Finding CIS vacuum leaks - the vacuum cleaner pressure test

[thomastee]

Hi all,
I wanted to share how I finally found several vacuum leaks in my 74 engined car, after spraying carb cleaner/ propane/ wd40 didn't detect anything. Tony (boyt911sc) on this forum suggested a pressure test so I wanted to see if a regular household vacuum cleaner would work as the source of pressure. Well, it does!
I took off the bag and connected the hose at the 'exhaust' end of the vacuum using overlapping strips of electrical tape to make this mostly airtight. I also taped over the hole where the power cable comes out - basically you need tape over anywhere you can feel air blowing out.

I then took off the rubber boot off the air meter/ throttle assembly and made a connector using the end of a plastic soda bottle to join the end of the vacuum's hose to the throttle intake, sealing things with masking tape.

Well, switching on the vacuum even at low power created plenty of pressure. I mixed washing up liquid with a bit of water in a sprayer and went over everything that carries air. Oh, don't forget the tennis ball in the exhaust. I found big bubbling action at 2 of the injector seals and at the pop off valve too. Now obviously it's tricky to access everywhere with the engine in the car but I think for the very little effort required, this approach is pretty good!
For reference the problem I was trying to solve was a 2 to 3k rpm bucking. Otherwise the car starts and runs well and fuel pressure and ignition tests have all been done first.
Best,
Thomas

[jeffs9146]

This is how I solved my problem!

Make sure to look very close at the air box where the top edges meet the bottom intake! If it leaks there you need a new air box!!

Also if there is a vacuume leak in the intake boot between fuel distributor and the throttle body the car wont run! So it would be better to hook the vacuume to the intake on the air box cover and and put the vacuume on medium or high!

[boyt911sc]

After all this time, a lot of people are still insisting using the propane gas, carb cleaner, starting build, etc. trying to locate these illusive sources of vacuum leaks in CIS in their troubleshooting. In most cases, you won't be lucky finding the leaks. An air compressor, a shop vac, house vac , or even an air mattress pump could do similar task. The trick as you have demonstrated is to create a positive air pressure inside the system (engine) and spray soapy water at suspected areas.

Like what you had experienced, not being able to locate any air leak does not mean you did not have any. So testing and verifying the absence of these air leaks could easily be done by any DIY'er like you or me with better accuracy.

After the vacuum leaks have been corrected, correcting/adjusting the air mixture is necessary. Not before. I thought I failed to convince you using this method because it has been a while since we last communicated. Keep us posted.

Tony

[dave 911]

That's pretty cool. We have a Dyson, I don't think there's an exhaust outlet on it, but I'll check.
If you use a compressor, is there a risk overpressurizing and doing damage? What's the max pressure that you need/should use?

[timmy2]

Originally Posted by dave 911

That's pretty cool. We have a Dyson, I don't think there's an exhaust outlet on it, but I'll check.
If you use a compressor, is there a risk overpressurizing and doing damage? What's the max pressure that you need/should use?

I kept mine under 10 psi and it worked well.

[Bob Kontak]

Originally Posted by boyt911sc

After all this time, a lot of people are still insisting using the propane gas, carb cleaner, starting build, etc. trying to locate these illusive sources of vacuum leaks in CIS

Stop confusing us with the facts, Tony.

[Craig_D]

Great idea and great job! That's now on my list of winter projects!

Cheers,

- Craig_D

[SReppel]

Don't know if any of you have access to any Ultra Sonic probes, but this is what I use to find leaks, both Vacuum and pressure. It can also be used to diagnose bad bearing etc.
I have used it several times to find vacuum leaks on different cars including my 78 SC.



It's not cheap, but luckly I use one for my job along with thermography equipment.

Here is a link if anyone is intrested in this type of equipment.
Ultraprobe® 2000

Scott
_________________
78 911 SC Targa
01 Honda S2000

[86911tlcab]

thanks for the idea...not sure why i never thought of that....smile

[Paulporsche]

Nice work, Thomas and Tony.

[thomastee]

Thanks guys, have re-sealed everything and am ready for another test tomorrow. I can't wait - the prospect of driving the car without the bucking is awesome!
Tony, no convincing was needed - I did try this a few weeks ago with the spare tyre compressor and that didn't work, I guess the volume of air was too low to maintain the right pressure, so I'd been scratching my head since then...
Anyway, will keep you posted!

[Tidybuoy]

Thanks for posting!

I have a 4 day weekend coming up, most of my family is leaving town so I basically have 3 days to myself - It's going to be a "Vern Simmons Weekend"

Checking my car for vacuum leaks is now at the top of my to-do list for this weekend.

[thomastee]

An update - the bucking is still there as I still have leaks at the injector seals. I did a bit more research though and this makes sense as I never replaced the injector sleeve seals, just the outer ones and it seems that the inner ones are pretty important. Especially since there is a lot of play when I wiggle the injectors. Soooo, next project, remove injector sleeves and replace them along with their seals.
I have read various articles and it seems most people recommend taking out the fuel injection entirely. Is this the only best way or is it safe to try dremeling off the notches on the intake runners and pulling out while everything is in the car?
Thanks in advance

[boyt911sc]

Originally Posted by thomastee

An update - the bucking is still there as I still have leaks at the injector seals. I did a bit more research though and this makes sense as I never replaced the injector sleeve seals, just the outer ones and it seems that the inner ones are pretty important. Especially since there is a lot of play when I wiggle the injectors. Soooo, next project, remove injector sleeves and replace them along with their seals.
I have read various articles and it seems most people recommend taking out the fuel injection entirely. Is this the only best way or is it safe to try dremeling off the notches on the intake runners and pulling out while everything is in the car?
Thanks in advance

Thomas,

Find a high temperature vacuum grease commonly used in laboratory using glass wares. I have one at home with a DOW brand name that could seal good. Apply a thin coat around the injector o-ring and sleeve. Replace the O-rings when you get a chance but the sleeves could be sealed using this technique or similar sealant.

Tony

[thomastee]

Yes, makes good sense Tony, I'll put in a temporary seal with a similar material. And if that cures the bucking I'll know it's worth fixing properly in the future.

[don gilbert]

Tony, that the best Idea ive seen on here. Thanks for sharing. Cant wait to see if I have any leaks left.

[hbueno]

Question: If I were to use a shopvac, could I connect the hose to the exhaust pipe instead of the throttle assembly?

Another thing, the engine compartment is so cluttered and tight that I'm not even sure I would know where to spray (and be able to reach) with the soap/water mixture. Are there any easy to follow pictures or diagrams that would point me in the right direction?

Thanks,
Hugo

[Jerome74911S]

Yes, connecting to the exhaust will work. I connected my (thoroughly cleaned) shop-vac that way. I believe that it was easier to connect than the other way around seen in this thread.

I found a lot of leaks around the pop-off valve, but no other place.

[Bob Kontak]

here is a link topictures within the Technical associatd with "vacuum leak". Should give you a start.

vacuum leak site:http://forums.pelicanparts.com/porsche-911-technical-forum/ - Google Search

Check out the Ghetto vacuum leak tester

[Bob Kontak]

Originally Posted by Jerome74911S

I found a lot of leaks around the pop-off valve, but no other place.

Spend some time around the back side of the air box with the bubbles. This is an area the starting fluid, etc. does not pick up very well.

Hose going to the brake booster can be a sleeper as well. It's long.