Vacuum Drops When Depressing Clutch Pedal

[TwoSnelz]

Thread title, is this something I should be worried about? What is it an indication of, if anything?

On startup the car is idling with about 17 in/hg. It used to be around 19, only recently dropped(last week or so) but its been colder here in the mornings.

Car starts and idles fine, when clutch pedal is depressed the vacuum gauge reads 15 in/hg.

I cant get my head around how the clutch pedal would affect engine vacuum. Will be checking for leaks tomorrow, just looking to see where I should start if this is a symptom of where my leak might be.

Thanks AZ

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[CELison]

Normal. The system is hydraulic and shares fluid with the brake booster which uses engine vac.

[TwoSnelz]

What do you think about my vacuum readings? I tend to see lower readings when its cold out, anyone else experience this?

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[Silverex]

How much difference we talking about? If it’s slight I’d caulk it up to the HVAC or fuel trims adjusting to different temps.

[TwoSnelz]

About 2in/hg

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[MetalMan]

Normal. The system is hydraulic and shares fluid with the brake booster which uses engine vac.

Sorry, but the systems are totally separated except for sharing a fluid source. Engine vac doesn't have any impact on the brake/clutch fluid or vice versa.


I cant get my head around how the clutch pedal would affect engine vacuum. Will be checking for leaks tomorrow, just looking to see where I should start if this is a symptom of where my leak might be.

TOTALLY NORMAL. Actually I'd be more worried if vac didn't change from depressing the clutch pedal.
You'll notice the clutch pedal has 2 sensors, upper and lower. The lower switch is used for starting, and the upper switch is used by the ECU.

At zero speed / idle, the ECU uses the upper clutch switch to change idle load parameters. So when you're just idling, the engine is at its lowest load point for fuel conservation. But when you press the clutch pedal, it increases engine load to give you an easier time taking off in first gear and reduce your likelihood of stalling the engine.

Less importantly for this discussion, when at speed the ECU also uses the upper clutch switch to disable cruise control, as well as change engine parameters surrounding a gear shift. I believe this signal is one of the signals for the ECU to open the N249 valve to evacuate charge pressure from the intake tract during a gear shit.

[TwoSnelz]

Makes complete sense, thanks for the detailed explanation!

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[MetalMan]

FYI, I'd expect vacuum to become greater with cooler weather. Cooler air is more dense. Just like if you take a sea level car to the mountains you'd expect lesser engine vac because the air is less dense at higher elevatiosn.

My point is that a boost leak test would be a great idea.

[Scotty@Advanced]

Vacuum is proportional to throttle angle. Monitor throttle % and you will see a corresponding throttle change when you press the clutch pedal.