What it is

When a TXV stops holding superheat steady, it fails in one of two general directions. Either it hunts — superheat swings up and down repeatedly and the valve never settles — or it sticks — it parks open (flooding the coil) or closed (starving the coil) and won't move. These look different on the gauges and on a thermometer over time, and telling them apart saves you from cutting out a valve that's actually fine.

The single best diagnostic tool here is not a snapshot. It's watching superheat over a few minutes.

How it works

A healthy valve constantly makes tiny corrections and lands back near its setpoint. Hunting is that correction loop gone unstable — the valve overshoots open, the coil floods, superheat crashes, the bulb cools, the valve slams closed, the coil starves, superheat spikes, the bulb warms, and it overshoots open again. The cycle repeats because the valve's response and the bulb's reaction are out of phase. You'll see suction pressure and superheat oscillate together on a regular rhythm.

A stuck valve isn't correcting at all. Stuck open, it dumps liquid and superheat collapses toward zero. Stuck closed (or badly underfeeding), the coil runs dry, suction pressure drops, and superheat climbs and stays high.

In the field

Put a thermometer clamp on the suction line at the bulb, gauges on, and watch for three to five minutes. Write down superheat every 30 seconds.

  • Numbers march up and down on a repeating cycle (e.g., 4°F, then 18°F, then 5°F, then 16°F): hunting.
  • Superheat pins low — 0 to 3°F — and the suction line sweats or frosts back toward the compressor, suction pressure is high: stuck open / flooding.
  • Superheat climbs high — 25°F, 35°F and up — and stays there, suction pressure is low, the coil may even start partially frosting at the inlet only: stuck closed / starving.

Then check the cheap stuff before condemning the valve:

  1. Bulb contact and insulation. Loose, oily, or uninsulated bulb is the number-one cause of false hunting. Re-clamp to clean copper and insulate.
  2. Equalizer line open and connected (external-equalized valves).
  3. Charge — verify subcooling. An undercharge can make a valve hunt because it's starved for liquid.
  4. Liquid line and filter-drier — a partial restriction upstream starves the valve and mimics a stuck-closed valve.

Normal values & targets

  • Healthy coil superheat: steady, roughly 8–12°F, drifting only slightly as load changes.
  • Hunting: superheat swings of 10–20°F or more on a repeating cycle, suction pressure oscillating with it.
  • Flooding (stuck open): superheat 0–3°F, suction line cold and wet all the way to the compressor.
  • Starving (stuck closed/restricted): superheat 25°F+, suction pressure low for the conditions.

Common faults & what they mean

  • Hunting that stops when you fix bulb contact: it was never the valve — it was the sensing element reading wrong.
  • Hunting that continues with a good bulb, good charge, clear equalizer: oversized valve for the load, or a valve with a worn/erratic power element. Mild hunting at very light load can be normal; severe, constant hunting is not.
  • Flooding with the bulb correctly mounted and charge correct: valve stuck open, oversized valve, or lost superheat spring tension. Flooding is the dangerous one — liquid floodback slugs and dilutes oil in the compressor.
  • Starving with charge and liquid line confirmed good: valve stuck closed, plugged inlet screen, lost bulb charge (a power element that's leaked is dead and the valve drifts shut), or wax/moisture freezing at the port.
  • Starving that comes and goes with a frost ball at the metering device: moisture in the system freezing at the valve orifice — change the drier and pull a proper vacuum.

Tech tips & gotchas

  • A snapshot lies; a trend tells the truth. One superheat reading can't distinguish hunting from a stuck valve. You have to watch it move.
  • Lost bulb charge fails closed. If the power element leaks out, there's no opening force, and the valve drifts toward shut — that reads exactly like a starving coil. A new power element (if field-replaceable) or a new valve fixes it.
  • Moisture mimics a stuck valve. Water in the system freezes at the orifice, the valve "sticks" closed, then thaws on the off cycle and works again for a while. Intermittent starving that improves after the system sits is a classic moisture clue — replace the drier and evacuate to a deep vacuum with a decay test.
  • Don't fight a hunt by cranking the superheat adjustment. Fix the cause (bulb, charge, sizing) instead of masking it. Over-tightening the spring just starves the coil.
  • Flooding back? Protect the compressor first. Liquid returning to the compressor is destroying it in real time. Don't run it long while you diagnose.

Safety / code notes

Recover charge per EPA 608 (40 CFR Part 82, Subpart F) before replacing a valve or power element. Use nitrogen while brazing. If you found moisture, a thorough evacuation to a deep vacuum with a standing decay test is the difference between a real fix and a callback.