Quick Answer:

If opening the main shutoff partway restores flow but fully opening it makes flow worse, the internal gate or seat inside the valve is likely damaged or loose, or debris is shifting into a blockage when the valve is wide open. Stop cycling the valve repeatedly, use the curb stop for an emergency shutoff, and arrange to have the main valve replaced by a plumber or your water utility.

Why This Happens

Household main shutoff valves age and wear. Inside the valve is a gate, disc, or seat that controls flow. If that internal piece breaks free, corrodes, or traps debris, its position can change as the valve moves. Partially open, it may sit in a spot that allows water to pass. Fully open, the loose piece or shifted debris can move into a path that chokes flow or causes rapid pressure changes.

This problem can also cause intermittent symptoms such as Sudden loss of pressure during showers when the loose fragment shifts under changing flow conditions.

Step-by-Step What to Do

Step 1 — Confirm the symptom

• Check multiple fixtures: test a cold tap, a shower, and an outdoor hose to confirm the pattern (flow when partially open, worse when fully open).
• Ask a neighbor if they have the same issue. If they do, it could be a service line or utility-side problem.

Step 2 — Observe while cycling the valve

• Get a simple pressure gauge or use the water meter gauge if available so you can watch pressure while you operate the valve.
• Cycle the main valve slowly while watching a gauge: if fully open worsens, the internal gate may be detached or debris is shifting.
• Do not force the valve or crank it quickly — slow is safer and gives a more accurate read on how the internals are behaving.

Step 3 — Use the curb stop for emergencies and plan replacement

• If the valve behaves unpredictably or you need to stop flow quickly, use the curb stop as the emergency isolation point. The curb stop is intended for emergency shutoff.
• Schedule a valve replacement with a licensed plumber or contact your water utility if the valve is on their side of the meter. This is not a repair to delay.

What Not to Do

• Don’t keep operating a failing main valve—use the curb stop and plan a proper replacement.
• Do not try to dismantle an underground or utility-owned valve yourself. That can damage the line or violate local rules.
• Avoid repeatedly forcing the valve open or closed; that can break stems or fling debris into your plumbing.

When to Call a Professional

Call a licensed plumber or your water utility if any of these apply:

• The valve gives inconsistent flow or pressure when moved, or the pressure gauge drops when it’s fully open.
• You see leaks at the valve, or the valve is hard to turn or feels loose — these are signs the internal parts are failing.
• Emergency isolation is needed and you cannot operate the curb stop safely, or you prefer the utility to handle replacement.

If the issue alternates between no flow and low flow after water comes back on, also review guidance for Water returns but pressure is low when deciding next steps.

Safety Notes

• Turn valves slowly and stand clear of fixtures and fittings in case something breaks or sprays.
• Do not dig for the curb stop or service box without calling your local utility for utility-locate services first.
• If electrical pumps or water heaters are involved, switch off their power before working on plumbing to avoid motor damage or hazards.

Common Homeowner Questions

• Q: Can I replace the main valve myself?
  A: Not recommended unless you are experienced; it often requires cutting the service line or coordinating with the water utility.
• Q: Is this an emergency?
  A: If you can’t stop water when you need to, or the valve is leaking, treat it as an emergency and use the curb stop until a pro can replace it.
• Q: Will flushing the system help?
  A: Flushing may move debris temporarily, but it won’t fix a detached or broken internal gate; replacement is the reliable fix.