Lexus automotive unknown refrigerant path

[CyFi6]

I am working on my 1999 LS400 AC system and there is a branch circuit that bypasses the TXV and leads to an extra port on the evaporator. I can't figure out what the function of this line is and I don't see anything similar on any other models. I originally thought that it may be an IHX for improved efficiency but based on the flow path it doesn't look like it.

There is a small "tee" off the liquid line (before the TXV) that feeds some sort of expansion device (this is separate form the TXV, see photo below).
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After going through the expansion device the line goes to a third port on the evaporator. The evaporator has three ports - one for the feed from the TXV, one for the suction back to the compressor, and one from this unknown expansion device. Here is a picture of the evaporator core.
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When I blow into the third port it seems to be directly connected to the evaporator outlet. Its almost as if its a bypass around the TXV and heat exchanger, perhaps an high pressure relief to the suction side? The hole in the "tee" leading to the expansion device is small, almost a pinhole, probably .035" in diameter, so there's not a lot of flow here.

I have a spare evaporator I can cut open if need be, I just cant figure out what purpose this extra line serves. Any ideas?

[seanf]

Could it be for maintaining enough refrigerant flow to the compressor when the evaporator load is low?
Is it a fixed capacity compressor?

[CyFi6]

That sounds like a logical explanation. It does have a fixed displacement compressor but it also has an evaporator pressure regulator that maintains a backpressure on the evaporator to prevent it from freezing and having to cycle the compressor as much. The TXV has an external equalizer tube that is attached to the compressor suction line after the EPR valve. Perhaps the TXV is capable of closing off to the point of almost completely blocking flow through the evaporator and a bypass is there to keep the compressor lubricated?

[seanf]

Not seen that sort of set-up on auto AC, but in refrigeration they use things like liquid injection to keep the compressor cool.

Is the bypass downstream of the EPR?
Does the TXV have a remote temperature sensing phial/buld?

[seanf]

Is this the set-up you have?

https://partsouq.com/en/catalog/genuine/unit?c=Lexus&ssd=%24%2AKwFoXE1iKi1oMCwYHzRQRDAkBAMdJC41OkN2MzkyOH9gb3B8ZWR1eGF-ZW1qZHwvL3g0eWZ_Fx4NajI_UngmIiszPxwlIW5yeGF8f25obWwyPTN2e340LnlmfiFdUAgaEX92e34zYC8renkJHhxqbAwuTFQK HBd_dnt-KGAvK3o1KTE2d3x8ZWR1eGF8bicAAAAA-8Pogg%3D%3D%24&vid=0&cid=4&uid=222178&q=

https://partsouq.com/en/catalog/genuine/unit?c=Lexus&ssd=%24%2AKwGAtKWKwsWA2MTw99y4rNjM7Ov1zMbd0que29Ha0JeIh5iUjYydkImWjYWCjJTHx5DckY6XioSEh97X28_NwtzW88-UiIbbiJOWgYGChofbx9WSkd3BkImUlLukq_P-lpmSkdnVyd_ZkOb384OD5puqoKn1-JaZkpHC1cnf2dzG2Nmek5bQgoHbiJOHyAAAAADcRmYY%24&vid=0&cid=4&uid=222182&q=

[CyFi6]

Not seen that sort of set-up on auto AC, but in refrigeration they use things like liquid injection to keep the compressor cool.

Is the bypass downstream of the EPR?
Does the TXV have a remote temperature sensing phial/buld?
The EPR is between the evaporator and the suction side of the compressor. Please take a look at page 9 and 11 for the EPR function. This is straight from Denso the supplier for my OEM
https://www.napabeltshose.com/~/media/napa/documents/ac-fundamentals-2-condensor-functions.pdf

The bypass inlet is in the liquid line before the TXV and goes into the evaporator. From there it seems to go directly to the outlet of the evaporator which eventually leads the the EPR and compressor.

