Could a vacuum leak do this?

[txredgt]

Kinda strange and I am a little curious as to if this is possible so here goes. My brakes are for sure on the way out. I have metal on metal contact on the right front and I don't think the driver side has too much longer as well. Those will be on the way out in a few weekends. The last few days have been cooler here in TX (around 45 in the morning) and the car is fine during the early part of the drive to work but after the motor is warm I am having an idle issue. It seems to stay around 1500-1800 and not go lower. It obviously sounds like a vaccuum leak but as I was at a stoplight today trying to figure out what it could be, I pumped and held the brake pedal down and it wasn't a pretty sound as far as the brakes are concerned and it definately sounds spongy. To my surprise though it seemed that if I put more pressure on the pedal it seemed like the RPM's slightly lowered. Could this be maybe because the master cylinder could be going/is gone? Any other ideas from the issues I've listed?

I will be putting new brakes on and I would be a little upset if I went through all of that with a cobra upgrade for it still to need a master cylinder.

 

[trinity_gt]

The brakes on your GT get their power assist from the power steering pump. Unlike many cars that use engine vacuum, the "hydroboost" system uses hydraulic pressure to generate assist. It's more complicated both mechanically and to plumb etc but the actuator is smaller and allows the tall, wide modular engines to fix in what is basically a Fox engine bay.

Pumping the brakes generates a hydraulic load on the power steering pump which may have been what you observed. It's similar to turning the steering wheel and loading the pump that way.

Your idle issue is likely a vacuum leak where air is getting around the throttle plate. If this has just started it's possible you've got an IAC (idle air control) solenoid issue.

 

[txredgt]

So same vaccuum problem is still around and it has done it more frequently now that it is slightly cooler. Pulled up to a stoplight and it stayed around 2K but when I turned the car off it didn't do it for the rest of the way during rush hour traffic. Could it be to do with the tune/ecu of the car?

 

[wmburns]

How to trouble shoot IAC & idle issues

For a high idle, some typical causes could be:

• Vacuum leak after the TB. Remember, the PCV is basically a controlled vacuum leak. When was the last time the PCV valve was changed?
• weak throttle return spring
• loose or worn throttle linkage
• TB idle set screw set too high allowing too much air to enter.
• bad IAC
• Bad TPS. Don't try to adjust the TPS voltage. 96+ MY cars do not use the TPS in the same way.
• If the speedometer doesn't work or is not accurate, this can cause high idle as well.

For more information on how to trouble shoot IAC idle issues:
Is your mustang stumbling, dieing at idle or idling rough? - Page 14 - Ford Mustang Forums

 

[txredgt]

I just find it strange that once I turned the car off and restarted it that the idle went back to normal. Wouldn't a vaccuum leak be present every time you start/run the car?

 

[wmburns]

Please go back and re-read the material provided. Especially the part of how the PCM learns a new "ratch" value from the TPS at every startup. Do you think that could offer an explaination of why it would change after a restart?

Also read the part about the role loose or worn throttle linkage can play when vacuum holds the throttle closed and then releases as vacuum changes.

I could also see a case were a sticky PCV value would get held closed under vacuum. Turning the motor off would release it. So this offers a explaination of how a vacuum leak could come and go.

 

[txredgt]

Please go back and re-read the material provided. Especially the part of how the PCM learns a new "ratch" value from the TPS at every startup. Do you think that could offer an explaination of why it would change after a restart?

I could also see a case were a sticky PCV value would get held closed under vacuum. Turning the motor off would release it. So this offers a explaination of how a vacuum leak could come and go.

Looks like I read what I wanted to read lol. Thanks for the help I will do some searching in regards to these issues.