09-15-2009, 07:55 AM
Officially Welcomed to the 'Fest
Location: Omaha NE
Join Date: Mar 2006
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:
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: