Mustang GT rough idle/AC stuck on defrost

[Ethan Branch]

Hello community, I am new here hopefully this is the correct forum. I am looking to see of any other people have had a sn95 with rough idle, or stuck AC thermostat on defrost. I have been told most likely this is a vacuum issues and they are possibly related. Any input or steps to go about checking would be great. Thanks for any and all peoples time.

 

[ttocs]

for the defroster issue, look on the passenger side firewall and you will see a vac line come out by the heater core in/out and it will be dangling down not attached to anything. Look on the drivers side and you will see the vac tree and there will be a line that goes up behind the engine and is also not connected to anything. Get a little thicker piece of vac line to use a coupler to connect them back together.

For the rough idle, while the car is running go and tap on the IAC valve with a screwdriver and then giggle the harness that connects to it and see if it stumbles at all.

 

[evilcw311]

Keep in mind that since you probably have a vacuum leak which is causing the defrost issue then there’s a chance that’s affecting the idle as well. Depending on how well it’s been maintained or not there is probably other vacuum leaks.

Easiest way to test is grab a cigar, pull the brake booster vacuum line, blown smoke into the line pretty hard and watch for smoke escaping. This will help your find the leaks. Replace the bad lines and you’ll see a difference.


Sent from my iPhone using Tapatalk

 

[Ethan Branch]

Thank you for for your suggestions!

 

[Ethan Branch]

I have an update for anyone who might be able to provide insight. After finally getting some time to inspect the engine bay for loose vaccine lines. I discovered the tree of lines coming from the drivers side firewall area. I noticed the AC line was missing it’s hose entirely. I checked around the engine bay and could not find a loose hose or anything dangling. On the opposite side near the passenger side
I did find a random electrical connection that I don’t know where that goes either. I’m guessing maybe this absent hose could be my problem. Where does it go to? I’ll try and post photos.

 

[Ethan Branch]

Anyone able to provide any input?

 

[Chip66]

The picture of the plug might be to the low-pressure switch on the dryer. I cannot tell because your hand is blocking the view of where the low-pressure switch is.

The vacuum line should enter the dash in that same corner. It is buried in under all the wires there. It may be easier to trace from inside the dash by taking out the glove box door. There is a vacuum reservoir right behind the door with a check valve.

If you have any open ports on the vacuum tree or elsewhere they will likely cause a rough idle. There should be no open ports.