Heating and Air Conditioning Refreshing

The HVAC system in my 300zx was in pretty bad condition. The heater core leaked, the AC blows only warm air, and the temperature reading is often inaccurate. I decided, since I have to replace the heater core, I might as well completely renovate the entire HVAC system. I have included heater box renovation, duct repair, sensor repair, etc on this page. Again, if anything is missing, please feel free to contact me.

Items needed

  • Spray enamel (flat black)
  • Open cell foam weather stripping (3/8" wide by 3/16" thick)
  • Number 3 steel wool
  • Silicon spray lubricant
  • Multi-purpose grease
  • Hi temp engine enamel (cast aluminum)
  • Metric o-ring
  • 5 minute epoxy
  • Soldering iron
  • Rosin core (60/40) solder
  • Ohm meter

Access the heater box

The heater core on my 300zx was leaking substantially; to replace it, the entire dashboard must be removed.

Rebuild the heater box

  • If necessary, replace the flap seals within the box
  • Remove the vacuum actuators from the right side of the box (see image below), including springs and vacuum manifold

  • Remove the vacuum actuators from the back side of the box, including wiring harness and sensor
  • Remove the six screws holding the two halves of the box together
  • Pull the box apart.
  • Replace the internal flap seals
  • Scrape of all of the old seal
  • Sand the metal flaps with 220 grit wet-or-dry sandpaper
  • Repaint the flaps with spray enamel (I used Duplicolor's high temperature cast aluminum engine enamel)
  • Replace the seals using open foam 3/8" wide by 3/16" tall weather-stripping
  • Lubricate all flap pivot points with a high-temp multipurpose grease
  • Reassemble the heater box
  • Replace all springs (this is not necessary, but I figure that the springs have probably relaxed a bit in the last 18 years)
  • Lubricate all external pivot points with multipurpose grease

Replace all vacuum lines

  • All vacuum hoses on the heater box should be replaced, regardless of condition. (You don't want to do this job again!)
  • Remove each vacuum hose and cut new hose to match it's length
  • Replace one vacuum hose at a time

Refinish the cross member and cross member center support brackets

  • Sand the cross member with 220 grit paper
  • Paint with spray enamel. I used your standard, run of the mill flat black enamel.
  • Reinstall the cross member in the vehicle. If necessary, loosen the lower steering column bolts to assist in replacement of the upper column bolts.

Rebuild the HVAC ducting

  • Thoroughly clean the ducting, inside and out, with a dishwashing detergent.
  • Trim off excess material from the original molding process (see image above)
  • Remove all traces of old weather-stripping.
  • Mask the area surrounding the metal attachment brackets
  • Refinish the metal brackets and any metal surfaces with spray enamel (I used a flat black spray enamel, matching the cross member finish)

  • Apply open cell foam weather-stripping to all mating surfaces of the duct

Sensor diagnosis and renovation

If your digital HVAC system is not working properly, refer to the Factory Service Manuals or to the Z31 Automatic Digital Climate Control Testing procedure and isolate any malfunctioning sensor. My system was functioning properly, however, the sensors were in need of some restoration.


  • Reinstall the HVAC ductwork; make certain that all of the joints in the ductwork fit securely and are properly sealed. If necessary, remove the 3/8" stripping and apply a thicker, open celled foam stripping.
  • Reinstall the center support brackets
  • Reinstall the dash
  • Test the HVAC unit to make sure that all positions are working properly.

Sources used on this page:

  • 1984-1988 Nissan 300ZX Factory Service Manual
  • Writen by: Raredare,