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Running issues after reattaching vacuum line

Alex05

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Madison, WI
The vacuum line that allows the HVAC to change output location (defrost, lower/upper, etc) sometimes falls off the manifold. This time, I didn't notice that it fell off because of some maintenance I was doing and it remained off for a while while I drove the vehicle.
After realizing that it had fallen off again, I reattached it with the motor running.
Immediately the motor bogged and was noticeably struggling with any throttle input. What did I do wrong?
 

CharlesHS

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The vacuum line that attaches to the Intake Manifold feeds more than just the A/C actuator motors; it connects to the Vacuum Accumulator in the Passenger fender well, and the Cruise Control Actuator Motor.
You may need to do a thorough inspection of the vacuum lines and actuator motors that the vacuum operates.


HVAC Vauuum Line Routing Jeep Wrangler.jpeg
 
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Alex05

Alex05

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Madison, WI
Thanks for this Charles!
With other instances when this vacuum line has fallen off, I have simply reattached it and not noticed any change at all in running behavior.
Is it possible this time that my PCM had enough time to adjust for the vacuum leak?
Also, I do not have cruise control. When you say inspect the vacuum lines and actuator motors, do you mean that I should be looking for vacuum leaks? Visually, the vacuum line is in great shape with no dry rot, cracks, etc, and is still nice and flexible. The HVAC actuator moves the airflow as it should (obviously, when the vacuum line is actually attached!)
Would it be worth trying a PCM reset?
 

CharlesHS

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Since the vacuum leak caused by the disconnected vacuum line is small... I doubt it would have much impact other than possibly a slightly raised engine idle RPMs.
IF the vacuum line has fallen off before then you should either replaced the rubber end piece that slides onto the intake manifold barb OR place a tie wrap around the rubber end piece to keep it attached to manifold barb.
It is doubtful you will notice much change by doing a PCM reset...
The motor bogging during throttle movement is probably the result of some other problems.
 
Last edited:
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Alex05

Alex05

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Madison, WI
Definitely seems like resetting the PCM sorted it out, but I'm certainly keeping an eye on it.
I could very clearly upset the running conditions by attaching the vacuum hose. After a reset and a bit of driving, with the hose reattached, the motor settled back into normal idle speed and throttle behavior.
As far as I'm aware, even one vacuum leak is enough to upset A/F ratio. I trust your insight Charles but it does seem like my PCM had time to adjust to more air coming into the manifold. A reset took the map back to a default state, and the computer has now started to adjust to driving conditions. I don't have any reason to suspect that the vacuum accumulator or the HVAC actuator have anything wrong with them, but seeing as I don't have a vacuum gauge, I wouldn't be able to test anyways.
This has happened to me before, but not nearly as dramatic as being able to very noticeably bog the motor when reattaching this hose; That's never behaved that way in other instances. The hose and connector aren't even dry rotted; Is there a reason why it's so easy to pull it off the manifold barb when there is no vacuum? A slight breeze could practically knock it loose.
 

CharlesHS

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Interesting that such a small vacuum leak caused that problem and the PCM could not adjust the to the MAP sensor and O2 sensors signal.
Thanks for the update on resolving the problem.
 

Steve04

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The pcm has what Chrysler calls adaptive memory. It had adjusted to having the vacuum leak probably by changing the iac position and going richer on the a/f . It would have adjusted itself back after you reattached the hose given enough drive time or pcm reset like you did. Find a way to secure the hose so you don’t keep having this happen
 
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Alex05

Alex05

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Madison, WI
Should I expect harder shifts while the PCM is re-adapting/learning?
Sometimes the 1-2 shift is hard, other times it's 2-3.
I never experienced hard shifts prior to resetting the PCM.
 
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Alex05

Alex05

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I’m guessing you have an 05 so tcm is part of the pcm and yes it should go back to shifting the same soon
Sounds good! I'm also aware that I can expect my '05 PCM to fail at some point. Fortunately this vehicle is very low mileage, but it sounds like that may not matter