Toyota 4 runner blows very cold air with no problem at in town speeds but on the freeway the a/c cuts out and won't come back on until the vehicle cools off. Good air flow out of the vents and low side press is 40. Vehicle doesn't overheat so don't know where to go.

 

Does the compressor run? (Hard to tell at Hwy speeds, but once you stop you can pop the hood and look.)

Also, testing only the low side pressure suggests you're trying to use some sort of "A/C in a Can". Get a real set of gauges and thermometer, so you can get actual data. Doesn't cost much. If you don't want to put in the investment of (little) money and (little) time, you're better off relying on a professional.

 

a) get a set of gauges
b) make sure you are NOT using "outside air" but are using re-circulation mode

Outside air tends to have more moisture in it, and this can cause more "ice" to form on the EVAP

BTW - Im a noob when it comes to AC.... take this post as a "guess" not advice

AFAIK this typically this occurs when the EVAP unit in the dash "freezes up".
It could be several things
-- expansion valve
-- EVAPerator
-- air (aka moisture) in the lines (mixed in with the R134a/R12) freezing up and blocking the EVAP/Expansion valve

 

Ice on (outside of) the evaporator should limit airflow too, which glenn says is not happening.

Ice in the refrigerant should block the TXV, so the pressure switch will turn the compressor OFF, and the high side gauge will read high. This is something worth looking for. (As in: get the gauges -- I liked these: https://www.harborfreight.com/ac-r13...set-62707.html , While you're at it: https://www.harborfreight.com/1-inch...ter-46586.html)

 

Thanks @scope103

I have a similar issue

My AC was converted to R134a from R12 (by me - it was bone dry)
I pulled the EVAP and cleaned out the mouse nest etc. Replaced the O Rings not he EVAP connection at the fire wall.
I did not touch the expansion device as I didnt have a spare and I didnt want to break stuff
(Yeah - I should replaced all the O rings - but I thought Id give it a shot)

It holds a vac OK

It blows at around 45F when its 95 out
after 1 hour on the highway, the temps out of the vent climb to about 60F
Air flow "appears" to be "the same as before" (i.e. no blockage)
When I stop to get gas / at the rest stop, and come back (to a water puddle under the drain), its back to blowing 45F

The truck is at the shop for other reasons - I told em to pull he R134a, vac it down and re fill to spec weight

 

Let us know how that works. It should address air/water in the system, but if you have junk in the expansion valve (or any other problem there) it won't help.

When I replaced my compressor, each vendor ominously warned that there was no warrantee on the compressor UNLESS I replaced the TXV (and flushed the system). I assume that's because when a compressor fails it is either caused by, or causes, crap in the system. My flush came out clean, but who knows?

Replacing the TXV is a much bigger job than vac-and-fill, because the cooling unit has to come out. And I don't have any way to know if your TXV is good or bad. But you've already pulled the Evap once -- you're an expert at it now!

 

I'm lucky in that my compressor never died and the system was always "sealed" (except not so sealed that the R12 didnt leak out)
So, I'm less worried about "junk" in the lines.

Yesterday - I did the "Sure Kill Expressway Rush Hour Commute" - I76 in the Philly area is known locally as the Schuylkill Expressway.
During rush hour, it takes approximately 1 hour to traverse the 16 mile section between the Blue Route (I476) an the Vine Street Expressway (I696)
While yesterday was cooler (mid 80F), the truck ran a 42Fin the vent, the entire time in traffic.
Cold enough that I turned the blower down to the #1 setting.

Me thinks I may leave well enohg alone for now

yeah - - and I know that the drain isnt clogged
and there is no more "mouse nest musty smell".