Need help from someone that gets vacuum - Pontiac GTO Forum
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post #1 of 14 (permalink) Old 05-01-2017, 01:21 AM Thread Starter
 
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Need help from someone that gets vacuum

I had my car on a scope and they said at idle, (I think) number 2 cylinder was dragging a bit. Whether it was 2 or 4, it's the cylinder where the intake manifold feeds the vacuum to the power brake booster. The tech thought this cylinder may be dragging due to a vacuum leak.

Pulled the hose off of the brake booster and the engine barely changed behavior. Shouldn't it stumble quite noticeably? He put his finger over the hose and the engine really stumbled.

Now, I am not a vacuum expert, but with the finger over the hose, that should be similar to the hose hooked up to the booster, right? So my thinking is that the booster has a vacuum leak and the engine has been adjusted to over compensate for that.

I have a noticeable 'whirring' sound coming from up in the dash, could that be the vacuum leak?

More concerning is that under a load (most apparent in 3rd or 4th gear) I have excessive pinging, could that also be caused by that vacuum leak?

I guess I am hoping that if there is a vacuum leak, this will cure all my problems. Really what I need is some form of confirmation that this booster is leaking and it seems to me that if you pull that hose out of the booster, the engine should run dramatically different, poorly or not at all.

I appreciate your thoughts!

Mike

Torque (tork), n: 1. The tendency of a force to rotate the body to which it is applied. 2. See also: GTO
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post #2 of 14 (permalink) Old 05-01-2017, 11:37 AM
 
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Yes it sounds like your booster has a vac leak,....the whooshing sound under the dash is an indicator of that as well. Of course there is a vacumn manifold that is under the dash that works the heater blend doors from your dash controls.

in the engine side you will see a vac hose going thru the firewall near the coil, above it. you can squeeze that off and deny the vacumn to that manifold, and see if any changes. That can also give a sound under the dash...

so if the whoosh is under the drivers side I would suspect leak at booster,...whoosh near glove box may be heater manifold.

you could also have a vac leak as tech said on the intake runner, where the gasket seals, often those are at the back of the intake as it gets hot there...

see if tech has a smoke machine he can throw on it to find such a leak..

you are right, the engine may be compensating for the vac leak....

a bad vacumn advance can with too much advance,....which they all seem to have....at the distributor..... can cause pinging at under light throttle as the can pulls in too much advance, and effects timing.

find that vac leak, and repair it, Booster Manifold or intake gasket are the main suspects....

then go back with timing.....
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post #3 of 14 (permalink) Old 05-02-2017, 12:29 AM Thread Starter
 
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Yes it sounds like your booster has a vac leak,....the whooshing sound under the dash is an indicator of that as well. Of course there is a vacumn manifold that is under the dash that works the heater blend doors from your dash controls.

in the engine side you will see a vac hose going thru the firewall near the coil, above it. you can squeeze that off and deny the vacumn to that manifold, and see if any changes. That can also give a sound under the dash...

so if the whoosh is under the drivers side I would suspect leak at booster,...whoosh near glove box may be heater manifold.

you could also have a vac leak as tech said on the intake runner, where the gasket seals, often those are at the back of the intake as it gets hot there...

see if tech has a smoke machine he can throw on it to find such a leak..

you are right, the engine may be compensating for the vac leak....

a bad vacumn advance can with too much advance,....which they all seem to have....at the distributor..... can cause pinging at under light throttle as the can pulls in too much advance, and effects timing.

find that vac leak, and repair it, Booster Manifold or intake gasket are the main suspects....

then go back with timing.....

Thanks! Ugh, sounds like it can be an event. I think the (whoosh, whirr - my son says it sounds like a UFO) is definitely coming from the driver's side behind the gauge cluster. I need to go sit in the passenger seat and see if I still feel the same way. I will look for that hose by the coil to test for the heater manifold vacuum.

I will ask shop if they have a smoker but I doubt they do. I can try the old school carb cleaner on the intake manifold to validate the manifold gasket. Smoker would be nice as I think I have exhaust leaks as well. We got some things to do, this car and me!

My pinging seems to be noticable under lower RPM in higher gear (under a load)

Thanks again, I just need to order a booster and get off my ass and start eliminating stuff.

Torque (tork), n: 1. The tendency of a force to rotate the body to which it is applied. 2. See also: GTO
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post #4 of 14 (permalink) Old 05-02-2017, 12:32 AM Thread Starter
 
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Sorry one more thing, do you know where I can locate a vacuum line diagram for my car? Again, its a 69 GTO with hidden headlights. I have a factory service manual, but did not see a diagram for vacuum.

Torque (tork), n: 1. The tendency of a force to rotate the body to which it is applied. 2. See also: GTO
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post #5 of 14 (permalink) Old 05-02-2017, 05:49 AM
 
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One of the guys here will have it start a thread asking for the diagram, seen them before.

The standard test for a bad brake booster is to pump the pedal 3 times with engine off to release any residual vac. Then put your foot on the pedal and start the car. Pedal should depress from engine vac.....if not, walk it back...check vac at check valve,...if ok booster is bad...

If bad at check valve, remove check valve from hose and check hose, if good replace check valve,.....

If bad at hose, check engine intake without hose, if good replace hose,....if bad...find vac leak in engine

Chugging in high gear at low RPM.s,......answer don't do that!......

