Vacuum Leak - A/C and Heat Stops Blowing When Accelerating

Hello everyone. First off, I drive a 1994 Buick LeSabre Limited. I have been searching on the interent for a while and have come across what I believe to be is a vacuum leak. Now that it is getting warm outside, I like to use the Air Conditioning, which is really nice. Except when I accelerate. Even when I'm moderatly increasing my speed, the amount of air that I'm getting out of the front vents will significantly decrease. This is getting to be quite annoying becuase it doesn't cool as well. Is it a vacuum leak? Where would anyone suggest to look for a leak?


Thank you and any help is appreciated.

Firstly, check and see if air is flowing properly out of any vents. If it is, check under your hood for the vacuum box on your firewall. Is it plugged in and are the connections all tight and is the hose in good condition? Check all other vacuum lines you see (there are not that many.) Get a vacuum gauge and test your vehicle with it. They are cheap to buy. If all is well, I would look at the blower motor, then the relay, wiring, resistor block, and control unit on the dash where you push the buttons.

You can test each thing by getting an official GM service manual. It will have the exact tests to run.

I just fixed it. The two way check valve was completley broken and I replaced a bunch of lines. It's great now. It seems as if it's even shifting better also. I'm estatic. Thank you so much!

I just fixed it. The two way check valve was completley broken and I replaced a bunch of lines. It's great now. It seems as if it's even shifting better also. I'm estatic. Thank you so much!

Thanks for posting to tell those who helped what you found. That will benefit them when other posters have the same question.

I just fixed it. The two way check valve was completley broken and I replaced a bunch of lines. It's great now. It seems as if it's even shifting better also. I'm estatic. Thank you so much!Good vacuum plays a major role in the in the way a transmission shifts, so you are not just imagining that. BTW, good find! :grinyes:

Thank you so much again everyone! I used to think that this transmission was just a pile of junk, but it's great. Thanks again.

This is my exact problem too, so I am glad I found the post. Can you give me a heads up though on where this two-way check valve is?

Well if you look at your engine, you will see two loop around lines, these are vaccum lines. I would replace them also. Anyways follow them toward your firewall and you should see a "bottle" like valve, this is the two way check valve. One end goes to your cruise control and the other goes toward your firewall. If you look at the picture attached, I have circled where you should look at first. If you go to Autozone or Advanced Auto, they probably won't know what this two-way check valve is. It isn't an automobile specific part, so their computer system won't do you any good. The valve is a cheap $3 part in the "Help" section of them stores, commonly stocked, right there on the shelf.

Thank you, Thank you, Thank you!!!! This "bottle check valve" advice was the best car advice I have ever had.:iceslolan. The 5 dollar part :2cents: at Pep Boys was installed, after confirming (by lightly testing the old vs. new by blowing through them). :runaround:Once it was replaced, my vacuum problems were fixed, the cruise control worked great, the engine shifted better, performance was a lot like it used to be. But, it still stuttered a bit when trying to go fast from a stop or when at a normal street crusing speed. The next fix that was found, was to remove the MAF - mass airflow sensor (very easy to remove using a secure TORX and phillips) and clean the little "crystal" sensors with an electronic parts cleaning spray and a very light brush. For about 10 dollars total, I now have a car that wouldn't and couldn't get up a hill very well, to one that runs like a tiger with its tale on fire (just like when it was new).:rofl: It jumps up from the start and will burn rubber, as best as it ever could (it is only a Buick, after all).

Can someone tell me where the check valve is on the 94 Lesabre Jaimey was talking about

Somewhere in the engine compartment, there is an emissions label showing all vac lines......follow the vac line from the cruise control to a tee/or the vac tank.....there should be a line to the cruise actuator, a line to the firewall(this feeds the HVAC system), and a line to the intake of the engine....that check valve is usually in the line between the intake and the the vac tank......

When the check valve is bad, the vac in the engine drops as you accelerate(vac is max at idle), and this affects the vac actuator position in the HVAC system.....so what the check valve does is prevent loss of vacuum stored in the tank when you accelerate....

Thank you so much, I replaced the 3 way valve and all the vacuum lines, but the a/c still Stops Blowing When Accelerating. Last time this happened my a/c went out. Going to clean the MAF Sensor. Any suggestions would be appreciated :smile:

Not sure what you mean by a three way valve.......the check valve is plastic and looks like 2-3 dimes stacked on one another....