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The evaporator absorbs cabin heat by boiling refrigerant
Liquid/vapour refrigerant enters after the expansion device at low pressure. As it absorbs heat from air, it changes phase and leaves as superheated vapour for the compressor. Fin temperature also condenses water from humid air.
Airflow, refrigerant mass flow and surface cleanliness all determine capacity. Replacing a leaking core without correcting contamination or control faults can damage the new system.
Evaporator-system components
| Component | Function | Typical fault | Effect |
|---|---|---|---|
| Core/tubes | Transfer heat and contain refrigerant. | Corrosion, crack or blockage. | Leak or poor cooling. |
| Expansion valve/orifice | Meters refrigerant into core. | Stuck, restricted or sensing fault. | Flooding/starvation. |
| Temperature sensor | Prevents icing and controls compressor. | Bias or wrong position. | Freeze-up or short cycling. |
| HVAC case seals | Force air through fins. | Missing foam or poor assembly. | Bypass and reduced output. |
| Condensate drain | Removes collected water. | Blockage, kink or displaced seal. | Footwell water/odour. |
| Cabin filter/blower | Supplies clean airflow. | Restriction or low speed. | Ice and weak vents. |
Fitment identification
Refrigerant and HVAC case revision matter
| Check | Variation | Mismatch risk |
|---|---|---|
| Refrigerant | R134a, R1234yf or other approved system. | Pressure/material incompatibility. |
| Drive side | Pipe and case orientation. | No connection or dashboard interference. |
| Climate zones | Single, dual, rear and heat-pump layouts. | Wrong core/manifold. |
| Expansion device | Block valve or fixed orifice. | Incorrect metering interface. |
| Sensor | Clip, pocket and calibration. | Icing or control fault. |
| Case seals | Foam profiles and drain arrangement. | Air/water bypass. |
Symptoms and alternatives
| Symptom | Evaporator possibility | Other checks | Urgency |
|---|---|---|---|
| Refrigerant loss | Core/manifold leak. | Every hose, condenser, compressor and service port. | Prompt. |
| Weak cooling | Blocked or starved core. | Charge, compressor, valve and airflow. | Diagnose. |
| Cooling fades then returns | Core icing. | Sensor, airflow and charge. | Prompt. |
| Water in footwell | Drain/case seal. | Body/scuttle/heater-core leak. | High. |
| Musty odour | Microbial soil on wet fins. | Filter, carpet and body leaks. | Hygiene service. |
| Oily dye at drain | Evaporator refrigerant/oil leak. | Product overspray and previous dye. | Confirm electronically. |
Refrigerant leak diagnosis
Recover and weigh remaining charge, inspect accessible joints and use electronic detection, nitrogen/hydrogen trace gas or approved UV dye within system guidance. Dye at a drain is useful but can be contaminated by prior service.
Do not exceed test pressure or use oxygen. A vacuum hold alone can miss leaks that close under vacuum and open under pressure.
Pressure and temperature interpretation
High- and low-side pressures depend on ambient temperature, airflow, humidity, engine speed and compressor command. Compare with service charts and record vent/core temperatures.
Low suction can mean restriction or low charge; high suction can mean valve/compressor issues. Pressure alone does not identify the evaporator.
Airflow and icing
A blocked cabin filter or weak blower reduces heat reaching the core, allowing the surface to fall below freezing. Ice then blocks more air. A misplaced sensor can fail to cycle the compressor.
Inspect blower speeds, filter direction, case doors and sensor data before opening the refrigerant circuit.
Superheat and refrigerant distribution
Superheat compares vapour temperature leaving the evaporator with saturation temperature at measured pressure. Interpreted with the system design, it helps distinguish a starved core from one receiving excessive refrigerant. Variable compressors and electronic expansion valves require controller data as well as gauges.
Uneven pipe or fin temperatures can indicate poor distribution, internal restriction or airflow bypass. Frost location is evidence, but running a core into heavy ice risks compressor liquid return and does not replace specified measurements.
Why evaporators corrode
The core remains wet after use and collects salt, pollen and organic acids from cabin air. Dissimilar metals, blocked drainage and aggressive cleaning products can start pinhole corrosion, commonly near tube-to-header joints or contaminated lower fins.
Replace the cabin filter and restore drainage/air seals so the new core does not remain in the same environment. Do not spray household disinfectant whose residues attack aluminium.
Heat-pump and EV applications
A heat-pump vehicle can reverse refrigerant flow or use several valves and heat exchangers for cabin and battery thermal management. The component called an evaporator in one mode may act as a condenser in another.
Service mode, valve positions and charge process can differ from conventional A/C. Use electrically safe oil, dedicated uncontaminated equipment and the exact thermal-system bleeding procedure.
Condensate drainage
Water beneath a vehicle after air-conditioning use is normal. If none drains while the case fills, inspect the drain from its outlet without pushing debris deeper. Avoid sharp tools that puncture the core.
Drain seals must pass through the bulkhead and prevent water returning around the tube during acceleration or cornering.