Here is a photo of the setup. The black foam wrapped line is the bypass line. The small open port is the liquid line feeding the TXV and the big open port is the evaporator outlet leading to the EPR and compressor inlet.
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The TXV has a remote sensing bulb clamped to the suction line exiting the evaporator. It also has a remote external equalization tube that connects to the suction line between the EPR and the compressor. Here is a picture of what it looks like. You can also see the sensing bulb and equalizer line in the PDF page 9 diagram.
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Is this the set-up you have?


https://partsouq.com/en/catalog/genuine/unit?c=Lexus&ssd=%24%2AKwFoXE1iKi1oMCwYHzRQRDAkBAMdJC41OkN2MzkyOH9gb3B8ZWR1eGF-ZW1qZHwvL3g0eWZ_Fx4NajI_UngmIiszPxwlIW5yeGF8f25obWwyPTN2e340LnlmfiFdUAgaEX92e34zYC8renkJHhxqbAwuTFQK HBd_dnt-KGAvK3o1KTE2d3x8ZWR1eGF8bicAAAAA-8Pogg%3D%3D%24&vid=0&cid=4&uid=222178&q=

https://partsouq.com/en/catalog/genuine/unit?c=Lexus&ssd=%24%2AKwGAtKWKwsWA2MTw99y4rNjM7Ov1zMbd0que29Ha0JeIh5iUjYydkImWjYWCjJTHx5DckY6XioSEh97X28_NwtzW88-UiIbbiJOWgYGChofbx9WSkd3BkImUlLukq_P-lpmSkdnVyd_ZkOb384OD5puqoKn1-JaZkpHC1cnf2dzG2Nmek5bQgoHbiJOHyAAAAADcRmYY%24&vid=0&cid=4&uid=222182&q=


No that setup is for the older version. Mine is a 1999, it is the same from 1995-2000. The older one did not have the bypass.

[CyFi6]

I looked up the catalog for my year and the diagram is missing the bypass circuit
https://partsouq.com/en/catalog/genuine/unit?c=LEXUS00&ssd=%24%2AKwEUIDEZQmMYQ3dAYU5KIExYeH9hEB8SEwEfLV0FVAMcBXoAU1MOBB0UXFhSSUY_Ck9FTkQDHBMMABkYCQQdAhkRFh QAU1MESAUaAx4QEBBMQk9bWVZIQmdbABwSTxwHAhUVFhIQT1NBBgVJVQQdAAAvMD9nagINBgVNQV1LTQRyY2cUF2wPPjQsYWx2Aw oHAlQcU1cGSVVNSgsAABkYCQQdABdbAAAAALGyOWk%3D%24&vid=0&cid=4&uid=222488&q=

But you can see the bypass here
https://partsouq.com/en/catalog/genuine/unit?c=LEXUS00&ssd=%24%2AKwFhVURsNxZtNgI1FDs_VTktDQoUZWpnZnRqWChwIXZpcA91JiZ7cWhhKS0nPDNKfzowOzF2aWZ5dWxtfHFod2xkY2 F1JiZxPXBvdmtlZGM8M1xxLysiOjYVLChne3FodXZnYWRlODQ6f3J3PSdwb3coVFkBExh2f3J3OmkmInNwABcVYGUbJ0VdEhUeAn F-dXV8ZzppcjshPz55dyhicTdwb3RlLwAAAACre52d%24&vid=0&cid=4&uid=222486&q=

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[seanf]

I think I would be trying to see where the bypass really goes. See if it really is just straight into the suction line at the evap outlet, or if there is a volume of the evaporator it flows through before joining the suction. You might be able to use refrigerant oil to fill the volume and see how much it takes.

[CyFi6]

The evaporator is a stacked plate design so I'm not sure what the best way to cut it open would be to be able to see the flow paths. I could fill with anything since I have an extra I don't need, but if I fill with liquid I won't know if its filling some separate volume or just filling the entire heat exchanger due to the outlet port also being internally routed to the heat exchanger core.

I think if I cut the heat exchanger in half perpendicular to the flow I will just see a bunch of channels without knowing which ones lead to where. Its all soldered together so the plates can't be separated. If I cut it inline with the flow then I will only be able to see one of the many channels.