Engine stalls when I connect brake booster vacuum hose to the intake manifold.

[bose213]

Hi!
I just got my 79sc sportomatic 911 and there was a very hard brake pedal. I figured out that the brake booster vacuum line had been connected wrong. As soon as I connected the brake line to the intake manifold, the brake booster started working and the pedal felt softer, however, the engine started to stall. After that, I pulled the hose out, started the engine, and then blocked the intake hose with my finger. The engine stalled immediately. What is the problem might be?
Thanks in advance.

[sigchuck]

The last part you mention - being able to start the car fine with the hose off (port presumably open) then stalling when you block the port - doesn’t make a lot of sense to me. Was the engine side port previously capped?

Hooking the booster hose up and having stall issues might be a vacuum leak in the soft hose that runs from the engine to the tunnel, or from the tunnel behind the pedals in the cabin/frunk.

It looks like the right hose on the Y in your picture is pulled off of the air box port quite a bit - any chance that might have just happened in connecting/disconnecting the booster line and created a new leak?

You might do well to get a smoke machine to check for leaks if nothing seems obvious.

[proporsche]

there should be only one hose pick up for the booster ,which is the pipe on the air box.The other one you have no idea...find where it is connected to and remove it.

Ivan

[boyt911sc]

Bose,

You have a wrong vacuum hose connection shown in the picture. Why is the “Y” plastic connection open to atmosphere? Where is the other vacuum hose from the “Y” connector goes to? The other hose goes to the side port of the air box.

There is a small rubber vacuum hose near the vacuum booster (trunk). This is where you perform a vacuum test for:

• To test the brake booster.
• To test the vacuum system to the air box.

Tony

[proporsche]

Originally Posted by boyt911sc

Bose,

You have a wrong vacuum hose connection shown in the picture. Why is the “Y” plastic connection open to atmosphere? Where is the other vacuum hose from the “Y” connector goes to? The other hose goes to the side port of the air box.

There is a small rubber vacuum hose near the vacuum booster (trunk). This where you perform a vacuum test for:

• To test the brake booster.
• To test the vacuum system to the air box.

Tony

Tony is that a frequency valve after the Y connection hose.On 78-79 Sc strange.Some kind of funny set up .i`d say

Ivan

also what are those A/C hoses doing in this set up??

[boyt911sc]

Originally Posted by proporsche

Tony is that a frequency valve after the Y connection hose.On 78-79 Sc strange.Some kind of funny set up .i`d say

Ivan

also what are those A/C hoses doing in this set up??

Ivan,

‘78-‘79 SC’s do NOT use frequency valves. Maybe the owner could give us some information about this strange vacuum set-up. I have no experience with Sportomatic transmission.

Tony

[john walker's workshop]

Someone maybe left the vacuum hose open and turned the air bypass screw way in to compensate, as well as richening the mixture to get it to idle. Need to redo those two adjustments.

[bose213]

It's my fault that I didn't give you a full description.
So, there is a sportomatic transmission and the clutch operates by the vacuum. The Y adapter connects the control valve (which operates the clutch), intake manifold, and brake booster. As you see in the photo, I pulled out the brake booster hose from the Y connector, because the engine stalls when I connect it.

The Y connector sucks in air a lot. When I connect the brake booster hose to that Y connector, the brake booster starts to operate and the brake pedal feels softer, however, the engine starts to stall.

The red line connects the charcoal canister with the side port of the air box.
The green line connects the intake manifold with the Y connector.
The blue line connects the control valve (which operates the clutch) with the Y connector.
The orange line goes to the brake booster and I suppose has to be connected to the Y connector.

[boyt911sc]

Bose,

Could you post a picture without the heater blower mount? I like to see the vacuum hoses hidden behind the metal heater blower mount. Thanks.

Tony

[bose213]

Originally Posted by boyt911sc

Bose,

Could you post a picture without the heater blower mount? I like to see the vacuum hoses hidden behind the metal heater blower mount. Thanks.

