Pressure testing leaking vacuum system on 3.2 Carrera?

[LukasM]

Hi guys,

I have an 1988 Carrera that I bought from a private seller a year ago, it was running but turned out to have lots of hidden problems that have stopped me from ever driving it, from a broken head stud, to a leaking clutch MC and worn out rusty pedal box, to a seized brake caliper and now all kinds of engine running issues. I'm really glad I paid an "air cooled Porsche specialist" good money to have a detailed PPI done beforehand...


Most of these issues are now fixed (head stud remains though....), however the engine runs badly and smokes a lot, looks and smells (I think, I've got a bad nose) like unburnt fuel. After reading a million threads on Pelican about the potential causes most likely being vacuum leaks, I have now found and replaced two hoses that had cracked open in multiple spots, the short one behind the intake going from crimped hard line to flexible, and a reducer hose that connects to the oil tank on the right.

I already built a smoke machine but had little success with it, almost nothing seeped through the cracks so it's hard to tell which other hoses might be damaged.

I have read on here that some people recommend pressurizing the vacuum system, by plugging up the intake and exhaust. Then you go around with a spray bottle with soapy water and should quickly find out if any bubbles come out.

What does everybody think of that idea on a 3.2 Carrera? I looked into it and could probably draw up an 80mm round plug for the intake boot in CAD and 3D print it, and then get a rubber cap with clamp for the exhaust. I guess one bar / 12 psi should do it?

Thanks for any help!

Cheers from Vienna,
Lukas

[Classic11]

How many broken head studs do you have? What is your plan to get it fixed? You can drive with a broken stud but you are not helping the head/cylinder seal and it could make your top end rebuild that will need to come at some point a lot more expensive.

[LukasM]

It has one broken head stud, on cylinder number 5, so at least it's somewhat supported by the cam tower on top. I do plan on rebuilding the 3.2 engine soon (it has 210.000 kms on it), but I need to get the car registered in Austria first, as I imported it from Luxemburg. You might have heard about TÜV and other strict tech inspections we have over here. The first one is by far the worst as it's by an official government agency, so the car basically has to be 100% stock to have any chance of passing. Once that's behind me a fairly stock looking cam'd twin plugged 3.4 should be able to pass the not so detailed annual inspections as well as they are mostly focused on safety critical things like suspension, brakes, etc....

I did a detailed check once I got it into my own garage, compression was between 175-185 psi and leak down 8-15%, interestingly enough the one with the broken head stud wasn't the worst at 180 and 13%.

Now if only I could get the Motronic to run nicely, even with some power lost...

[boyt911sc]

Lukas,

Using a smoke generator to locate your hard to find vacuum leak sources is more efficient than pressurized air and soapy water. I used the soapy water method for many years before but after trying the smoke generator using 2~3 psi., never used the soapy water again. You don’t need a very high air pressure. All you need is a steady supply of pressurized air with smoke. The trick is how to isolate the system to be tested from atmospheric condition.

Tony

[HaroldMHedge]

When I pressure checked my 87 intake system. I removed the air filter housing and bolted a plate to the air flow meter with an air fitting quick disconnect. This worked well but I did not find any leaks.

Then I tried my friend's smoke machine. I finally found the leak, but it took a while before smoke started showing up around #1 intake manifold at the head. I had loose intake manifold bolts.

[targa80]

In my opinion using a smoke machine to test for vacuum leaks is absolutely necessary and if set up properly the test will show any leaks via the escaping smoke. I posted a thread on my setup and testing of my 3.0 motor it can be found by doing a search on my username.

[targa80]

Here is the thread I previously posted.

https://forums.pelicanparts.com/porsche-911-technical-forum/1081238-blown-air-box-repair-upgrade-spider.html