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g/auto • Posted by u/diesel_dan •

Intermittent no-start on a 2019 Ram 2500 6.7 — 14 days of nothing, then it did it in the bay Diagnostics

Fleet truck, intermittent crank-no-start. Maybe once every ten days. Driver says it cranks normally and just does not catch, then twenty minutes later it starts fine.

Two weeks of it sitting in my bay behaving perfectly. Datalogger on it the whole time.

Today it finally did it with me standing there. Captured:

  • Crank RPM normal, 210 RPM
  • Cam and crank signals both present and correlated
  • Rail pressure during crank: 4,100 PSI (needs about 5,000 to fire)
  • Fuel temp 34°F, ambient 39°F
  • After 20 min in the warm bay: rail pressure 6,200 PSI on crank, fires immediately

So it is a rail pressure problem that is temperature dependent. The obvious answer is the CP4 giving up, but the truck runs perfectly once started and has no other symptoms, no metal in the filter, and the fuel is clean.

Before I sell this fleet a pump, what else produces low crank rail pressure only when cold?

💬 8 replies

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u/admin_jake ADMIN •
✓ Accepted Solution

The reply above is on the right track and there is a five minute test that settles it before you take anything apart.

Clear bottle test. Splice a clear section of line into the suction side between the tank and the lift pump, cold-soak the truck overnight, and crank it while somebody watches the clear section. Air on the suction side is visible. You will see bubbles marching toward the pump or you will not.

If you see bubbles: it is a seal, a cracked line, or the filter housing, and you can find it with a low pressure air test on the suction circuit with the tank isolated.

If you see no bubbles and rail pressure is still low: now measure suction vacuum. Restriction and aeration are different failures. A partially blocked tank pickup screen — which stiffens and blocks worse when the fuel is cold and waxy — gives you low rail pressure with no air at all. On a fleet truck with unknown fuel history I would bet on the pickup screen before I would bet on the pump.

Either way you have a fifteen dollar answer or a four thousand dollar one, and you can tell them which before you order parts.

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✓ Solution
u/diesel_dan OP •

Clear line went in this afternoon. Cold soaking it tonight, cranking it at 6am. Will report back.

Also the waxing point — this is a Northern fleet and I do not know that anybody has been checking the CFPP on what they are buying. If the pickup screen is loaded with wax that is a fuel purchasing problem, not a truck problem, and that is a very different conversation with the fleet manager.

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u/scan_tool_sam •

That is the conversation worth having. One truck with a cold no-start is a repair. A fleet buying summer fuel in October is going to give you eleven more of these and they will all get diagnosed separately by somebody who does not know about this one.

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u/scan_tool_sam •

Agreed on checking the supply side, and be specific about when you measure. Put a gauge on the inlet and log it through a cold crank event. You are looking for the supply pressure dropping off as the pump tries to draw.

Also: check the fuel filter housing for a cracked or hardened o-ring. Cold makes rubber shrink. A seal that is fine at 70°F can let air in at 35°F, and air on the suction side means the high pressure pump is trying to compress a foam. Perfect match for your symptom — only cold, only during crank, disappears once it warms and the seal expands.

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u/diesel_dan OP •

Vacuum on the suction side is something I have not checked. If it is drawing air the pump would still show a normal-looking return volume, which is why the pump looks fine to me.

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u/wrench_monkey •

Low crank pressure only when cold with a healthy running pump is usually not the pump. Check the low pressure side first — lift pump output during crank, cold. If the pump is not getting fed it cannot build.

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u/road_warrior_rex •

First year here — the thing I am taking from this thread is that "captured it in the act with a logger" is what made the whole diagnosis possible. Two weeks of the truck behaving is two weeks of no information. I would have started replacing things on day three.

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u/wrench_monkey •

That is the actual lesson and it is worth more than the diagnosis. Intermittent faults are won by instrumentation and patience. Everybody who parts-cannons an intermittent does it because waiting feels like not working. It is not.

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