Vacuum Testing of Manifold Hoses
>when the car is running I hear a suction noise somewhere from underneath
>the intake manifold I looked as best as I could and did not see anything
>loose, but this Saturday I'll take it apart. any suggestions before I start
The sound could be a vacuum leak. There are hoses under the intake manifold that can be hard to see, and, unfortunately, difficult to replace. A vacuum leak will result in unmetered air getting into the engine without running through the air flow meter. This is called "false air" and will result in the fuel computer setting the wrong air/fuel mixture. Here's one methodyou can use to check for leaky hoses:
Remove the intake tubes, air filter, and air box. This will expose the air flow meter. You need to make an airtight seal on the top of the air meter. A plastic cap used when shipping a new air meter is perfect for this. Alternatively, you can put a sheet of heavy plastic over the opening. Either way, use a large radiator clamp around it to make an airtight seal. Unhook the large vacuum line that runs between the engine and the brake booster. Pressurize the engine intake area by exhaling into the hose. (I keep a new, clean hose of the proper diameter in my toolbox for this purpose.) The pressure should hold. If you can hear air escaping, you have a vacuum leak. Unfortunately, this can often be one of the hoses underneath the intake manifold, requiring removal of the manifold to gain access. If there are no leaks, you will probably hear/see a bit of "fizzing" as a tiny bit of oil/bubbles comes out the black plastic plugs on the sides of the engine. At least that's what I see when doing this test on my 1987 S4.
Thanks to Jacques Vuye for teaching me this procedure last Summer!
'87 928S4 5-sp, Venetian Blue
This works even better if you have someone who is willing to exhale their cigarette smoke into an open vacuum line. This will allow you to visually see the leak (That's if you could see the hoses under the intake)
>Next, I want to check for false air leaks in the intake system. The shop
>manual shows a way to do this by attaching a cover over the mass air flow
>sensor and pressurizing the intake to about 0.5 bar.
I did this bout a year ago. It worked really well and I was able to pinpoint the false air leak.
Get an old inner tube and some large hose clamps.
1. Cut a piece out of the inner tube that includes the inflation valve large enough to go over the Mass Air Sensor, stretch it around a little and clamp it on.
2. To get around the issue of the possibility of an open valve and any resultant leak, I cut out pieces large enough to go over the tail pipes. These I also stretched and clamped into place.
3. I attached a bicycle pump to the inflation valve at the MAS and pumped it up until I could see the inner tube bulge out.
4. Listen for leaks. The inner tube should hold its shape with the outward bulge. Initially mine did not. I used a collection of screws and bolts to plug hoses that led to things that leaked. On my car there is some sort of venturi thing going to the brake booster, I had to plug this initially.
Hope this helps, my leak turned out to be in the throttle body butterfly valve shaft. I was able to rebuild the washers and spring washer to seal the shaft.
It wasn't as hard as I thought it would be. I think somebody else posted this a while back, that's how I learned to do it this way.
Ken Postma '86 928 S
928 Tips Home Greg's Home