What PoolBoss Says
A pool pump usually will not prime for one of four reasons: the water level has dropped below the skimmer mouth, a strainer basket or impeller is clogged, the pump lid O-ring has failed, or a suction-side fitting is drawing air. Bubbles still swirling in the basket after 30 seconds mean air is getting in, not purging out.
A pump that will not prime is the most common equipment failure on a route and the one most likely to turn a 25-minute stop into an hour. The homeowner version of this checklist tells you what to look at. It does not tell you what to look at first, and it never tells you when to stop.
That ordering is the whole job when you have twelve stops left. What follows puts the four causes in time order, cheapest and fastest first, explains what bubbles in the strainer basket are actually telling you, and draws the line where a four-minute O-ring becomes a pressure-test quote and you move on.
At a glance
Key takeaways
- Check in time order: water level below mid-skimmer, a clogged basket or impeller, a failed lid O-ring, then a suction-side air leak.
- Bubbles still swirling in the basket after 30 seconds mean air is entering the line, not purging out of it.
- Trickle a hose over each suction joint from the pump backward; bubbles thin within seconds when you find the leak.
- Shut a pump down at about 90 seconds without prime, because running dry glazes the shaft seal and can distort the impeller and volute.
- Carry lid and union O-rings, a spare basket, and thread sealant, and most no-prime calls become a four-minute fix.
- Anything needing the wet end opened or the motor touched is a scheduled quote, not a route repair.
- Kill the breaker rather than the timer when you leave a pump off, and add chlorine so the pool does not go green.
Why won't a pool pump prime?
Check the four causes in this order, because it is the order that costs you the least time. Water level first: if the surface has dropped below the middle of the skimmer mouth the pump is pulling air and nothing else you do matters. Then the strainer basket and the pump lid, which take about 60 seconds together. Then the lid O-ring, the single cheapest failure on the pad at $4-8 for the part. Last, a suction-side air leak at a union or threaded fitting, because that is the only cause that needs real hunting.
The impeller sits behind the basket and clogs on the debris the basket missed - a hairball, a leaf stem, a chunk of cured putty. You can usually feel it. Kill the breaker, pull the basket, and reach the impeller eye with a finger or a bent piece of wire. If it will not spin freely by hand, that is your prime problem, and it is a five-minute clear rather than a repair.
One field shortcut is worth having: fill the strainer pot with a hose before you start. A pump that primes with a full pot and then loses it is telling you air is entering somewhere downstream of the water you just added, which points at the lid or the suction plumbing rather than at the pool's water level.
Bubbles after 30 seconds mean air is still getting in
Trapped air purges. A leak does not. When you restart a pump after opening the lid, the basket churns with bubbles for the first 20-30 seconds while the system pushes out the air you just let in, and that clears on its own. If the basket is still swirling with fine bubbles after 30 seconds of steady running, air is entering the suction line continuously, and you are hunting a leak rather than waiting out a purge.
Find it with a garden hose. With the pump running, trickle water slowly over each suction-side joint, starting at the pump and working backward toward the skimmer: the pump union, the threaded fitting into the pump body, the valve stems, then the plumbing. When water gets drawn into the leak the bubbles in the basket stop or thin noticeably within a few seconds. Shaving cream does the same job and is easier to read on a vertical fitting, because it gets pulled into the gap and leaves a visible dimple.
Two details save a wasted trip. The pump lid O-ring is the most common culprit and the easiest to misjudge, because a dry, flattened, or hairline-cracked O-ring will seal well enough to hold water with the pump off and still leak air under suction. A threaded fitting into the pump body is the second most common: PVC and the plastic housing expand at different rates, so a joint that was tight in April can be drawing air in July. Neither failure arrives without warning, which is why the pad check that catches this a week earlier is worth the ninety seconds it adds to a stop.
