How muc hvacuum should I have????

[Mike93]

In my continued effort to fix my crap running car...more here...I'd like to test vacuum to rule this out. I have a gauge that reads to 30, so what are acceptable numbers??? Also, if there a best or better spot to take the reading from???

 

[SHOZ123]

15-18" and steady when idling and warm. Get it directly off the manifold.

 

[Mike93]

15-18" and steady when idling and warm. Get it directly off the manifold.

Thanks, just tested it and it was pretty steady at 18".

 

[plethaus]

I would like to test this on my own car, where do you hook up the gauge to test?

 

[yamahaSHO]

I would like to test this on my own car, where do you hook up the gauge to test?

On the intake after the TB.

 

[plethaus]

On the intake after the TB.

Can you explain more? Sorry, I've never tested vacuum before.

 

[SonicRiot]

You are testing manifold vacuum after the throttle body to give yourself a window into the condition of the engine. At idle, the car should be pulling about 18 in-hg of vacuum. If it's significantly more vacuum, you may have valvetrain issues such as sticking valves. If you have significantly less vacuum at idle, you most likely have some power issues related to anything from vacuum leaks to bad rings or even timing issues.

If the car passes a vacuum test at idle but still has misfire or driveability issues otherwise, you may have just narrowed down your search for troubles.

A vacuum test at idle is a simple way to determine the basic condition of the engine without going into compression or leakdown tests which often take a little more eqipment and time.

Does that help plethaus?

 

[Mike93]

At idle, the car should be pulling about 18 in-hg of vacuum. If it's significantly more vacuum, you may have valvetrain issues such as sticking valves.

At idle I was steady at 18...but when I'd crack the throttle it went immediately to zero and then when it was returning to 18 it would go past that until 23-23 for a second before settling back at 18...is this a normal scenario? Or may it be the result of a mistimed engine???

 

[plethaus]

You are testing manifold vacuum after the throttle body to give yourself a window into the condition of the engine. At idle, the car should be pulling about 18 in-hg of vacuum. If it's significantly more vacuum, you may have valvetrain issues such as sticking valves. If you have significantly less vacuum at idle, you most likely have some power issues related to anything from vacuum leaks to bad rings or even timing issues.

If the car passes a vacuum test at idle but still has misfire or driveability issues otherwise, you may have just narrowed down your search for troubles.

A vacuum test at idle is a simple way to determine the basic condition of the engine without going into compression or leakdown tests which often take a little more eqipment and time.

Does that help plethaus?

Actually, yes, thanks for the explanation! I was also just looking for a simple "where do I plug in a vacuum gauge to perform the test", LOL!

Thanks

 

[yamahaSHO]

Whenever you open the throttle, you're going to lose vacuum. WOT should indicate atomoshpheric pressure (or zero) on an NA motor. When you let off the throttle, the motor is spinning faster so you'll read more vacuum until it settles down to idle speed.

 

[Mike93]

Whenever you open the throttle, you're going to lose vacuum. WOT should indicate atomoshpheric pressure (or zero) on an NA motor. When you let off the throttle, the motor is spinning faster so you'll read more vacuum until it settles down to idle speed.

Just as I had suspected, thanks for the clarification