Quest of The Vacuum Leak

So I am tracking down a vacuum leak on my '99 cab. I started the car and pulled a vacuum line off and the car did NOT change the way it was running at all. A bad sign (or a good sign in that my theory of a vacuum leak is correct). Also the pull of the vacuum seem quite weak when I put my thumb over the end of the line. There definitely was vacuum, it just didnt seem very strong. I also opened the oil filler while the car was running. The engine immediately started to stumble (a good thing). I could also feel a lot of vacuum pulling on the filler ***. I've read that spraying carb cleaner in places where the leak could be is one method. I'm a bit nervous about doing this... not really into using my newly installed fire extinguisher. Also, a lot of the vacuum lines are hidden back on the engine top. I could also just start replacing vacuum lines with new lines and new fittings. Really wish I had a smoke machine. Anyway, any ideas on where to go next on my quest to find the holy vacuum leak?

It's honestly easier to just replace the damn things. Measure the fittings, buy some bulk vacuum line at the local auto parts store, and go to town. By the time you find one leak you could have them all replaced. Remember if you have one leak, you'll have another before long. However, if you're really determined to hunt for leaks, get a propane torch like you use for soldering copper pipe and run it unlit around your connections. Carb cleaner works too, but can be pretty nasty on painted and delicate surfaces.

I tried the starter fluid to find vacuum leaks on my 944. It worked great, until the liquid started pooling, the burst into flames and melted a some vacuum lines. So I ended up changing them all. How did you to come find that you have a vacuum leak? My 99 occasionally searches a little bit on idle. But it can be anywhere from cold start up to warm start up, or even waiting for a light.

The car is throwing a check engine light with a code that indicates a problem with the secondary air injection system. I have tested all of the parts of the SAI but everything seems to be working correctly. The last thing left is a vacuum leak. Also, when I take off a vacuum hose, the engine does not run any differently. I also think the idle is a bit rough and it searches a bit but I have nothing to compare it to. Yes, I guess I will go with just starting to replace the hoses.

I don't know what the spec is, but you can get a vacuum gauge pretty affordably and check vacuum level at idle.

Are you sure the SAI is good? I was throwing the same type codes, but my SAI was awfully loud in addition. I just purchased a good "preowned" one from a porsche dismantler for $50. Solved the error codes and was much quiter to boot. I would think the emission/air injection CEL would reference the SAI pump itself. If you were throwing codes like p1124 and 1126, this would lend me to believe it was a true vacuum issue. At least it was on mine.

I think there is a valve on the front top of the engine that the vacuum hoses connect to. Perhaps that is the culprit. It is similar to the one on the rear of the engine just above the alternator.

Notice there are two number 18s. That's the valve I mentioned. 108-05.gif

Hi, been watching for awhile now but first time posting. I have an 04 C4S cab that I purchased about 18 months ago with PSE ... love it. About 2 days after purchasing it, the CEL came on throwing P0491/P0492 codes. I also noticed that I couldn't hear a difference in the exhaust sound when the PSE was switched on/off. With a vacuum leak being one of the potential causes for these codes, I invested in a brake bleeder vacuum pump like one in the picture below. brake bleeder vacuum pump.jpg Using the pump I was quickly able to identify the PSE solenoid as the culprit. When I isolated the vacuum line going to it, the feed coming from the engine would hold a vacuum, but when I tried to pull a vacuum on the solenoid, it would not hold. The output of the solenoid goes to 2 vacuum actuators on the PSE mufflers. To be sure the leak was not in these lines I simply disconnected them and sealed the switched output of the solenoid. It was the body of the solenoid that was leaking. I isolated the solenoid (as I had also found used Fabspeed mufflers which I installed) and haven't had a CEL since. Hope this helps.

Thought I would clarify with a pic ... Starting at the check valve (item 17 in the previous vacuum circuit drawing), connect the hand held vacuum pump and see if the line will hold a vacuum. In the photo below, my circuit was able to hold a vaccum photo (3).jpg If it does not, move to the first junction (item 20 in the previous vacuum circuit drawing) and check each line coming out of the junction (i.e 28, 31 and feed going to 6). In my case, it was one of the solenoids (similar to #18) that was faulty. Steve

Thanks Steve. I just checked out Bleeding kits on Amazon. Its amazing what you can get for $35 these days!

Yah, we have a great place for these sorts of tools here in Canada called Princess Auto. You can order on line and they have stores in most major cities. When my daughters were small, they were pretty excited to be going there with me ... they weren't so impressed when they got there, especially with the smell of hydraulic oil and tire rubber. Good times.

OK. So the resolution of this was... I found a vacuum hose that had slipped off. I ended up just replacing all of the vacuum hoses so the problem doesn't happen again.