Rig Surges or Pulses Instead of Running Steady
Why this matters
A pulsing, uneven stream is a different failure from the steady pressure drop covered in this trade's other troubleshooting case, and it is easy to treat both the same way because they both feel like "not enough pressure." Pulsing points somewhere the steady drop does not: something in the system is cycling, catching and releasing, rather than simply producing less flow. This case follows a rig that was correctly diagnosed and correctly repaired once, and pulsed again anyway, because the first repair fixed a real problem that turned out not to be the only one.
The first visit: a legitimate diagnosis
Weeks before this call, the same rig came in pulsing at the gun, a rhythmic kick in pressure roughly in time with the engine, not a smooth steady stream. The tech at the time ran the unloader's trigger-cycle test and found a slow, uneven response, pressure creeping and dropping unevenly on release rather than snapping cleanly to bypass. That is a real fault, and the fix was correct: the unloader's seal and seat kit was replaced. The rig went back into service, pulsing gone, and stayed quiet for several weeks.
Nothing about that visit was wrong. The unloader genuinely was worn, it genuinely was contributing to the pulsing, and replacing it genuinely fixed the symptom, for a while.
The recurrence, and the wrong first instinct
The rig comes back pulsing again, and the obvious assumption is that the new unloader has already failed, either a bad part or a bad rebuild. That assumption is worth resisting before acting on it: a part failing again within weeks of a correct rebuild is possible but uncommon, and jumping straight to "replace it again" without re-testing risks throwing a second part at a problem the first replacement never fully solved.
The unloader is retested using the same trigger-cycle procedure from before: pressure snaps cleanly to bypass within about a second of trigger release and returns promptly to full working pressure on re-pull, with no creep through an extended hold. The unloader is healthy. Whatever is pulsing this time, it is not the part that was just replaced.
Reconsidering what "fixed" actually meant
The pulsing before the first repair likely had two things happening at once: a chattering unloader seat, and something else contributing a smaller pulse underneath it. The unloader's chatter was the louder, more obvious signal, and replacing it removed enough of the total pulsing that what was left fell below the threshold anyone noticed, especially on a mild day with lighter demand on the water supply. That is not the same as removing the underlying second cause. It only stopped being loud enough to hear.
This time, the water supply gets a harder look than it did the first time around, when the unloader's obvious chatter made it the clear leading suspect and nobody chased the smaller contributor underneath it.
Testing the supply, not just glancing at it
The supply hose from the spigot looks fine by eye: no visible kink, connected at both ends, running to a garden-hose bib rather than a dedicated commercial tap. Looking fine is not the same as delivering enough flow, so the tech runs an actual flow test: fill a five-gallon container from the open spigot end and time it. The container fills in 62 seconds.
Five gallons in 62 seconds works out to roughly 4.8 gallons per minute. The pump on this rig is rated to draw 5.5 gallons per minute at full output. The supply is delivering about 87 percent of what the pump wants to pull, a real and measured shortfall, not a guess. A pump asked to draw more water than its inlet is supplying does not simply run at reduced output the way a restricted nozzle would; it periodically runs the inlet dry for a fraction of a cycle, pulls a slug of air along with the water, and that intermittent air ingestion is what produces a pulse rather than a steady reduction. That mechanism, cyclic, not constant, is also why this symptom is pulsing rather than the steady drop the sibling article covers: a constant shortfall of the same magnitude with no cyclic air ingestion would more likely show up as reduced but stable pressure, while a supply that is borderline enough to intermittently run dry produces exactly the rhythmic catch this rig is showing.
The supply hose itself is checked next and turns out to be an undersized garden hose, roughly a half inch inside diameter, rather than the five-eighths inch minimum the pump manufacturer's inlet chart calls for on this model. A half-inch hose can flow enough for many household uses without anyone noticing a restriction, but it is tight enough at this pump's draw rate to be the borderline supply the flow test just measured, especially once ordinary line-pressure variation at the spigot is factored in.
