Vaccuum Guage

[Charlie]

Completely non boat related. Sorry. But I seem to get the best answers to motor questions here. My son just bought a Subaru WRX. It is a tight engine compartment. A real PITA to work on. There is a problem with the engine so I am running a vaccuum test on it. I plugged into the brake booster and at 3000' elevation I am getting 17hmg. Adjusting for altitude that is 20 hmg. All good. I run a snap throttle test and it climbs up to 24 hmg but doesn't drop like I am accustomed to. I have tried two different gauges and both react the same. Is this a problem? Next test would be to do a leak down test but I have to take apart a fair bit of car to get to the spark plugs.

[a64pilot]

I don’t understand what your asking.
Are you saying at idle you get good vacuum, but if you punch the accelerator and let it return to idle, the same vacuum isn’t measured?

WRX is a turbo isn’t it?

[Charlie]

Originally Posted by a64pilot

I don’t understand what your asking.
Are you saying at idle you get good vacuum, but if you punch the accelerator and let it return to idle, the same vacuum isn’t measured?

WRX is a turbo isn’t it?

Yes WRX is a turbo.

When you snap the throttle the vacuum should decrease dramatically. The butter fly valve opens increasing the size of the passage between the intake manifold and the engine. Then with more fuel the vacuum increases above standard. Then with the throttle back to idle the gauge should drop back to the original reading. In my case neither the drop at the opening of the throttle nor the drop after the engine settles back to idle are occuring.

After explaing this I wonder if there is a check valve inbetween the intake and the brake booster. That would explain no drop in vacuum, right?

[shakey doug]

NASIOC Rex forum has access to info from street to track, you will need to identify your son's car more accurately ie. GC8, spec C, EJ 20 STI ete.

[leftbrainstuff]

Originally Posted by Charlie

Completely non boat related. Sorry. But I seem to get the best answers to motor questions here. My son just bought a Subaru WRX. It is a tight engine compartment. A real PITA to work on. There is a problem with the engine so I am running a vaccuum test on it. I plugged into the brake booster and at 3000' elevation I am getting 17hmg. Adjusting for altitude that is 20 hmg. All good. I run a snap throttle test and it climbs up to 24 hmg but doesn't drop like I am accustomed to. I have tried two different gauges and both react the same. Is this a problem? Next test would be to do a leak down test but I have to take apart a fair bit of car to get to the spark plugs.

A vacuum test will not indicate engine condition. A compression test or leak down test will.

If you"re looking for a vacuum or induction leak then you try pulling diagnostic codes (mostly useless or find it the old fashioned way.

Has the vehicle been modified? Perhaps by someone who shouldn't touch things?

Where are you measuring?

[Lepke]

The is usually a check valve before brakes or any sensor or other item needing constant vacuum.
Old vacuum wipers stopping when accelerating was fixed by adding a air tight can and a check valve between the wipers and engine.

[Wotname]

Originally Posted by Charlie

Completely non boat related. Sorry. But I seem to get the best answers to motor questions here. My son just bought a Subaru WRX. It is a tight engine compartment. A real PITA to work on. There is a problem with the engine so I am running a vaccuum test on it. ........

It might be helpful to know the actual symptoms of the problem.

[a64pilot]

Originally Posted by Charlie

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After explaing this I wonder if there is a check valve inbetween the intake and the brake booster. That would explain no drop in vacuum, right?

Every vehicle I have ever had that used vacuum to provide brake assist had a check valve, you can “prove” it easily.
With the engine off, pump the brake pedal, after a couple of pumps you can definitely feel the brake assist go away.
Being a turbo is another clue to having a check valve, cause under boost that vacuum booster can’t do much.
In fact a turbo vehicle that uses vacuum to do things like position air handler doors etc., will usually have a significant sized accumulator and a check valve of course.
I had a friend years ago that had an old deuce coupe with a Pontiac 389 Tri power in it, when you floored it, vacuum would go to nothing and the windshield wipers would come to a dead stop. Back off of the throttle going fast and they would go so fast it seemed they would tear up.
Vacuum windshield wipers of course.

[Charlie]

Originally Posted by shakey doug

NASIOC Rex forum has access to info from street to track, you will need to identify your son's car more accurately ie. GC8, spec C, EJ 20 STI ete.

Thanks I will try that website.

Originally Posted by leftbrainstuff

A vacuum test will not indicate engine condition. A compression test or leak down test

I agree that a leakdown test is more accurate. OTOH a vacuum test is an old school way og getting a quick idea of the condition of the valves, vacuum leaks, broken lifter springs, rings.

Originally Posted by Lepke

The is usually a check valve before brakes or any sensor or other item needing constant vacuum.
Old vacuum wipers stopping when accelerating was fixed by adding a air tight can and a check valve between the wipers and engine.

Thanks this is the kind of information that I was curious about.

There was a check valve. I moved the vacuum gauge to the other side of ghe hose and it worked correctly.

Originally Posted by Wotname

It might be helpful to know the actual symptoms of the problem.

Hesitates on rapid accelration. Throws a lean code.

Originally Posted by a64pilot

Every vehicle I have ever had that used vacuum to provide brake assist had a check valve, you can “prove” it easily.
With the engine off, pump the brake pedal, after a couple of pumps you can definitely feel the brake assist go away.
Being a turbo is another clue to having a check valve, cause under boost that vacuum booster can’t do much.
In fact a turbo vehicle that uses vacuum to do things like position air handler doors etc., will usually have a significant sized accumulator and a check valve of course.
I had a friend years ago that had an old deuce coupe with a Pontiac 389 Tri power in it, when you floored it, vacuum would go to nothing and the windshield wipers would come to a dead stop. Back off of the throttle going fast and they would go so fast it seemed they would tear up.
Vacuum windshield wipers of course.

I believe that the vacuum is all used for engine performance on this engine.

I have ordered a smoke machine to locate the vacuum leaks. The vacuum hoses are brittle, non elastic, so I think I will replace all of them. Will be quite a chore. Makes doing bilge yoga look like a treat.

Thanks for all the help.

[a64pilot]

It it goes lean during high fuel delivery, start with a fuel filter and or other things that may restrict fuel flow.