Automotive Air Conditioning Information Forum

Hello forum. Back with another AC project.

2000 Mitsubishi Montero Sport 3.5L 235K miles. Vehicle was purchased 8 yrs ago with inop AC. The owner (Uncle) took it to a shop 7 yrs ago to have the AC repaired. A new compressor, drier and exp valve installed. AC worked ok for a brief period then quit. New Compressor and Drier installed and AC worked again then quit. The vehicle was up north in the Dakotas so AC wasn’t a necessity, but vehicle back in PHX now and I took on the project of getting the AC working.

I hooked a gauge set up and got the comp to engage, but made literally no pressure or suction. Evacuated the system and began tearing into it.
Replaced the Condenser, drier, compressor, idler pulley and belt. Flushed all the lines, blew dry with nitrogen. Charged with 24oz 134A and 8oz PAG46 as per the service sticker.

My garage temp 71*F
Engine at 800 RPM
Vent temps 51*F
Low side 17-45 (cycling every 20-25 seconds)
High side 110-160 (cycling)

The Evaporator and Expansion Valve I did not replace and could not flush. I’m thinking there is a defective expansion valve or blocked evaporator. When the compressor engages, the low side just dives down until the low pressure switch cuts the compressor off. I added 2 more oz of 134a and it made no difference.

I’m going to discharge the system and replace the evaporator and exp valve tomorrow.

A few observations:

It sounds like the system had a leak when your uncle owned it. Even after two rounds of repairs and component replacements, the system still quit after a short time. Since it spent a considerable time in the Dakotas, it's possible that one or more of the metal refrigerant lines corroded through due to the ice-melt chemicals used on the roads in the winter. Carefully inspect all the metal lines for any signs of corrosion or pinholes, especially the lines down low, replacing any that are suspect.

An ambient temperature of 71°F is on the cool side for MVAC performance testing. It would be best to wait until the temperature rises to at least 80°F outside. Then, go to work.

It's difficult to judge the pressures at idle speed. Set the RPM at about 1,800 while testing and diagnosing because the compressor isn't pumping much of anything at only 800 RPM.

Since you didn't flush the old evaporator, you probably had about 1-1/2 ounces of oil remaining in it. So if the under-hood decal calls for 8 ounces, and that's what you put in, I would guess that you're a bit over the optimum amount now. An overage of ~1-1/2 ounces might affect system performance. So if you do install a new evaporator tomorrow, don't add any oil to it and the total oil amount should then be correct.

Mark86 wrote: ↑Fri Feb 10, 2023 8:38 pm The Evaporator and Expansion Valve I did not replace and could not flush. I’m thinking there is a defective expansion valve or blocked evaporator. When the compressor engages, the low side just dives down until the low pressure switch cuts the compressor off. I added 2 more oz of 134a and it made no difference. I’m going to discharge the system and replace the evaporator and exp valve tomorrow.

The TXV might be malfunctioning, as you mentioned, so changing it out couldn't hurt. Hopefully, you have a new OEM TXV on hand. Some aftermarket TXV's have been known to give problems in the past.

Lastly, if any oil comes out during the refrigerant recovery phase, be sure to add back an equal amount.

Repeat your test at 1500-1800 RPM, which is standard RPM for AC testing. A compressor at the RPM you posted is doing almost nothing, so that test is invalid.
Report back

OK, John and I were typing at the same time....

I had wedged the throttle open with a screwdriver and that produced 2000 rpm. The results were the same, but the cycling happened faster. The car is pretty much rust free, and I recovered almost a full charge. I pressurized the system with 250 lbs of nitrogen to test for leaks.

The expansion valve is is the bulb type, which is mounted to the evaporator.

I dumped the oil out of the new compressor and measured. It was factory filled with 5 oz. I put 4oz back in, 3oz in the condenser, 1oz in the drier,

If the exp valve was not functioning correctly or partially blocked, or a partially blocked evap present as a deep dive on the LP side? It’s presenting as the low side is being starved of flow.

1. To repeat, testing at under 80* can be misleading, there is really no work to be done by the system. I for one would wait for warmer weather to test before going further. It is possible the charge needs to be adjusted but you can't tell at such low temps.
2. It is not recommended to pressure test at more than 100 psi

In Phoenix, you won't have to wait long before the weather warms up

JohnHere wrote: ↑Sat Feb 11, 2023 12:52 pm In Phoenix, you won't have to wait long before the weather warms up

True, but it was amazing today.

Got the evap out, which was relatively painless. About 20 min to have the case on my workbench. New Evap and Exp Valve installed. I didn’t add any oil to the Evap, as pointed out I didn’t compensate for that on the initial oil charge. About 2 table spoons came out of the old evap. Vacuumed down for and hour as I buttoned up the project. Recharged with 24 oz of 134a.

Duct temp is 47 and evap temp is 39*. There is a small removable cover that exposes the outlet side of the evap. I was able to stick a remote probe in there and read the temp off my VOM.

I didn’t write any pressures down, but there was no deep dives on the low side. As suggested I will wait for a warmer day.

Thanks for help!

Mark86 wrote: ↑Sat Feb 11, 2023 10:01 pm Got the evap out, which was relatively painless. About 20 min to have the case on my workbench. New Evap and Exp Valve installed.

Very good. I've spent the better part of a day removing evaporators from some vehicles.

Mark86 wrote: ↑Sat Feb 11, 2023 10:01 pm I didn’t add any oil to the Evap, as pointed out I didn’t compensate for that on the initial oil charge. About 2 table spoons came out of the old evap. Vacuumed down for and hour as I buttoned up the project. Recharged with 24 oz of 134a

The oil amount should be just about right now.

I forgot to ask earlier how you're recharging it, and with what. Some auto parts and big-box stores often carry recharging "kits" that contain sealer and other additives. Sealer, in particular, is something you definitely don't want in the system. Professional MVAC shops recommend installing only pure refrigerant and checking for leaks with an electronic "sniffer."

Do you weigh-in the refrigerant precisely, using a scale—while the system is under a well-held vacuum—or do you use two 12-ounce cans? Using a scale is a much more accurate method of charging because a 12-ounce can often doesn't contain 12 ounces. Also, based on what you said, I'm a bit confused about how you evacuated it.

Mark86 wrote: ↑Sat Feb 11, 2023 10:01 pm Duct temp is 47 and evap temp is 39*. There is a small removable cover that exposes the outlet side of the evap. I was able to stick a remote probe in there and read the temp off my VOM.
I didn’t write any pressures down, but there was no deep dives on the low side. As suggested I will wait for a warmer day.

As mentioned previously, vent temperatures and system pressures don't mean much when the ambient temperature is below ~80°F. The fact that the LP side didn't drop while watching the gauges is a good sign, though.

I charge with a 30lb cylinder, and a refrigerant scale. I have a vacuum pump, which for my temp/altitude pulled 27.5”. Vacuumed for an hour.