Symptom to cause to verdict
Most pump calls land in one of five symptoms, and each has a default first cause and a default verdict. The verdict column is the one that matters on a route: fix means a parts-in-hand repair you finish at the stop, and quote means you stop, note it, and it leaves as scheduled work.
| Symptom | Most likely cause | Verdict at the stop |
|---|---|---|
| Will not prime, bubbles in the basket | Low water, clogged basket or impeller, failed lid O-ring, or a suction-side air leak | Fix, unless the leak is underground or in the skimmer |
| Hums but will not turn | Failed start capacitor, or a seized motor or impeller | Quote - capacitor work is bench or licensed-electrician work |
| Screeching or grinding | Worn motor bearings | Quote - a bearing noise is a motor rebuild or replacement |
| Water dripping between pump and motor | Failed mechanical shaft seal | Quote - a seal replacement means pulling the wet end |
| Runs quietly but flow is low | Clogged filter, clogged impeller, or a closed valve | Fix - read filter pressure against that pool's clean baseline first |
Stop at 90 seconds and rebook rather than cook the motor
A pump that has not pulled water within about 90 seconds of steady running should be shut off. The mechanical shaft seal is cooled and lubricated by the water moving through the volute, so running dry lets the seal faces overheat and glaze. A glazed seal weeps, and a weeping seal drips onto the shaft and eventually into the bearings, which is how an $8 O-ring call becomes a wet-end teardown. On a hot pad the plastic volute and impeller can soften and distort as well.
A humming pump deserves the same discipline on a shorter clock. Every second it hums under load without turning is heat into a stalled winding, so cut power within 5-10 seconds rather than letting it sit while you think it through. A pump that hums and then starts after you nudge the shaft through the back vent is telling you the start capacitor is going, and that is scheduled work rather than something to keep nudging every week.
The 90-second rule is also a route rule. Past that point you are not troubleshooting, you are gambling forty minutes of a day that still has eleven stops on it. Leaving the pool safe takes about two minutes: kill the breaker rather than just the timer so an automated schedule cannot start a dry pump at 6am, close any suction valves you opened, and add enough chlorine to hold the water until the repair. A pool with no circulation in July loses its residual fast, and coming back to green water turns a pump quote into a pump quote plus an algae recovery nobody is paying you for.
What you fix at the stop and what you quote
The line sits at parts you can carry and work that does not open the motor. Lid O-rings, drain plug O-rings, a spare strainer basket, union O-rings, O-ring lubricant, and thread sealant all live in a small bin on the truck and turn most no-prime calls into a four-minute fix. Anything past that - the mechanical shaft seal, the impeller, the motor, the start capacitor - means pulling the wet end or opening the motor, and that is bench work you schedule rather than route work you improvise.
What makes the second category profitable is the record behind it. If you record what you found at the pad on every visit - basket condition, filter pressure before and after, a photo, and a note - then the third no-prime call on the same pool is not a mystery, it is a documented pattern with dates on it. That is the difference between saying the pump acts up sometimes and saying this pump has drawn air at the union on three visits since June, and only one of those two sentences sells a repair.
The same history is what makes quoting the repair fast, because the evidence is already written down before the conversation starts. And when a motor is genuinely finished, the replacement is also the moment to raise efficiency: recommending a replacement pump like-for-like on an oversized single-speed leaves the customer's largest available saving sitting on the table.
Two McAllen pools, the same bubbles, two different verdicts
A three-truck operator running 175 pools across McAllen and Edinburg hits this symptom twice in one week, and the two stops end very differently.
On the first, a Tuesday pool in McAllen, the strainer basket runs half full of swirling bubbles that never clear. Water level is fine, the basket is clean, the lid O-ring looks serviceable. The tech runs a hose slowly back along the suction line and at the second union the bubbles thin and stop. A $6 O-ring out of the truck bin, four minutes, and the pump pulls a solid prime. The stop runs about ten minutes long and the day absorbs it.
On the second pool the symptom is identical, but the hose test walks the whole suction run and nothing responds. At 90 seconds with no prime the tech shuts it down at the breaker, notes the basket condition and what was ruled out, photographs the pad, and adds chlorine to hold the water. It leaves as a pressure-test quote. That stop cost twelve minutes instead of the forty it would have taken to keep chasing a leak that is probably underground, on a day that still had eleven stops on it.
