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Jaybird61, ASE Certified Master Technician
Category: Mercury
Satisfied Customers: 1711
Experience:  Automotive Instructor at Palm Beach State College
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I am working on a 2005 Grand Marquis with an intermittent

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Hi, I am working on a 2005 Grand Marquis with an intermittent rough idle. Check engine light is off. There are no stored codes. I replaced vacuum line between throttle body and purge solenoid? This seemed to help. The engine ran smoother. I was also trying to get the climate control to shift between floor, vent and defrost. This has been an intermittent problem for the past year. It seems to work better when the weather is warm. In the dead of winter I have no floor heat. I think all these issues are related to poor vacuum. Should I replace vacuum hoses between throttle body and tee that branches off to Brake booster?? Where do I start?

Hi, My name is Jay

The Tee where it connects to brake booster, PCV and throttle body will get a small hole in it where it connect to the throttle body. Check it there for a bad vacuum line. It comes as an assembly from store or dealer. On the a/c you need to check the small vacuum lines where they go into the firewall for the a/c and trace back to vacuum canister and to the engine and repair any cracked lines.

Customer: replied 1 month ago.
To get to the T do I need to remove the cowl at the edge of the windshield? Do I need to remove the windshield wipers to get the cowl off?

No you do not. Follow the hose from the brake booster and where it connects to the intake back side.

Customer: replied 1 month ago.
Hi Jay, Should I cut-off the vacuum line at the T and then install a new hose up to the throttle body? I can barely get my hand in there! See attached photo.

The vacuum tee is the part that goes bad. I would not recommend cutting it out until you know if it leaking and you have the replacement. It will show small holes and will leak vacuum if it is bad. It will be soft and mushy if it is bad.

Customer: replied 1 month ago.
Can I plug the port at the throttle body to see if it changes how the engine idles? I realize this would disable the power brakes and climate control?
Customer: replied 1 month ago.
I have a hand vacuum pump. If I plug the hand pump into the vacuum line that goes to the T should it hold vacuum if I pump it up??

You can plug it to test idle and see if it helps but will not have power brakes. Also the other side goes to the PCV valve and this can cause rough running if blocked. You can apply vacuum to the brake booster but it will take a large amount of vacuum to pull a vacuum on the brake booster. You will need to disconnect and block booster vacuum line.

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