A/C Diagnosis Help

[phil527]

Looking for some advice on diagnosing an AC problem - I believe it is my compressor- but just wanted some added feedback before I spend the bucks on another compressor.

 

98 OBW, 186K. A/C has always struggled on this car, but being in WNY it is never really taxed that much. This summer has been putting it to the test.

 

I am only getting about 15-20 degrees drop in ambient air temp. Worse cooling at idle, gets better when driving down the road. About a month ago I pulled the evaporator box to make sure it was clean, changed the expansion valve while I had it out( all new orings)- fair amount of leaves and crud in there, new receiver/dryer as well, sprayed down and blew out the evaporator, evacuated, verified it held vacuum and recharged.

 

It ran better for a while (ambient temps were in the 70s when I did this), but now that temps are in the 90s it is obvious it is still not up to snuff. I put the gauges back on and recorded some readings. Ambient air temp was around 85-90. At idle low pressure was 38, high 176. If I run the throttle up to 2500 or so the pressures will go to 20/220.

 

I don't know what the normal low/high pressures should be at idle. I am thinking the 4:1 high to low is not enough at idle - indicating a failing compressor???? System is leak tight, I feel heat all over the condenser at high RPM, it does not seem to get too hot when running at idle though.

 

Does this point to a failing compressor? What should the low/high pressures be when running at idle?

 

Thanks

[smackvt]

did you add the right amount of r-134 ? its labled on the rad spt low side sounds a little to low im thinking it should stay around 40 psi when running dont forget the comp oil

[phil527]

Yes, tag lists 19-23 oz, I put in right around 22 oz when I filled after evac. Added some oil with new accumulator and valve.

 

The 40 was at idle, and compressing up to around 175 about a 4:1 ratio. As soon as I give it a little RPM it goes to 20/200 (10:1 ratio). I don't know if this behavior is normal or if I should be seeing a better compressing ratio at idle. Coincidentally, the poor performance is only at idle. When driving at higher RPM I can get better output, not perfect but better.

 

Also, static pressure of the system, when off and allowed to equilibriate is about 75 psi at an 80-82 degrees ambient temp.

 

I have been hunting around for some normal high/low pressure values for 134a and have not found them yet.

[mdjdc]

You may want to check the cable for the heater to the blendoor. I just looked at a friend's system and she said that it just slowly got to the point that it wouldn't cool. The freon was perfect so we looked under the dash and saw that the cable casing was moving with the heat selector. It was not closing and causing the system to push hot air with the A/C. Perhaps yours is doing something similar or the cable is not allowing the door to copletely close.

 

Check it out.

[phil527]

I did check the blend door operation when I had the evap core out.

 

My other thought would be the condenser not working well enough. From other sites I have looked at, the compressor pressures do not seem too far off normal. I am going to pick up an IR thermometer and try to see what the temperature drop across the condenser is. I don't believe it is plugged, otherwise I would have higher output pressures.

 

With and idle output pressure at 175 it puts the gas at 121F, which is only about 30 degrees above ambient. That is not a lot of difference, maybe an dirty (internally) condenser is not cooling the gas enough?

 

It would be a cheaper part to change I guess.

[BigMattyD]

You can check to see if the fins on the condenser are dirty or bent. I noticed on mine, that the bottom few inches has a ton of gravel, road dirt, bugs, etc . that have kind of clogged up and bent the lower few inces of the fins.

 

I have a fin straighetner that I got from Harbor Freight. I need to get out there and deal with it.

 

Matt

 

03 Outback 2.5 SOHC AT

[nipper]

1.5 lbs of 134A

Readings are at idle.

 

i am afraid to ask the next question, how did you release the coolant the first time. Why did you just start replacing parts?

 

Outside temp 91 degrees

low is 30=40 high is 170-210 (Per the FSM)

 

 

R134A is week at idle, but you should be getting more cooling. YOur numbers are a biit on the low end of the tolerance.

 

Did you evacuate the system before you refilled it since you had the system open?

[phil527]

It has been running marginal for some time now and figured I would start with the simple things. I cleaned the exterior of the condenser, and figured I would pull the evaporator and clean it( I did find a fair amount of leaves and crud in there).While I had the evap core out, I replace the expansion valve and dryer, used all new o-rings when I put it back together.

 

I had the system discharged by a shop this spring when I was in for tires/inspection. I have a vacuum pump, pulled vacuum for an hour, closed valve and verified no leak back over 30 minutes. I am pretty confident the system was tight. Added correct amount of refrigerant. At the time outside temps were around 70 F and the system seemed pretty cold - thought that did the trick. But as the outside temp has been pushing in the 90s it is clear it is still not optimal.

 

I did some more checks last night. I used an IR thermometer to double check my pressure readings and found a few more things out:

 

Ambient air 85F

Static pressure car off 100 PSI both low and high (matched temp and pressure tables for 134a)

At idle low/high pressure 35/165

at 1500 RPM 27/195 PSI, I checked the compressor outlet line temp and it was 128 F (again right where the T and P tables say it should be for 195 psi).

 

At the upper inlet of the condenser it is 128 F, at the lower outlet it is 100 F .

 

With this running, the ac on recirc and the fan speed on 2 or 3 I get about 56 or 57 degree air coming out. This is only about 30 degrees below ambient. I also noticed that the compressor never cycles off which leads me to believe the cooling output is not up to par.

 

I am now thinking that maybe the compressor is OK but I am not getting enough cooling out of the condenser? What should the temp drop be across the condenser? Is there a way to test the efficiency of this component?

 

I have seen posts describing misting some water on it while it is running to see if the added cooling improves the system operation.

Edited July 19, 2011 by phil527

[nipper]

Hate to tell you this, but for 13a you are normal. Nothing cools like freon. 134a a 30 degree drop is about right, as freon was 40.

 

Some cars really do not perform as well as the old system because they need larger components, and there is not much room under the dash. If you googlr thr issue (and i did and it really was hard to find a true paper on it) there are a lot of people complaing the conversion does not cool as well as freon. In addition some sites explain that larger parts (and higher pressures) are needed to (by at least 12%) to make up the difference. If you look under the dash, there is no room under there to begin with.

 

YOu are normal. There are some other people that have noticed this on here, lost posts due to a system crash. Newer subarus were desighned with this in mind. At speed it cools getter. YOu really need to crack open the windows, let the AC push the heat out, then close them and let the car cool off.