly like this on a cold morning: extra seconds of cranking with no codes, because no circuit faults are set when a heater element just burns open. The white smoke puff at catch is the fuel that burned slowly and incompletely in cold air — the signature of an unassisted cold start.

Against: On these engines the aids usually fail as a rough, smoky first minute, not only as a long crank — and 4–6 s at only 40°F is milder than a full aid failure usually looks. It is also an assumption so far, not an observation: nothing has verified the circuit.

#3 Rail pressure bleeding off — excessive injector return flow or CP3 output loss

For: At 118k the injectors have wear, and a pump or injectors that return too much fuel simply need longer to bring the rail from zero to the injection threshold at every start. It gets worse gradually, which matches the two-year trend.

Against: Leak-off is a hot-oil problem more than a cold one: thin, hot fuel escapes worn clearances more easily, so the classic presentation is long HOT restarts. A strictly cold-only symptom argues against it. It takes #1 if hot restarts ever get long too.

#4 Fuel draining back to the tank overnight — lost prime at rest

For: First-start-of-the-day-only is the textbook signature of a lost prime: the rail and lines drain, and the first crank is spent re-priming. Lengthening over time fits a check valve wearing in.

Against: The 6.7's lift pump is in the tank, pushing rather than pulling, which makes drain-back far less likely than on vacuum-primed systems — and a lost prime would care about parked-at-lunch hours too, not just overnight. Warm restarts being instant argues against it.

#5 Fuel waxing/gelling or water in the fuel

For: A cold-only, seasonal, gradually worsening symptom is at least the right temperature family, and a partly clogged filter path can starve pressure build at crank.

Against: At ~40°F ambient, blended diesel is nowhere near its gel point — waxing lives around 0–15°F — and a month-old filter set plus normal running power once started both argue against it. Drops to a real suspect only if this truck winters far colder.

? Does a 6.7 Cummins have glow plugs, or is it a grid heater?

No glow plugs on this engine — it uses an intake-air grid heater to warm the charge, and the two get blamed interchangeably. A grid heater relay or circuit that quits shows up as exactly this case: extra seconds of cranking on cold mornings, no codes, fine the rest of the day. It is worth verifying before any part is bought, because a grid heater that fails on is a melting hazard, not just a hard-start nuisance.

? How much rail pressure does a 6.7 Cummins need to start?

Techs work from a threshold of roughly 4,000 psi at the rail during cranking — below that the ECM will not command injection and the engine just spins. Most low-rail-pressure no-starts on this engine trace to injector return flow or the rail relief valve rather than the CP3 itself. The useful check is actual versus desired pressure on one cold crank: if actual lags desired for several seconds, the fuel side is the story.

? How do I tell the lift pump from the injection pump on a 6.7 Cummins?

Measure fuel supply pressure at the CP3 inlet: the in-tank lift pump should deliver roughly 10–15 psi at idle, and a reading near zero under demand indicts the lift pump side, not the high-pressure pump. A dead lift pump will not directly kill a CP3, but it lets air into the lines — and lost prime after sitting overnight is a classic cold-only hard start that looks exactly like this case.

? How fast, and at what voltage, does it need to crank when cold?

These engines want a brisk cranking speed — on the order of 150–200 rpm — and the electrical system has to hold about 9.6 V or better under cranking load. Because the batteries work as a pair, load-test both cold before trusting either: one new battery next to a four-year-old one usually leaves cranking speed slow. Weak batteries also depress rail pressure while cranking, which is why the electrical check comes before any fuel part is bought.