The difference between those two outcomes is not skill. It is having a stopping rule, and having somewhere to put what you found.
FAQ
Frequently asked questions
How long can a pool pump run dry before it is damaged?
Assume damage can start inside the first minute, and treat 90 seconds as a hard stop. The mechanical shaft seal is the part at risk: its two faces are cooled and lubricated by water moving through the volute, and with no water they run dry against each other, heat, and glaze. A glazed seal weeps, and a weeping seal eventually drips into the motor bearings. On a hot pad the plastic volute and impeller can soften and distort as well. Some pumps survive several dry minutes with nothing to show for it and others are damaged in well under one, because the variable is heat rather than a spec you can look up. That uncertainty is exactly why the stopping rule is worth having, since you never find out in advance which pump you are standing in front of.
What should I carry on the truck for pump problems?
A small parts bin holding roughly $40-60 of stock covers the large majority of no-prime calls. The core list is pump lid O-rings in the two or three sizes you actually meet on your route, union O-rings, drain plug O-rings, a spare strainer basket, O-ring lubricant, PTFE tape and thread sealant, and a strap wrench for a lid somebody overtightened. Add a short length of garden hose if the truck does not already carry one, because that is the leak-finding tool. What is not worth carrying is anything that requires opening the motor or the wet end. Capacitors, seals, and impellers are bench parts, and stocking them mostly tempts you into starting repair work at a stop that is already running long.
How do I prime a pump that sits above the pool's water line?
Fill the strainer pot by hand and give it more time than a flooded-suction pump. A pump mounted above the water line has to lift water rather than receive it, so it must first evacuate the air in the whole suction run before it can pull, and the 90-second rule needs a little grace on those pads. Cut the power, open the lid, fill the pot to the top with a hose, reseat a lubricated O-ring, and close the lid firmly by hand. Then start it and watch the pot. If the water level drops and stays down, it is pulling and you can let it work. If it drops, empties, and stays empty, air is entering somewhere and no amount of refilling fixes it - the suction side has to be sealed first.
Should I repair a pump motor or replace the whole pump?
Compare the repair against the replacement and let the condition of the wet end decide. A motor replacement on an otherwise sound pump commonly runs somewhere around half to two-thirds of a new pump installed, so a motor swap on a three-year-old unit with a healthy volute and impeller is usually worth doing. On a pump past about eight years, or one with a cracked volute, a worn impeller, or a second failure inside a year, replacing the whole unit is the better call, because otherwise you are mating a new motor to parts that fail next. The other input is running cost. A failing oversized single-speed is often the moment the whole pump gets replaced rather than repaired, because the efficiency difference changes the math on the customer's side of the quote.
What do I tell the customer when I have to leave the pump off?
Tell them three things in this order: that the pump is off and why, what you did to keep the pool safe, and what happens next with a date attached. Something like - the pump is drawing air and will not hold prime, I shut it off at the breaker so it does not run dry and wreck the seal, I added chlorine so the water holds without circulation, and we will be back Thursday with a pressure test. That framing matters, because a customer who comes home to a silent pump and no explanation assumes something got broken during your visit. Put the same note in writing on the service record too. A message that lands before they get home does far more for the relationship than the same information delivered after they have already called you.
How do I keep a pump from losing prime overnight?
A pump that primes fine but sits empty the next morning almost always has a small air leak or a failed check valve rather than a pump fault. When the pump shuts off, water in the suction line drains back toward the pool through whatever gap exists, and a leak far too small to break prime while running is more than enough to empty the line over eight hours. Check the pump lid O-ring first, then the check valve if the pad has one, since a valve with a worn flapper or a stuck spring lets the whole column drain back every night. On an above-water-line pad this is the single most common version of the complaint and a failed check valve is the usual answer. If the line holds when capped but still drains in service, the leak is in a fitting rather than the valve.
