HVAC Anomaly

[Rich]

Here is a synopsis:

1997 Eldorado. 137,000 miles. While driving with the A/C on, the cold air stops coming out of the A/C vents and defaults to the defroster vents. Manipulating the HVAC controls has no effect in correcting the problem. The head unit continues to indicate airflow from the face vents while the actual airflow is from defrost. The problem usually corrects itself within a few minutes and redirects to the face vents. This is obviously not a compressor problem as cold air is still being produced as I could feel a slight cold airflow from the side of the console where the carpeting is loose. No relevant codes are set to indicate an electrical fault. The problem doesn't occurr specifically during high vacuum or low vacuum, it seems pretty random. Usually occurrs around town rather than at constant highway speeds, usually. Interestingly, this issue NEVER occurred over the winter, even when redirecting heat through the face vents.

Corrective measures: I suspected a vacuum leak. From online research, I discovered that the main vacuum line runs from under the motor beauty cover on the passenger side of the motor to a one way check valve, then to a "T" fitting by the fire wall on the passenger side of the car. Removed the beauty cover and made sure the connection was secure, which it was. From the "T", one side runs through the firewall providing cabin vacuum, the other toward the front of the car to the accumulator tank under the battery tray. Most of the vacuum lines are a type of hard black plastic. One eight inch section of the relatively soft rubber vacuum line between the check valve and the "T" was completely degraded. When I handled it, it started to crumble. It was obviously still maintaining some vacuum, but its integrity was severely compromised. The part is no longer available from the dealer, so I replaced it with vacuum line I purchased at Autozone. I also replaced the other rubber vacuum lines described above, whetther they looked degraded or not. The lengths of hard plastic vacuum line look like new. I then replaced the check valve described above with a new AC Delco unit. Unfortunately, this seems to have increased the frequency of the problem over the past few days so I put the original valve back in today. I also shortened one of the soft vacuum lines by a few inches since it appeared to be longer than necessary. I'm not even sure the problem is vacuum related. I'm at a total loss and completely frustrated at this point. If any of you folks have any advice, I would be very grateful.

[KHE]

There is a vacuum line that runs under the battery tray that has been known to degrade. When it does, the air outlet will be in the defroster outlet. It is possible there is a connection in the line that is intermittent/opens up under certain conditions. Have you checked this line?

[Rich]

I actually read what you just mentioned online somewhere. I couldn't get under the car myself due to clearance, but about a month ago I took my car to a shop (general automotive) and described my problem. They were nice enough to put my car on a lift and we all (three of us) walked under and looked at the accumulator with a flashlight without removing the apron. As you describe, what I read was that there was a section of vacuum line at the accumulator that is known to degrade. But our cursory examination didn't show any soft vacuum line, only the hard plastic line which appeared to be intact. Perhaps a more thorough examination of this area will be required. Based on my explanation of the symptoms, do you feel that the source of the problem is indeed vacuum?

[KHE]

Yes - I would bet the problem is vacuum related. The line you need to look for is the one that runs under the battery tray.

[Cadillac Jim]

When the car is having the problem, pull over and put the car in Park, then press the emergency brake. Then, put the car in Drive or Reverse, and if the parking brake releases automatically, you have vacuum through the A/C system under the dash.

If you do NOT have vacuum through the AC system under the dash, look for hoses pulled off the connectors or cracked. You can get to them easily enough once you remove the covers under the dash on both sides. It's been awhile but there may be covers on the sides of the radio but I don't think so.

If you DO have vacuum through the AC system, look for a problem with the A/C programmer. It operates the doors that control the temperature and where the air goes. Likely, there will be an OBD code if there is a problem with the controller, one of the actuators, or a hose between the controller and an actuator. Sometimes the actuators are called servos, or vacuum servos.

[KHE]

What the service manual calls "actuators" in this vintage of cars are electrically driven. The devices that direct the air to the floor, dash or defrostor vents are vacuum driven. Most of the time, when there is a complaint that the A/C or heat only comes out the defroster vents, it is the vacuum line that runs below the battery tray.

[Rich]

Thanks for your opinions as they give me avenues to continue to explore if the problem persists. The line under the battery tray will be the next thing I will look into replacing. My emergency brake has been releasing sporadically for a few years now but I though it was the emergency brake diaphram.

Since reinstalling the old check valve yesterday along with shortening the length of hose I first replaced, I took a shakedown cruise today to re-assess the HVAC performance. 84 degrees F with strong sun in NY today. I will make no claims at this time of having corrected the problem, but the airflow didn't divert for the entire ride. Local driving, highway driving, up long steep hills, down long steep hiils, I had strong airflow from the face vents the entire time. Very different from the past few days. A note regarding the length of hose I shortened. I installed a replacement length of hose from Autozone the same length as the degraded OEM hose that came with the car, but the replacement hose has a very slightly greater internal diameter. Perhaps shortening the length yesterday by half might have increased vacuum? Not sure, but the airflow from the face vents today seemed stronger than what I might have become accustomed to. Maybe the baffle wasn't fully opening for some time now, maybe opening only 75% or 85% and I didn't notice? Maybe I'm starting to imagine things. The emergency brake also released all three tiimes I shifted out of park. For the past two days, I had to manually release it every time. All I can say is that it seems fine now. If or when the issue recurrs, I'll have to get the car on a lift and examine the line under the battery tray.

Again, thanks to all for your very valued advice. My frustration has been abated and I have a game plan if it acts up again. Will keep you all posted.