Did a smoke test, question about results

[stupidwhitecat]

I’ve been having issues with cold starting and a slight miss at idle, assumed there is a vacuum leak. Bought a smoke machine and have some questions about the results. I used various locations to check for any leaks, below are the results:

Through “snorkel” on airbox cover: leaking around the perimeter of the cover, 2 larger areas on the left and right side. When you remove the cover and look with the filter installed in the cover there are a couple gaps where the plastic is slightly warped. How snug should this be ? I previously purchased new straps. I do notice that at idle if I put some downward pressure on the cover it will drop the idle slightly. I wonder if it is not sealing tight enough ?

Through pop off valve: Small amount from injector seals 4 and 6.

Vacuum hose on distributor: none

Exhaust: none

So I’ll be replacing the injector seals and re-testing. Wondering if I need to be worried about the fitment of the airbox lid ?

[boyt911sc]

Originally Posted by stupidwhitecat

I’ve been having issues with cold starting and a slight miss at idle, assumed there is a vacuum leak. Bought a smoke machine and have some questions about the results. I used various locations to check for any leaks, below are the results:

Through “snorkel” on airbox cover: leaking around the perimeter of the cover, 2 larger areas on the left and right side. When you remove the cover and look with the filter installed in the cover there are a couple gaps where the plastic is slightly warped. How snug should this be ? I previously purchased new straps. I do notice that at idle if I put some downward pressure on the cover it will drop the idle slightly. I wonder if it is not sealing tight enough ?

Through pop off valve: Small amount from injector seals 4 and 6.

Vacuum hose on distributor: none

Exhaust: none

So I’ll be replacing the injector seals and re-testing. Wondering if I need to be worried about the fitment of the airbox lid ?

Stupid,

You are doing the smoke test incorrectly. Inject the pressurized air with smoke to a section that is under vacuum when the motor is running. The snorkel and filter areas are not under pressure or vacuum when motor is running.

Tony

[walt]

The brake booster line has been used in the past.

[stlrj]

" Small amount from injector seals 4 and 6."

When engine is running, it's under negative pressure and injector seals were designed for negative, not positive pressure. Smoke machines produce positive pressure so leakage is normal.

Cheers,
Joe
87 Carrera

[boyt911sc]

Originally Posted by stlrj

" Small amount from injector seals 4 and 6."

When engine is running, it's under negative pressure and injector seals were designed for negative, not positive pressure.Smoke machines produce positive pressure so leakage is normal.

Cheers,
Joe
87 Carrera

Joe,

A good o-ring will prevent the unmetered air from going inside the motor. A smoke machine needs only a very low pressure (1~2 psi.) to detect an air leak. If air or smoke gets through the opening, cracks, space, etc. it will show during the test.

I totally disagree with your assessment that the air leak around the injector o-rings is NORMAL. This is false and misleading. You will never have a good running CIS if you allow these leaks to persist. There should be no smoke coming out of the air box, intake runners including rubber boots, injector o-rings, pop-off valve, vacuum lines, etc. during a smoke test. The POV works better under vacuum. This is how you test the integrity of your vacuum system.

What the smoke test does is locate the hard to find leak source/s. If the smoke could escape from the system under test, imagine what will happen to the ambient air if sucked by the vacuum producing motor.

Summary:

A smoke generator is very effective in locating those hard to find air leak sources in a CIS motor particularly underneath the airbox. This is more efficient than the pressure air and soapy water method. Fix all sources of air leaks and your CIS will be running reliably for a long time. It is the lack of understanding about CIS that makes people hate this system in some way.

Tony

[stupidwhitecat]

The testing I did through the pop off valve and the vacuum line off the distributor should be sufficient, no ?

I did the test around the airbox lid because I was just curious how tight of a seal it created. I know it is not supposed to be air-tight, but I wasn’t sure if my lid was warped or not.

My 74 doesn’t have a brake booster.

I’m new to Porsches, but everything I’ve read so far definitely recommends replacing injector O-rings if they are leaking or suspected. Even the how-to article for doing a smoke test on this site uses this as an example. I’ll be replacing them, quick and cheap reassurance.

I believe when I redo the test after replacing the o-rings I'm going to use the plug on the driver side of the airbox. That way I can also test the pop off valve o-ring and lid to make sure it is sealing. When testing through the POV itself there was no leaking around the epoxy at the base.

[stlrj]

Originally Posted by boyt911sc

Joe,

A good o-ring will prevent the unmetered air from going inside the motor. A smoke machine needs only a very low pressure (1~2 psi.) to detect an air leak. If air or smoke gets through the opening, cracks, space, etc. it will show during the test.

You will never have a good running CIS if you allow these leaks to persist. There should be no smoke coming out of the air box, intake runners including rubber boots, injector o-rings, pop-off valve, vacuum lines, etc. during a smoke test. The POV works better under vacuum. This is how you test the integrity of your vacuum system.

What the smoke test does is locate the hard to find leak source/s. If the smoke could escape from the system under test, imagine what will happen to the ambient air if sucked by the vacuum producing motor.

Summary:

A smoke generator is very effective in locating those hard to find air leak sources in a CIS motor particularly underneath the airbox. This is more efficient than the pressure air and soapy water method. Fix all sources of air leaks and your CIS will be running reliably for a long time. It is the lack of understanding about CIS that makes people hate this system in some way.

Tony

Tony,

Sorry for the misunderstanding. I couldn't agree with you more how vacuum leaks are very detrimental. What I was trying to convey was that positive pressure no matter how slight would cause the appearance of a leak from the outside but be totally sealed when under negative pressure.

How is that possible? Well, the plastic sleeve (bushing) also has an exterior "O" ring that seals it against the manifold. But the bore it fits in is slightly funnel shaped so the slightest amount of positive pressure is enough to displace it and cause it to leak similar to a one way valve.

A very clever design but very deceptive.


Cheers,

Joe
87 Carrera