Cold-Start Rough Idle After Cylinder Replacement
A 2011 dedicated CNG van (OEM bi-fuel-converted, now dedicated) comes in with a complaint of rough idle and occasional stall for the first 30 seconds after a cold start, then runs normally. The customer recently had one of the roof-mounted Type 4 cylinders replaced at another shop. Static pressure at the tank is 3,200 psi and rises to 3,400 psi when the engine bay warms. The technician notes the fuel rail pressure holds at 1.8 psi above manifold pressure at idle but drops 0.6 psi during the first 20 seconds of a cold start before recovering. No DTCs are stored, fuel trims are normal once warm, and a soap-bubble test at the regulator and injectors shows nothing. The replaced cylinder's manual shutoff valve was reportedly left in the fully open position and the high-pressure line was reused.
1. What is the most likely cause of the cold-start pressure dip, and what should the technician check first?
Read the worked answer
A 0.6 psi dip in rail pressure above manifold pressure during cold start points to a supply restriction or a slow pressure build in the high-pressure side, not a leak (the soap test was negative and the system holds pressure warm).
Key reasoning:
- The symptom is temperature-dependent - it occurs only when cold and self-corrects as components warm. That pattern fits a restriction that changes with temperature, such as a partially closed manual valve, a deformed/reused high-pressure line, or a restricted filter/coalescer in the regulator.
- The line was reused during a cylinder swap. A reused high-pressure line that was kinked, over-torqued, or has a collapsing inner liner can restrict flow when cold and relax when warm.
- The manual valve reportedly left fully open is normal - but verify it is fully open, not partially backed off, and that the new cylinder's valve outlet is the correct type.
First checks, in order:
- Confirm the manual valve is fully open and the valve outlet fitting is correct.
- Inspect the reused high-pressure line for kinks, sharp bends below minimum bend radius, and crush damage.
- Check the high-pressure filter/coalescer element for restriction.
- Verify the regulator's cold-start enrichment function and that the pressure sensor is reading correctly (compare with a known-good gauge).
Do not chase the injectors first - the warm behavior and negative soap test argue against an injector leak.
2. The technician wants to replace the regulator to fix the dip. Argue for or against that step given the evidence.
Read the worked answer
Argue against replacing the regulator as a first step.
The evidence points upstream of the regulator:
- The dip is transient and cold-only, recovering within ~20 seconds. A failed regulator usually produces a steady pressure error, not a self-correcting dip.
- The system holds pressure warm and passes a soap test, so there is no external leak driving the dip.
- The timing correlates with a recent cylinder/line service, which is the strongest clue. The most probable fault is a restriction introduced during that work.
A regulator replacement would be a parts-cannon diagnosis: it may mask the symptom temporarily (new diaphragm, new filter) while leaving a kinked or restricted line in place, which can later fail under full-flow demand.
Better sequence:
- Verify valve fully open and line condition.
- Flow-test the high-pressure side (or measure pressure drop across the filter and line).
- Only replace the regulator if the upstream side is proven good and the dip persists.
Document the reasoning in the repair order so the customer understands why the line/filter is addressed before the regulator.