Its that time again. Car will start with no issue. Let the car warm up at idle and it reaches a point when it stumbles, then searches to a proper idle. This is the only time during any given day that there is an idle issue. its not an ETM issue. Can it be the temp sensor, or maybe the rear O2 sensors? The front ones were replaced. I look for vacuum hoses leaks but don't find many hoses to check. The car is a 2001 2.9 S80 with 130,000 miles.

 

I pulled the air box off and observed the mechanism that switches cold air to hot air. It only stays in the position for hot air only. I'm thinking that this maybe my issue when the engine thinks that it will switch from hot air to a colder air and it doesn't happen, then it stumbles as it corrects itself. Any help?

 

Disconnect the metal tube going to the air box and valve and see if the stumble stops.

 

Are you refering to the fresh air tubing that runs over the radiator? I have and there is no suction or air flow when I place my hand over the outlet.

 

No your car should have one off the back of the air box going to the exhaust area.

 

Yes, I will try to disconnect to see its response. Can you explain the function of the vacuum activated device on the air box? Mine can't be functioning correctly.

 

When the car is cold the valve switches and opens to receive hot air from the back hose going by the exhaust to help warm the motor up faster. When the motor is warm it switches to receive the cold air from by the radiator.

 

Thats what I thought. It looks like my car will only get hot air! Is there a downside to this?

 

The car might be more slugish.

 

I thank you for your responses!

 

Is it possible that this air box has a thermostatic plunger that is bad? Is there a part number I can order? How do you remove without breaking it?

 

As of now, I have removed the swinging door to allow cool air in all the time. I will update if anything changes with performance and/or stumbles.

 

Update:
I've played enough with this car but now I think I've solved my stumbling idle. It will stumble when the engine and air temperature is somewhere below 50*F, but only when depressing the accelerator for the first time. Most of the time it will be in the morning after sitting over night. After this initial stumble, the car runs great. But it would happen every morning.
At first I thought it was my MAF and replaced that, but it still stumbled.
I then noticed the air assembly and the system of vacuum hoses and a diaphram to control the flow of air between hot and cold. I would apply vacuum to the hose and the diaphram would not respond. There seems to be a thermostat on this air exchange plunger, similar to a cars water thermostat, that seemed rusted and inoperatable.
I replaced the entire air assembly with a used one that came with this exchange contraption. I applied vacuum to the replacement diaphram and it responded! After one week in use, the car has never stumbled. A mystery solved? I hope so!

 

Congrats!