Open Road

In another thread the topic of blocking-off the rear A/C came up. The purpose of this thread is to discuss the methods of doing so.

There's exactly one "right" way to do this:

Order the front-only engine bay A/C lines, pull the engine and replace the dual-AC lines with the front-only A/C lines. Then remove all the rear A/C components and fabricate a plug for the floor pan to replace the fitting that couples the under-car A/C lines to the interior rear A/C lines.

There are other methods that are almost-right:
- Attempt to replace the dual-A/C lines with front-only engine bay A/C lines without pulling the engine.
- Cut-off all of the under-car branch of the dual-A/C lines and figure out a way to cap them off - in situ.

Obviously none of those methods are "reversible" for trivial values of "reversible" and none of them are particularly easy.

For now, I will post on two reversible methods:

#1 - remove the under-car lines and plug all four "holes" thus opened with purpose-built plugs
#2 - modify the under-car lines by plugging their ends and reinstalling them.

The first method is the most expensive in terms of dollars but is 100% reversible.

The second method is the most expensive in terms of time, the least expensive in terms of immediate dollars, but does require replacing the under-car A/C lines to reverse the modification.

Both of these methods will result in some refrigerant oil pooling in the the bottom of the engine bay lines after the branch to the under-car lines. It should, however, be noted that oil always pools in the under-car lines and accumulates over time as the rear A/C sits unused. I do not believe that either of these methods will result in more oil pooled than if the rear A/C was intact and continued to be unused(*).

* My '91 GT's rear air is 100% removed as of 20 years ago. I used a variation of "cut off the branches" and method #2 below and its A/C continues to function perfectly (and with the OE almost-40-year old compressor.)

Method #1

Disclaimers:

To date, I have NOT used this method. Until recently I was not aware that purpose-built plugs were readily available. So, while I provide links to parts below that I think are correct, I am not 100% sure that they are the correct sizes. In particular I'm not completely certain that the #10 fitting is correct. I am sure that #6 is correct, that #8 is too small and that #12 is too big. However, I don't have anything #10 on-hand with which to test as of this post.

I provide links below to coldhose.com. I do this because I have ordered lots of A/C bits from them over the years. As a result they are familiar to me. I have always received timely service and good communication. That written, after Covid their prices went through the roof. I provide these links for reference pictures. It is highly likely that lower-cost alternatives can be found.

Summary of steps:

Remove the under-car A/C lines. To plug the engine bay lines, you will need a #10 female plug and a #6 female plug:

https://coldhose.com/products/6-female- ... 8820345087
https://coldhose.com/products/10-female ... 8819886335

These work just like the junctions for the removed hard lines: Lube the new o-ring with refrigerant oil or Nylog, install the plugs, turn down the threaded fitting and torque to factory spec.

At this point with those two plugs, the rear A/C is effectively blocked off.

However, if you leave it this way the guts of the rear A/C components are open to atmosphere, and in particular, the under-car fitting is open to everything that might get tossed up from the road. Those bits won't stay "good" for very long that way. If you want a completely reversible modification you need to plug the under-car fitting also.

You, again, need #6 and #10 but the type of plug is very different.

This is what I find on the coldhose.com site that appears to be correct:

https://coldhose.com/products/1999-2003 ... 4150044927

The first to attempt this method should contact coldhose and verify that the above are a #6 and #10 and if not, what part number should be ordered instead.

Note that you will need bolts that are longer than stock to fasten these plugs to the under-car fitting. How much longer, I don't know.

Method #2

This method is something I have done on numerous occasions (with proper communication and discussion with my clients.)

Disclaimer: At the time of this post I don't have a loose set of under-car lines to determine with certainty the exact sizes of tools and other bits. As a result I have not listed sizes. For the moment folks will have to use there own wherewithal to do that. I am however, certain that the proper bits use SAE sizes and not metric sizes.

Summary of steps:

Remove the under-car A/C lines and clean them thoroughly inside and out.

Tap both ends of each line to cut threads for grub screws on the inside of the lines.

Clean the lines again to remove all debris.

Coat the newly-cut inside threads with JB weld and turn in the grub screws until they are 3 or 4 millimeters deeper than the ends of the lines.

The next day, after the JB weld dries, apply another coat of JB weld over the grub screws to make a "solid plug" over them.

Allow to dry for a day, then install the under-car lines with new (lubricated) o-rings.

You'll need at least two taps: One for the #10 line and one for the #6 line. Best would be four taps: A starter tap for each size and a blind-hole (bottoming tap) for each size.

And you'll need two grub screws of each size that corresponds to the taps.

Tap the ends of the under-car lines on the "engine end" with the starter taps.

Tap the ends of the under-car lines on the "interior end" with the starter tap until it bottoms and then use the bottoming tap to complete the bottom threads.

Well done David. A very thorough discussion of optional procedures. Thanks.

one thing to add about replacing the lines with the non rear air lines. getting the rear lines out with the engine in the car is just not possible but if you are going to the non rear air you can bend and break off the T part of the lines that are going down the passenger firewall, as you won't need them. then you can install the non rear air lines with the engine installed no problem. I forget the size of the hole under the passenger side but I did find a rubber plug that fit perfectly.

If you're thinking you're going to save the under car lines...

Even with the proper crow's foot I was unable to loosen the connections that are between the engine and the firewall heading down. I used a cutoff wheel. Even off the car these proved difficult to separate.

Fox_ wrote: ↑Sat Feb 21, 2026 10:55 am ... the connections that are between the engine and the firewall heading down. I used a cutoff wheel. Even off the car these proved difficult to separate.

Ummm... what exactly are these "connections between the engine and firewall heading down" of which you write?

There are junctions under the car. And there's the junction at the fuel cooler. No other A/C junctions between the engine and firewall ("heading down" or not) on an unbutchered 928.

worf wrote: ↑Sat Feb 21, 2026 4:40 pm Ummm... what exactly are these "connections between the engine and firewall heading down" of which you write?

There are junctions under the car. And there's the junction at the fuel cooler. No other A/C junctions between the engine and firewall ("heading down" or not) on an unbutchered 928.

Interesting. My car had junctions that looked factory. Perpendicular to the ground in the vertical section some distance below the tees for the under car lines.
Here's what's left of the low pressure side that I'm talking about. The two are still stuck together. Took a breaker bar and a socket to break the nut loose.

Fox_ wrote: ↑Sat Feb 21, 2026 5:29 pm My car had junctions that looked factory.

Ok. I see the problem now: Me. I wasn't paying attention to your model year. I thought it was an '87+. And I've not, yet, ever worked on a pre-'87 with rear A/C!

Pre-'87 lines shown first, '87+ lines shown second. The '87+ lines are more-or-less straight and the coupling is easily accessible from underneath.