Leaks: finding and fixing them

ZF63A ZFMarine

How to find the source

• Touch up the housing with a little gray primer so you can see the fluid. On MerCruisers everything's black, which makes a leak hard to spot.
• Sprinkle a little baby powder around the transmission or engine. An old commercial fisherman gave Chris that tip. As it leaks, it leaves a nice little trail showing where it's coming from.

Where we've found leaks on a ZF

• The vent. A customer's ZF63A would dribble a little fluid out of the vent area after about an hour of running.
• The output shaft seal on ZF63IV units. Chris calls that seal, and the shaft wearing under it, the biggest problem with these units. On earlier boats with normal stuffing boxes, water sprays up on it. It gets corroded, probably from salt water, and that eats the seal surface. Then the seal leaks and the transmission fails. One ZF63IV core that came from the Great Salt Lake had no seal surface left on the output shaft at all, and the bolts were badly rusted.
• The coupling-spline O-ring on a ZF63. It hardens and gets stiff with age, and that gives you a second leak path besides the seal itself. Chris's take: it's one of the few things he doesn't love about the German engineering on these.
• The filter cover on ZF45 through ZF85 units. When you change the filter, make sure the body of the cover sits flush with the transmission housing. If it isn't flush, most times it'll suck air and neutral the transmission out. The transmission will just stop after it heats up.
• A flattened filter cover O-ring. The O-ring should be round. When it goes flat it doesn't make good contact with the housing, air gets sucked in, the pump neutrals out, and you're dead in the water.

ZF63IV 1.55 (1.6) Core, Filter Cover O Ring

ZF63A

Where we've found leaks on a Velvet Drive

• Front and rear output seals. A bad seal, a misaligned coupling wearing the shaft rough where the seal rides, or a bore that's gone rough will all leak. Fix the actual cause behind it, or it'll leak again.
• Gaskets. Loose bolts, a bad gasket, or a mating face that isn't flat. See why you never paint the coupling face for the flat-face side of this.
• O-rings swollen by additives. In old Velvet Drives, transmission additives, stop-leak or synthetic fluid swell the O-rings until they pop.
• The breather. If the breather cap doesn't move, replace it. Oil coming out of the breather is a different problem: the factory ties that to overheated fluid, the wrong oil level or water in the oil.

Losing fluid with no visible trace points to the cooler or the cooler lines, not a seal. On a 5000 or Liberty, a bad cooler can send fluid into the engine's cooling water or overboard. Track that down before you start pulling seals.

What doesn't work

You can usually tell where a customer thought the leak was by all the silicone smeared on the bottom. It never works. Don't waste your time gobbing silicone on the outside of a transmission, an engine or anything else.

Fix and prevention

Once you've traced the leak, fix the actual source. On ZF units, replace the filter and O-ring yearly, or at least every two years, and make sure the filter cover sits flush. On old Velvet Drives and Paragons, use Dexron III and skip the synthetic fluid, additives and stop-leak.

When to call

If you can't trace a leak back to its source, call (804) 333-3677, Monday through Friday 9 to 3 Eastern, or email sales@halemarine.com.

Related reading: water in your transmission fluid and filter locations by model.