You will get it do the basic checks then dive in those boosters are not that hard to change,......

One thing at a time....!
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post #6 of 14 (permalink) Old 05-02-2017, 11:19 PM Thread Starter
 
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One of the guys here will have it start a thread asking for the diagram, seen them before.

The standard test for a bad brake booster is to pump the pedal 3 times with engine off to release any residual vac. Then put your foot on the pedal and start the car. Pedal should depress from engine vac.....if not, walk it back...check vac at check valve,...if ok booster is bad...
Thanks Lemans Guy. I will try to find time this weekend to do some searchin! Interesting related to your test for bad booster. When the car is running, I get great vacuum (and recall the sound from long ago or Whoosh) when I press the brake. All good I assume, but I am pretty sure the whirring sound is coming from behind the gauge cluster which points to a leak at the booster. Now, what I find as interesting and perhaps another tell of a bad booster, is that once the engine is turned off, within seconds, I may get just part of an assisted pedal (as you say pump three times to release residual vacuum). I don't even get one pump. Does that mean the vacuum is leaving the system immediately and that would be another indication of a leak in the booster?

I know I should just stop analyzing and buy a new booster. After that, timing and carb can be analyzed better.

Thanks again!
Mike

Torque (tork), n: 1. The tendency of a force to rotate the body to which it is applied. 2. See also: GTO
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post #7 of 14 (permalink) Old 05-03-2017, 09:09 AM
 
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If leaking from the booster it is only a booster leak or bad check valve...remove the check valve and blow through it from the booster side no air should come through....it is a one way check valve to hold Vacumn...

Of course make sure you are getting Vacumn before you check about losing it...

If you change the booster you should change the check valve and the hose...

So check the check valve it is ez and or start with a new one...

Some hot cams pull very low Vacumn and those cars need a separate pump to keep vac in the booster.... it if your vac is 13 or so that is plenty...

Let us know what you find out and good luck!
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post #8 of 14 (permalink) Old 05-03-2017, 04:41 PM Thread Starter
 
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Originally Posted by Lemans guy View Post
If leaking from the booster it is only a booster leak or bad check valve...remove the check valve and blow through it from the booster side no air should come through....it is a one way check valve to hold Vacumn...

Of course make sure you are getting Vacumn before you check about losing it...

If you change the booster you should change the check valve and the hose...

So check the check valve it is ez and or start with a new one...

Some hot cams pull very low Vacumn and those cars need a separate pump to keep vac in the booster.... it if your vac is 13 or so that is plenty...

Let us know what you find out and good luck!
Starting to wish I lived in Nashville!

1. located the 'whirring' sound as coming from the center/passenger side. Actually it sounds like it could be both, BUT when I move the HVAC top lever from "off" to "inside, vent, AC or DeIce" the sound stops. Based on your earlier comments, that would indicate a leak in the line or manifold in the HVAC.

2. I still question the brake booster. I get killer vacuum on the brakes - stops on a dime and that whoosh sound on the pedal. However, once I turn off the car, there is not even one assisted push of the brake and no whoosh of expelling vacuum. I took off the check valve and I could blow back through it as if out of the booster. Oddly, I could not blow toward the booster (do I not have enough 'blow' to simulate engine vacuum?)

What still makes me wonder is that when I pull the check valve off the booster when the engine is running, you can barely tell a difference in how it idles. If you weren't paying attention, I doubt one would even know. If I put my finger over the removed check valve, it sucks onto my finger and the engine behavior is notable.

I'll keep checking stuff out an I will replace the check valve and start looking at the HVAC vacuum. I just don't want to change that booster and find out it was not the problem.

Thanks! Keep the cards and letters coming!

Torque (tork), n: 1. The tendency of a force to rotate the body to which it is applied. 2. See also: GTO
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post #9 of 14 (permalink) Old 05-03-2017, 08:44 PM
 
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Keeps you will get it, you can clamp off that vac line to the blend doors right in the engine compartment, just take that out for the rest of the booster sound and loss of vac......it only effects the blend doors..

It does sound like your booster is leaking vac as you should not have a loud whoosh at all, that is the leak....also check valve and booster should hold residual vac for say 24 hours.....

If it just holds for a few minutes or seconds that is a leak and if you here it under the dash that is the booster,...only other vac under dash is the blend doors and the vac manifold to them unless you are running a Vacumn gauge,...like I do.

Clamp the blend door vac line near firewall,....is whoosh still under dash,...sounds like a booster leak,...won't hold residual vac,...check valve or booster leak...

So you can just buy a check valve you may need afresh one anyway,...and recheck,..still there change the booster.,at which time you want a new check valve and hose, clamp it tight.

Also you may be right that someone adjusted the carb fuel air mixture to compensate for the added air through the booster leak,...they may have richened it up,.....know when you shut off that excess unmetered air with your finger,.....the mixture is too rich and you get a stumble...

Fix the leak, then you can move on....you are thinking right and will get it set up..

Good luck we are pulling for you!
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post #10 of 14 (permalink) Old 05-03-2017, 11:31 PM Thread Starter
 
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Thanks so much! It will take me some time to run all this down, but I appreciate the pointers!

Mike

Torque (tork), n: 1. The tendency of a force to rotate the body to which it is applied. 2. See also: GTO
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