Bimmerfest - BMW Forums - View Single Post - Found this Ingenious VACUUM LEAK Testing Trick
View Single Post
Old 09-15-2009, 06:55 AM
cn90 cn90 is offline
Officially Welcomed to the 'Fest
Location: Omaha NE
Join Date: Mar 2006
Posts: 14,364
Mein Auto: 1998 528i 5-sp 140K+
Found this Ingenious VACUUM LEAK Testing Trick


I came across this thread for a 1998 740i but you can virtually use this technique for any car:
One thing I'd do different is:
- Block the Inlet to Air Mass Meter with a Plastic Lid and Duct tape.
- Use the Brake Booster Hose Inlet as sucking point
NOTE that you need:
1. Stethoscope
2. Leave the vacuum machine outdoor to keep noise out of the garage.

Here is a repost of this thread:
Please don't laugh, but here's a technique that I found to be quite effective to diagnose the dreaded "fuel trim" errors in a 1998 740i (pre-9/98). First, many thanks to "Phil in Philadelphia" for his suggestions. I considered a few other possibilities, but it seems the most common cause of the fuel trim errors is an intake manifold vacuum leak, after the MAF.

I took out my stethoscope with the engine idling, but even at idle there are too many legitimate noises in the engine compartment to hear a leak.

I tried spraying starter fluid, jetting propane, and spraying water at suspected leak locations. None of these affect the idle, I found, enough to be useful. Other articles say use propane and monitor the O2 sensors, but that seemed like a pain. So I had an inspiration:

I parked my car in front of the garage and hooked a garden hose to my vacuum cleaner inside the garage:

Then I made an adapter for the other end of the hose to go into the MAF. This was from a new MAF cover but one could use anything with some gray tape:

Then I closed the garage door to muffle the sound of the vacuum and took out the stethoscope and listened in the usual places, on a quiet afternoon, engine off!! If one of the cylinders is near TDC with the intake and exhaust valves open you may have to graytape the exhaust pipes to get a better vacuum, but I didn't need to. I first checked the vacuum by removing one of the black plastic tubes at the top of the OCV -- clear vacuum just like with the engine running -- loud hissing in the steth. Then I replaced that and I checked all around the throttle assembly, the rear OCV area, even down by the intake manifold gaskets. Found a HUGE hissing sound from underneath the throttle body-manifold gasket. Very clear leak.

I got a new gasket and six new torx bolts and removed the throttle body. Believe the earlier posts and my earlier cussing that those ten-year old torx will strip! My technique is to remove everything to get some room, then clean the torx heads three times with carb cleaner and a toothbrush (need a mirror to do that for the one in the middle underneath), then with a good T30 on a 3/8 ratchet, push with all your might and crack them loose.

Here's what I saw. The gasket on the left bottom clearly lost its seal:

I cleaned everything, put in the new gasket, and reset the light. Fuel trim looks good and no light!!



Having said that, if you have more than 100K miles, it is a good idea to refresh all rubber parts in the Air Intake System (Intake Manifold Seal, all gaskets at Throttle, Vacuum Elbows, Vacuum Hoses etc.). Info is here for 1998 528i:
Attached Thumbnails
Click image for larger version

Name:	vacuumHookup.jpg
Views:	75187
Size:	120.7 KB
ID:	201530   Click image for larger version

Name:	hoseIntoIntake.jpg
Views:	107250
Size:	144.7 KB
ID:	201531   Click image for larger version

Name:	intakeManifoldLeak.jpg
Views:	72768
Size:	138.4 KB
ID:	201532  
Reply With Quote