Tony

Hello Tony. Yes, no problem, but unfortunately I can do it only tomorrow. I live in a different time zone. Thanks

[bose213]

Here is a better photo.

The red line connects the charcoal filter with the side port of the air box.
The green line connects the intake manifold with the Y connector.
The blue line connects the control valve (which operates the clutch) with the Y connector.
The orange line goes to the brake booster and I suppose has to be connected to the Y connector.

[JSV798]

Originally Posted by john walker's workshop

Someone maybe left the vacuum hose open and turned the air bypass screw way in to compensate, as well as richening the mixture to get it to idle. Need to redo those two adjustments.

I think this is what you need to look at. Reconnect the brake booster hose and make these adjustments: your engine is getting way too much air.

My car was originally a sporto and I always wondered what that metal tank was at the back of the engine; thought it might be a vacuum reservoir for the sporto transmission?

My car also has no vacuum advance on the distributor; is your car like that?

Roy

[bose213]

Originally Posted by JSV798

I think this is what you need to look at. Reconnect the brake booster hose and make these adjustments: your engine is getting way too much air.

My car was originally a sporto and I always wondered what that metal tank was at the back of the engine; thought it might be a vacuum reservoir for the sporto transmission?

My car also has no vacuum advance on the distributor; is your car like that?

Roy

Hello Roy.
Sorry, where is the air bypass screw located as well as the mixture screw?

Yes, you are right about the metal tank. It's for the clutch.
My car has a vacuum advance on the distributor.

[JSV798]

Idle speed adjusting screw is on the throttle body. The mixture adjusting screw is on the air flow sensor but needs an exhaust gas analyser. You could probably get the engine to idle by upping the idle speed and go from there. You will find the Bentley manual essential to get to grips with the fuel system.

Roy

[boyt911sc]

Artur,

Post the video of the motor you sent me. There is something terribly wrong with your CIS. The plastic “Y” should NOT be open to atmosphere. It should be closed or plugged. The fact that the motor runs and stalls when you partially blocked the “Y” tube tells me someone had adjusted the mixture very RICH to compensate for the massive air leak.

I have two (2) complete ‘78 SC motors sitting on engine test stands in the garage with the vacuum tubes from the side of the air boxes plugged. That is how I start and tuned the CIS motors. Go back to the basic setting.

Test run the motor without the brake booster and clutch vacuum systems connected to the airbox. Get the motor to run and idle well both cold and warm condition. You need to know where the by-pass air and mixture set screws locations. Do you have a reference maintenance manual for your motor?

Don’t worry, there is plenty of help available in this forum. Do not be afraid to ask even if it sounds silly. We all started like you and gained more experiences as time went by.

Tony

[bose213]

Originally Posted by boyt911sc

Artur,

Post the video of the motor you sent me. There is something terribly wrong with your CIS. The plastic “Y” should NOT be open to atmosphere. It should be closed or plugged. The fact that the motor runs and stalls when you partially blocked the “Y” tube tells me someone had adjusted the mixture very RICH to compensate for the massive air leak.

I have two (2) complete ‘78 SC motors sitting on engine test stands in the garage with the vacuum tubes from the side of the air boxes plugged. That is how I start and tuned the CIS motors. Go back to the basic setting.

Test run the motor without the brake booster and clutch vacuum systems connected to the airbox. Get the motor to run and idle well both cold and warm condition. You need to know where the by-pass air and mixture set screws locations. Do you have a reference maintenance manual for your motor?

Don’t worry, there is plenty of help available in this forum. Do not be afraid to ask even if it sounds silly. We all started like you and gained more experiences as time went by.

Tony

Thank you Tony! I appreciate that.

Here is the video of my problem.

https://youtu.be/bYAseaZSTH8?si=Kx0Maejerlz5zIwL

[boyt911sc]

Artur,

Congratulations! You guys did a good job making this CIS motor to run well. Please post the video you sent me and share your work with the rest of the world. Thanks.

Tony