We’ve seen this problem several times and it’s usually misdiagnosed or not repaired correctly. If you have a BMW model 740i, 740il, 540i, 745i, or 745il with the 4.4 liter V8 (M62) engine, and you have a coolant leak from a small weep hole in the front timing cover, be very careful that it’s not misdiagnosed as a water pump or valley cover problem.
What you probably have is a front timing cover to engine block coolant leak. BMW engineers were smart enough to know their design for sealing the water passage from the front timing cover to the engine block would probably not last forever, so they made provisions for a cavity to catch the coolant leak and eject it through a weep hole at the upper right corner of the timing cover. The cavity and weep whole keep the engine coolant from leaking into the engine oil that would probably lead to engine failure if not detected in time. The repair is quite labor intensive. Most manuals seem to estimate the repair time at around thirty hours. Alldata repair information service shows the timing cover remove and replace time at 30.6 hours. There are also other labor operations that should be done at the same time as the timing cover; for example, replacing the valley block cover. The cover isn’t re-sealable and should be replaced. Part of the labor for the cover is overlapping (removing the intake manifold), but replacing the cover itself isn’t included in the front cover time.
From the factory, the timing cover has just a very light sealant on the surface where the coolant passes from the cover to the engine block. BMW now offers sealing strips to better seal this surface. The sealing strips are made of metal that’s laminated with a light coat of rubber-like sealant. We also use a small amount of sealant in the corners of the gaskets to ensure proper sealing.
Great care must be taken to ensure that the parts are thoroughly cleaned. At Autobahn Performance Inc. all technicians are instructed to clean parts with mineral spirits in a parts washer first, then all sealing surfaces are cleaned by sanding with 440 grit wet or dry sand paper in a figure eight pattern. At Autobahn we rarely use surface preparation pads or discs; they’re too aggressive (even the fine or soft rubber ones) and end up removing surface material and rounding off the edges of the surface. Also, with a perfectly ground flat sanding block, imperfections in the sealing surface can be easily seen and usually can be corrected or eliminated. Contrary to most technicians’ belief, it’s not much slower to prepare the surface by hand than it is to use a die grinder and high speed surface conditioning pads (“cookie monsters”, Scotchlock pads, or green and yellow 3M surface conditioning pads). The final cleaning step is to wash the parts in hot soapy water and then dry with a hot plate and compressed air. This step removes any contaminants or oils from the surface and allows the sealant to adhere to the surface properly.
M62 weep hole at top left of timing cover. Radiator fan throws coolant upwards and all around area, making the leak appear to come from other locations. The weep hole can be seen without any dissasembly (with small flashlight). Components removed for picture.
This picture shows the cavity tthat leads to the weep hole. Cavity is vented to atmosphere (at weep hole) and is not supposed to have any fluid in it unless the primary seals have failed.
Front covers removed. 4 timing chains visible
Autobahn Performance disassembles parts into divided bins for organization
Zymo aqueous based enzyme cleaning machine on left. Safety clean mineral spirits machine on right