Posted: 6/23/2015 3:54:21 PM EDT
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My 2004 Chevy has 178,000 miles on it. The air conditioner was working fine last summer as long as the truck was not idling.
I put one of those crappy low side parts store gauges on it an it was reading about 100 PSI at idle. I suspected an overcharge so I had some refrigerant removed from the system. (Only a couple Ounces) After this the A/C was a little warmer all the time but even more so at Idle. I checked the condenser and it is relatively clean and the fan clutch is engaged so I don't think it is an air flow problem. Saturday I hooked the low side gauge back up and it was reading fairly normal around 35 PSI. I added a couple ounces back to the system, and it began cooling a little better at Idle, but as I increased the RPM's up to around 2,000 the pressure began surging between 20 PSI and 120 PSI on the low side. While driving down the road I could feel the air get colder, then hotter in cycles as the pressure surged from low to high. So? Do I have a bad orifice tube, too much refrigerant, or possibly a bad compressor? I don't think its the compressor as I had originally suspected, and like I said all I had was the low side only gauge, but the gauge showed the compressor pulling the low side down fairly well. |
| You NEED to get a FACTORY service manual and see what it says. But it sounds to me like something is plugging up the system and periodically opening and closing. My guess would be a partially plugged orifice in the expansion valve. Here |
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Quoted:
My 2004 Chevy has 178,000 miles on it. The air conditioner was working fine last summer as long as the truck was not idling. I put one of those crappy low side parts store gauges on it an it was reading about 100 PSI at idle. I suspected an overcharge so I had some refrigerant removed from the system. (Only a couple Ounces) After this the A/C was a little warmer all the time but even more so at Idle. I checked the condenser and it is relatively clean and the fan clutch is engaged so I don't think it is an air flow problem. Saturday I hooked the low side gauge back up and it was reading fairly normal around 35 PSI. I added a couple ounces back to the system, and it began cooling a little better at Idle, but as I increased the RPM's up to around 2,000 the pressure began surging between 20 PSI and 120 PSI on the low side. While driving down the road I could feel the air get colder, then hotter in cycles as the pressure surged from low to high. So? Do I have a bad orifice tube, too much refrigerant, or possibly a bad compressor? I don't think its the compressor as I had originally suspected, and like I said all I had was the low side only gauge, but the gauge showed the compressor pulling the low side down fairly well. You need a set of gauges to monitor both high and low side pressure. The cycling could be a stopped up orifice causing the high pressure switch to cut out, or it could be an undercharge causing the low pressure switch to cut off the clutch. The surging from 20psi to 120psi was most likely because a safety switch cut the compressor off. Most likely is an undercharge. But you need to check the high side pressure just to be safe. |
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Does it sound like the blend doors are actuating and the airflow through the ducts changes slightly when the engine RPMs change and cause the warmer air?
Could be vacuum related, since those blend doors in the HVAC box are usually vacuum pot operated. Maybe the heat door is opening during idle. I'm totally guessing. |
| No, it's still blowing strong. I'm thinking it's low on refrigerant now that I have read some of the others posters comments. Makes sense, have to remember the pressure RISES as the compressor kicks off on the low side. Wasn't looking at the situation from this perspective. |
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Quoted:
No, it's still blowing strong. I'm thinking it's low on refrigerant now that I have read some of the others posters comments. Makes sense, have to remember the pressure RISES as the compressor kicks off on the low side. Wasn't looking at the situation from this perspective. Also, when it's very hot and humid outside, the low side (and high side pressures) will be higher than usual. Especially when the engine is hot too. The tell-tale that always lets me know my AC is getting low on refrigerant (because I have developed a leak, whether it be a big one or small), is that the AC takes longer to get cold after you start the engine. It'll eventually take longer and longer to get cold and blow warmer and warmer until it finally just stops blowing cool air at all. |
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Quoted:
No, it's still blowing strong. I'm thinking it's low on refrigerant now that I have read some of the others posters comments. Makes sense, have to remember the pressure RISES as the compressor kicks off on the low side. Wasn't looking at the situation from this perspective. Watch for the AC Clutch to cycle. If it's popping on/off, and the pressure rises when it's off, then it's probably low (yes, as mentioned above, you should check the factory manual and hook up a high and low side gauge)...most of the ones I've seen have a cut-off for the AC when the coolant is low... |
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Ok, just got back from under the hood and a test drive.
Added about 3 ounces of R134a to the system. Hooked the cheapy low side gauge back up. Pressure held constant at 45 psi at 2000 rpm. Took it for a test drive with the gauge still hooked up. At idle the pressure would climb to 100 psi then drop back as rpms increased. But never surged as before. The a/c would get warm at idle, but remained cold at higher rpms unlike before. In conclusion: I believe it was low on refrigerant causing the compressor to kick off as the cutoff switch activated, also I believe my compressor is getting weak and will eventually have to be replaced. This should at least get me through the hottest part of the summer on longer road trips I just will get a little warm at stoplights and in the drive through. 
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Does this vehicle have a fan clutch? It might not be pulling enough air through the condensor at idle while stopped. Can you safely get an idle RPM at cruising speed? Does the air stay cold in that case? Maybe try a fan in the driveway or spray water throgh the grill. Does it have a temp gauge or just an idiot light? Hot radiator heating the condensor, hot day, high humidity, no enough air through the condensor, etc. It sounds like you are close to the solution. ETA: 2004 Chevy what? FWD or RWD? Electric fan or mechanical? Pic of engine bay?  |
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2004 Chevy Truck. 4 Wheel Drive, 5.3 liter V-8, belt driven fan with clutch.
The fan is spinning fine. No airflow restrictions. As long as RPM's are kept high it works fine. I suspect a weak compressor. The problem has been solved to my satisfaction, not that I wouldn't listen to more advice. I think if I do anything else it will be to evacuate the system and start over with a new charge to know exactly how much refrigerant is in the system. Thank you ARFCOM experts!
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2004 Chevy truck is subject to a reed valve problem.
The compressor is mounted too low. A reed valve was factory installed in order to keep the compressor from slugging liquid refrigerant (bad idea). Unfortunately, these valves were faulty. Slugging liquid refrigerant results in compressor lockup, belt squeal and poor cooling performance. I don't believe your shitty gauge reading. Get some real ones (high and low side). They are next to the noise and smoke generators at HF. |
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Quoted: 2004 Chevy Truck. 4 Wheel Drive, 5.3 liter V-8, belt driven fan with clutch. The fan is spinning fine. No airflow restrictions. As long as RPM's are kept high it works fine. I suspect a weak compressor. The problem has been solved to my satisfaction, not that I wouldn't listen to more advice. I think if I do anything else it will be to evacuate the system and start over with a new charge to know exactly how much refrigerant is in the system. Thank you ARFCOM experts! |