Odour and cleaning
Organic material on damp fins supports odour. Replace the cabin filter and use a vehicle-approved evaporator cleaning method with controlled application and complete drainage.
Do not flood the case, spray flammable aerosol into a running blower or mask coolant/mould odours with fragrance.
Refrigerant and environmental safety
| Hazard | Control | Avoid |
|---|---|---|
| Pressure | Recover with rated equipment. | Opening a charged line. |
| Frostbite | Eye/skin PPE and controlled couplers. | Contact with liquid refrigerant. |
| Flammability | Ventilation and refrigerant-specific equipment. | Ignition near R1234yf. |
| Toxic decomposition | Keep away from flame/hot work. | Smoking/brazing leaking system. |
| Environment | Recover, recycle or dispose legally. | Venting to atmosphere. |
| High voltage | Correct EV oil/equipment and isolation. | Conductive contamination. |
Dashboard and restraint-system removal
Evaporator access often requires steering column support, airbags, consoles and cross-car beam removal. Power down restraint systems, wait the specified time and store airbags correctly. Mark every earth, harness clip and fastener.
Do not hang the steering column from its joints or power an airbag connector with test equipment.
Opening the HVAC case
Recover refrigerant and drain coolant if the heater matrix shares the case. Cap every pipe immediately. Remove the case without bending aluminium tubes or tearing bulkhead seals.
Open clips and screws in order, noting foam and door positions. Hidden screws forced apart will crack the case and create later air leaks.
Contamination and compressor failure
If a compressor failed internally, metal and degraded oil can lodge in evaporator passages. Some modern microchannel components cannot be reliably flushed and must be replaced within a defined contamination scope.
Follow the system repair matrix for condenser, valve, receiver-drier and lines. Never circulate debris into a new compressor.
O-rings and refrigerant oil
Use the exact O-ring material and size, lubricated with the specified clean refrigerant oil. Colour is not enough. Keep caps on until assembly to limit moisture.
Balance oil according to measured recovery and component replacement data. Too much reduces cooling; too little damages the compressor.
Installation controls
| Stage | Correct practice | Failure prevented |
|---|---|---|
| Compare core | Match dimensions, pipes, valve and sensor. | Case non-fit. |
| Fit seals | Restore every foam strip and drain guide. | Air/water bypass. |
| Mount sensor | Place at exact fin/pocket depth. | Icing. |
| Close case | Check doors move before tightening. | Trapped flap and cracked plastic. |
| Connect lines | New O-rings, natural alignment and torque. | Leak and tube stress. |
| Restore dash | All earths, airbags and beam bolts to spec. | Safety and rattle faults. |
Evacuation and charging
Pressure-test with approved dry inert gas, repair leaks, evacuate with capable equipment and confirm vacuum stability in context. Charge the exact refrigerant mass by calibrated scales, not sight glass or pressure guess.
Small systems are sensitive to charge error. Account for service-hose volume according to equipment procedure.
Commissioning
Verify blower, blend and mode doors, compressor command, pressures, vent temperature and evaporator sensor. Check the drain produces water without wetting the cabin.
Scan HVAC, restraint and body modules, run actuator calibration and confirm no refrigerant or coolant leak after a heat/cool cycle.
Common mistakes
Do not vent refrigerant, charge by pressure alone, mix oils, omit the drier after prolonged opening, reuse flattened O-rings, leave case seals out, misplace the sensor or force dashboard clips.
A stop-leak product can damage recovery equipment and is not a structural repair for a corroded core.
UK legal and safety considerations
F-gas service and refrigerant handling must comply with applicable qualification and environmental rules. Air-conditioning also supports demisting, so poor function can affect visibility.
Water near electrical or restraint components and refrigerant leaks need prompt repair even when cabin cooling is optional.
Practical evaporator FAQs
Q: Does poor cooling prove evaporator failure?
A: No. Check charge, compressor, valve, condenser and airflow.
Q: Is water under the car normal?
A: Yes after humid A/C use; cabin water suggests a drain fault.
Q: Can refrigerant be vented before removal?
A: No. It requires legal controlled recovery.
Q: Does a vacuum hold prove no leak?
A: Not alone; some leaks behave differently under pressure.
Q: Can compressed air test the core?
A: No. Use approved dry inert gas and rated equipment.
Q: Why does airflow fade after cooling?
A: Evaporator icing may be blocking the fins.
Q: Must the expansion valve be renewed?
A: Follow failure cause and vehicle repair scope.
Q: Can any green O-ring be used?
A: No. Size and refrigerant material must match.
Q: Is stop-leak a lasting repair?
A: No. It can contaminate service equipment.
Q: Why replace the receiver-drier?
A: It absorbs moisture and may be saturated after opening/failure.
Q: Can EV refrigerant oil be substituted?
A: No. Electrical insulation requirements are critical.
Q: Does odour mean the core leaks?
A: Usually not; check moisture, filter and microbial soil.
Q: How is repair verified?
A: Leak test, exact charge, pressures, temperatures, airflow and drainage.