AC Evaporator Replacement

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By Nick Chrimes, BaySHO Performance.

If your AC system has a leak but you can’t find it, even with a UV light to make the dye glow green, it may be the evaporator that’s leaking.
The only part of the system you can’t see to check.

First off, the dash has to be pulled out far enough to remove the plenum (evaporator housing) behind it.

Empty the system of any remaining refrigerant by loosening the high pressure port valve near the battery. It will come out as a gas, not a liquid.

Disconnect the heater hoses (no need to drain the coolant) and the two AC lines from the firewall, and separate the upper and lower speedo cables.
You will need to remove the Y Pipe to get at the lower AC line.
You won’t be able to completely separate the lower line from the evaporator if this is an MTX as the rear exhaust manifold is in the way.
As long as you get it started, it will separate when you pull the plenum out.

Follow the shop manual for dash removal. There are several steps it describes that don’t need to be done as the dash only has to be angled sufficiently to remove the plenum.
Those include stripping down the steering column and disconnecting the main harness from inside the engine compartment:

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Remove the plastic cladding over the driver’s side doors as it will get in the way. A T50 Torx bit will be required to unbolt the seat belt from the B pillar.

Complete following the manual. No need to disconnect the antenna cable, but detach the harness along the front of the (removed) glove box to give the cables enough slack.
This is a two person job, one at either end of the dash. You will need a crowbar to lift the dash over a metal lip just to the left of the steering column opening:

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Disconnect the hoses and wiring, undo the plenum and remove it. There are three nuts holding the plenum to the firewall with large integral washers, as seen in the pic below.
One at the top and two at the bottom, one of which has to be reached through the passenger side wheel well.

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As an aside, take a look at the recirculating door / vacuum pump mechanism. You might find that half of the plastic stud holding the pump arm to the door lever has split:

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If so, screw a small nut onto the remains of the stud and secure it with a blob of glue from a glue gun. This can be done simply by dropping the glove box door:

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If you have a ’90 and are also replacing the Blend Door Actuator, be aware that it may have an ’89 BDA with the harness hard wired in.
This can be checked before you start by lowering the glove box door:

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The replacement will more than likely look like this:

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So make sure you get the harness as well if yours doesn’t have one. F0DZ-19D887-AC.

The top of the plenum is going to have to be cut to remove the evaporator. Here’s the instructions from the shop manual:

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The cuts will be top, bottom and right between the grooves. Internal hinge on the left:

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The semicircular protrusion bottom right can be safely cut off as it’s not used for anything.
You will also have to cut straight down through the large anchor tab at the bottom and the two tabs on the right:

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Those two tabs will have a bolt and spring nut through them to keep the lid closed. I used M6 1.0 x 10mm valve cover bolts with a spare couple of spring nuts from a SHO.
So a hole will have to be drilled. Put the spring nut in place and drill through it with a 3/16” bit.
That won’t damage the threads if you start at the threaded end of the nut. Remove the nut and drill out to 7/32” for the bolt to go through:

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The manual calls for the lid to be cut 1/16” beyond the line of the hinge. The outer tab shows where to stop:

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Cut as much as you can with a Dremel and large cutoff wheel. Finish off the cuts with a minihack:

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Peel back the top and remove the evaporator. Carefully peel the insulation off with a putty knife. A hair drier will soften the glue. Use a glue gun to attach it to the new unit.

Pull the plastic plug off the lip of the bottom evaporator tube but leave it in place. These are tough to move: you won’t be able to do it once the plenum is installed:

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Unit ready to be reinstalled. Note the orientation of the bolts and spring nuts holding the lid closed:

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Two people are going to be needed. One inside the car and one looking in front of the firewall top and bottom to give guidance.
Remember that the rear exhaust manifold gets in the way of getting the lower AC hose being clear of the evaporator pipe,
so the helper needs to be under the car to guide the hose onto the pipe before bolting up. Installation of the hose can be completed once the plenum is fully installed.

Next is to reinstall the dash per the manual. This is also a two person job. Replace all remaining parts and you are done.
You should have a professional recharge done to evacuate the system and remove all moisture.

Parts I installed in a '90 May 2021:

AC Evaporator (Four Seasons) 54534 $95 Autozone

Heater Core (SPI / Duralast) 398315 $45 Autozone

Blend Door Actuator F0DZ-19E616-A SHO Source