I didn't see this question asked after a search so I thought why not. I have a 97 GMC Sonoma with 90k miles that I barely drive. Since the hot summer is coming fast I went to use the AC and I am only blowing warm air from the defrost and floor vents. After searching the site I did run across a question regarding the Sonoma's brother, the s10, that mentioned to test the vacuum lines. I was curious to know if checking vacuum lines can be done by spraying a soapy solution with a spray bottle while the truck was running do the trick?

As stated in the comments, the doors were working intermittently at times when it would blow through the dash and cycle to only the defrost and floor. First step from past experience I did examine the vacuum lines such as next the canister. I did notice they appeared to by dry rotten so I went to O'reily's and purchased new vacuum lines for the location for the top right driver side where a T is located. After replacing some of the vacuum lines the issue proceeded to be less then a month later completely went out.

My interest is trying to figure out where to test the vacuum lines properly. Sometimes, as it could be a related issue, there is a bubbling noise coming from the passenger side dash which I believe may be the heater core but I am unable to determine if the heater core goes through the vacuum system.

Pic of location:

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Per request, the controls are electrical. Here is a pic of the controls. I do plan this weekend to further test the system:

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Do note that no matter what position the AC controls are in they will blow out through the defrost and floor vents.

  • Are you talking about checking some specific vacuum connection, or finding a vacuum leak anywhere on the engine? Commented Apr 27, 2016 at 7:12
  • 1
    Are you talking about vacuum pipes that are used to move the flaps that control the flow of hot/cold air into the cabin, or are you talking about using a vacuum pump to test if your A/C refrigerant pipes are leaking? Spraying soap fluid on vacuum pipes will just cause the fluid to be sucked into a leak, you wont see bubbles forming.
    – HandyHowie
    Commented Apr 27, 2016 at 7:45
  • 1
    The Sonoma uses vacuum to switch the ac-heater doors. Some vacuum supply lines break under the hood, look for a vacuum line that penetrates the firewall in the engine compartment, then back trace it to where it gets its vacuum supply from the engine or brake booster, you should find the line broken near the supply connection.
    – Moab
    Commented Apr 27, 2016 at 14:08
  • @Darth_Vader Followup. So what were the results of the heater control lever thump thump test? Photo of the control head? Question for you: instead of the current title for this thing, why isn't it "How do I fix my Heater & A/C in my car, suspect vacuum problem?" I know why folks with one point don't answer followup troubleshooting questions. You don't get that excuse.
    – zipzit
    Commented Apr 28, 2016 at 20:07
  • I will be testing the entire system this weekend and looking into getting dye to put into the system to verify no leaks are present. If you feel the title could be better by all means modify it, I will not cut your head off with my light saber. As far as the controls they are electrical and no matter what position the knob is they blow through the defrost and floor. Commented Apr 28, 2016 at 20:38

3 Answers 3


On the 97 Sonoma the controls are electronic and send a signal to a vacuum switch block which will send vacuum to various valves.

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This is from a corvette but is similar.

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While the switches on the control panel can fail more commonly the vacuum lines, check valve or vacuum reservoir will fail first.

Locate the vacuum source line and measure engine vacuum. If vacuum is low or non existent check the check valve for sticking.

Also check the vacuum reservoir using a vacuum pump. Leaking on the seams of the reservoir is common.

So funny enough a Buick with the same problem rolled in today.

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Vacuum line was burnt looks like the engine caught on fire and this wasn't fixed.

Everything working OK now.

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Heck yeah!

Short answer: Replace the check valve.

Long answer: There is a check valve between the vacuum reservoir and the rest of the system. What that does is help the vacuum reservoir hold more vacuum. Your engine won't have enough vacuum "force" to move the heating ventilation doors without the reservoir. The check valve is that little black and white thing about 9" below the round ball vacuum reservoir. It allows air to move in one direction but not the other. One thing to note: When the parts are in good working order, and with the engine shut off you should be able to change HVAC modes two or three times using the residual vacuum in the reservoir.

But there's more in play here. The "warm air" comment throws me. I'd like to see a photo of the controls on the inside of the car. I believe that on that vehicle vacuum is only used for the recirculation door and for the mode doors inside the heating / ventilation / air conditioning (HVAC) system. I thought that car uses a metal cable to control temperature.

Can you describe exactly what is happening inside the car? When you move the temperature lever from left to right (engine off) rapidly do you hear a thump thump noise at both ends of travel? If not that's bad. So here's how that works. Your engine flows hot coolant to the heater core all the time winter and summer. They use a small plastic door to control air flow thru that heater core to control the temperature of the output air. In the winter you want all the air to flow thru that heater core. In the summer you want no air to flow thru that heater core. Remember in your vehicle ALL the air always flows thru the air conditioning evaporator core. If you want cold air conditioning its critical that the heater vent door stays closed. Hot air will totally negate any air conditioning cooling.

Note there were some cars that had a vacuum controlled temperature doors but I don't think your was one of them.

I'd really like to see a photo of your HVAC controls and the results of the full heat / full cold temperature lever thump thump test.

  • 2
    on the 97 the temp door is controlled by a electric motor.
    – Ben
    Commented Apr 28, 2016 at 21:03

with the car running, use an aerosol can of carb cleaner with the straw in it to spray around the vacuum lines and fittings. When you get to the leak point you will hear the rpm change. move slowly and use spurts.

  • I do remember this approach but shouldn't it also be noted that carb cleaner is flammable and it should only be done on a cold motor? Commented Apr 27, 2016 at 20:34
  • 1
    Yes the carb cleaner is flammable, that's part of what makes this work. As always, be careful with flammable materials around a warm engine.
    – Jrican
    Commented Apr 27, 2016 at 20:40
  • I have done this for years, it works well, if its not dangerous i wont use it.
    – Moab
    Commented Apr 27, 2016 at 20:48

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