What actually needed fixing
The undersized supply hose is swapped for a five-eighths inch minimum hose per the pump's inlet spec, and the flow test is repeated. The same five-gallon container now fills in about 48 seconds, which works out to roughly 6.25 gallons per minute, comfortably above the pump's 5.5 gallon-per-minute draw rather than a hair under it. The rig is run for a full ten-minute continuous test at working pressure. No pulsing, no rhythmic kick, steady pressure throughout.
The unloader replacement from the first visit was not wasted work and does not get undone; it fixed a real, separate fault. What was missing the first time was checking the supply with an actual flow measurement instead of a visual glance, once the louder unloader chatter had been dealt with and stopped drowning out the smaller signal underneath it.
Why it took weeks to resurface
The flow test above was run midday, on a warm afternoon, at the same house where the pulsing had shown up. That timing is not incidental. Municipal supply pressure at a residential spigot is not constant through the day; it drops when demand across the neighborhood is highest, commonly in hot weather when irrigation systems, other households, and the property's own indoor use are all drawing from the same main at once. A supply that measures acceptable on a cool morning with light neighborhood demand can measure well below that same number on a hot afternoon, and a marginal half-inch hose that was close enough to adequate during the mild weeks right after the unloader repair can fall meaningfully short once seasonal demand rises. That is a plausible reason the problem stayed quiet for weeks and then came back, rather than evidence that anything changed about the rig itself in between. Whenever a supply-related flow test is run, running it under conditions similar to when the symptom actually occurs, not on the easiest, quietest morning available, gives a truer reading.
What changes the answer: a well or holding-tank supply
Where the source is a private well or a holding tank rather than municipal service, the fix is different even though the mechanism is the same. Upsizing the hose does nothing if the well pump itself cannot sustain 5.5 gallons per minute at the pressure it is delivered at; the flow test still applies, but a shortfall found there points at the well pump's own capacity and pressure-tank settings, not at hose diameter. Confirm which kind of supply is feeding the rig before assuming a hose swap is the fix, since applying the municipal-supply correction to a well-limited property fixes nothing and sends the tech back out for the same complaint.
Why the repair-then-recurrence pattern is worth naming out loud
A rig that pulses, gets a legitimate repair, runs clean for weeks, and pulses again is not evidence the repair was wrong. It is evidence that more than one cause was present and the loudest one got fixed first, which is normal and often unavoidable when two symptoms sound identical to the person standing at the gun. The lesson for the next call like this one is procedural: when a previously repaired rig comes back with the same symptom, retest the part that was replaced before assuming it failed again, and if it tests healthy, treat the recurrence as a sign to look at what else could produce the same symptom rather than repeating the same fix on faith.
What would point somewhere else
A pulsing rig on a dedicated hard-plumbed water source, rather than a garden-hose spigot connection, makes an undersized supply hose an unlikely cause and shifts suspicion back toward the unloader or toward air being drawn in at a loose inlet fitting instead. A pulse that speeds up or slows down with engine throttle rather than staying at a constant rhythm points toward a belt or pulley issue affecting pump speed rather than a supply or unloader problem. And a rig that pulses only when a downstream chemical injector's metering valve is open, and runs steady with it closed, points at the injector drawing air through a worn check valve rather than at the water supply at all.
The record this call leaves behind
The service note for this visit names both causes, not just the one that was fixed today: the original unloader replacement stays in the record as a legitimate, separate repair, and the supply-hose swap is logged as its own fix with the before-and-after flow readings attached. A tech who reads this rig's history next season and sees only "pulsing, fixed" twice, six weeks apart, with no detail, has no way to tell whether the second visit means the first repair failed or means a second cause was found. Writing down the actual flow numbers each time turns that history into something the next person can trust instead of something they have to redo from scratch.
References
- Pump manufacturer inlet-supply specification (minimum hose diameter and supply-flow requirement, model-specific)
- See related: Pump Loses Pressure Mid-Job
- See related: Unloader Valve and Pressure Regulation Service