2008 Nissan Sentra Air Conditioner Problems

[joshuawes1984]

I have a 2008 Nissan Sentra, 164K on the clock, and I am having some AC issues that I'm trying to get lined out. I have owned the car since new in 2008 and the AC has always worked well up until recently.

The problems began with the airflow from the vents gradually decreasing the longer the car ran. The fan speed remained constant, but the air flow actually weakened. After shutting the car off for a few minutes, starting it back up, resulted in the airflow being completely restored. I chalked this up to the evaporator behind the dash freezing up, blocking airflow, thawing, rinse and repeat.

Some Googling resulted in the expansion valve being the likely culprit here, though I did not verify with manifold gauges beforehand (rookie mistake, I guess). Replacing the expansion valve appears to have resolved the issue (idling the car for about 15 minutes, and driving the car) and the airflow seems to be fine now. I'd really like to drive for longer, 30-40 minutes to really confirm though. I DID pull a good solid vac on the system for about 45 minutes, and it held vac for an hour with no change.

What is concerning to me are the AC pressures and the amount of R134a that the system consumed. Low size is hanging about 25 PSI and the high side is in the 170-ish range at idle. When raising the RPM to about 1500-ish, the low side drops to around 18-19 PSI and the high side climbs to 185-190. This is with am ambient temp of 85-ish degrees. I really am feeling like the low side is too low, but the high side is just about right. Obviously in hotter temperatures the pressures will be a little higher. The Nissan workshop manual suggests 18 oz of R134A, however with 18 oz charged back in the system, the low side was quite a bit lower, and the high side was in the 140-ish range. I added quite a bit more R134a and that brought the pressures on up slightly to where they are now.

I am experiencing the low side/suction line frosting heavily now. With the fan speed on low/medium, the line frosts up pretty fast (less airflow through the evap), and when I turn the speed to high, the line thaws out for a couple minutes before frosting back up yet again. I'm thinking this is not normal, and will result in the evaporator freezing up (again), putting me back at square one.

Verified stuff
- The radiator/condenser fans operate properly and pull a ton of air.
- The compressor is engaging, but does not seem to cycle (at idle anyway). It kicks on and stays on as long as the "AC" button is pressed.
- I have verified there are no leaks in the system (sniffer and UV dye).
- I have verified the cabin air filter is not clogged or causing a restriction (it's new).
- No abnormal noises from the compressor when it is running.
- Static pressures (once stabilized) are about 100-110 at about 80 degrees ambient.
- Vent temperatures with the fan on max is about 38-39 degrees, and it holds this temperature.
- Vent temperatures with the fan at 50% fall to about 35 degrees.

Service performed
- Replaced expansion valve.
- New-o-rings.
- Vac'd system for 45 minutes.
- System held vac for 1 hour.
- Charged system back w/ 18 oz R134a.
- Noted pressures slightly low, added more R134a to compensate.

My questions are,

1. As the Low/suction line is frosting heavily, does this indicate I still have an underlying issue somewhere, and what does that point to?
2. Are my pressures normal for 80-85 degree ambient air? (20-25 / 150-170)
3. Have I potentially overcharged the system trying to compensate for the lower pressures? I can obviously pull some ref back out if needed.
4. Am I still potentially undercharged? (I don't see HOW, but considering the suction line is frosting..)

 

[Chris142]

Evaporator temp senser

 

[joshuawes1984]

Evaporator temp senser

There isn't one, unless I'm missing where it's located.

 

[Marcozi]

There isn't one, unless I'm missing where it's located.

I'd be very surprised if your truck doesn't have an evap temperature sensor.

 

[mk378]

It's a common misconception that a TXV is designed to regulate to a particular temperature. What they actually do is regulate the pressure - temperature conditions at the outlet to ensure that it is fully evaporated and no liquid exists there. Liquid entering the compressor will damage it.

Something else has to prevent the evaporator from getting too cold in absolute terms and having water freeze on the outside. The main way especially in more basic systems is a temperature sensor. The other way is to not let the low side pressure get too low with a variable compressor.

 

[joshuawes1984]

Just an update, I think I found the problem.

Number one, the manifold gauges that I *had* likely were not accurate. I discovered this by buying a new set from HF (they were on sale!) and comparing readings.

I decided then to just start from scratch and completely re-evacuate and re-recharge the system using my NEW gauges. I rented a vac pump again and this time drove the car to heat things up under the hood a little bit. I pulled a LONG vacuum on the system this time (about an hour) and then refilled. No surprise that the exact amount of 134A needed put the pressures pretty spot on.

I'm thinking that I may have still had a little moisture/air in the system (low low side reading and normal-ish high side) and after the deeper vacuum and warmer underhood temp, that boiled it off and properly evac'd the system.

Low side now is about 30 PSI and the high side was in the 160-170 range, and the suction line is NOT frosting over anymore. It's cold and wet, but not frost/iced.

The discharge air doesn't seem AS cold, but it's still plenty cold to cool you down. Just isn't blowing total ice, but that's okay. I've read that it can be too cold and cause problems. Discharge temp is right at about 40 degrees F.

Everything seems fine now. Drove it about 60 miles, AC wide open, and it never slowed down the first time.

 

[joshuawes1984]

Another update, finally resolved.

The culprit all this time was the AC compressor.

What tipped me off is that both high and low side pressures were low, with the correct R134a charge. Overcharging the system slightly resulted in slightly higher pressures - and masked the problem of a weak compressor. With the CORRECT amount of R134a, the pressures were very low, and resulted in the cold side line freezing up.

Replacing the compressor, doing another vacuum/evacuate, and recharging again with the proper amount of R134a resulted in pressures right on the money per the service manual.

I'm just glad I caught it before the compressor self-destructed and sent it's shrapnel all throughout the AC system. I guess that's what I get for ignoring the obvious. I should have paid attention to what those gauges were telling me when the system was charged properly. Lesson learned, the gauges only matter when the system has the exact correct amount of refrigerant in it.