Lower unit leak

gonzz · Jan 23, 2011

I have a 79 Merc 150 that has a lower unit leak. The oil is leaking near the prop and I think it's the prop shaft seals. Does the carrier have to be removed to change these? Also what other seals should be changed if I drop the lower unit. Thanks for any info.

RadarRick · Jan 23, 2011

Re: Lower unit leak

I can't help with your question about seals but, just to be sure, this is gear oil that's leaking, right? (not unburned oil from exhaust)

gonzz · Jan 23, 2011

Re: Lower unit leak

Yes gear oil. It's settling in here. Is this the prop shaft seals?

Mi duckdown · Jan 23, 2011

Re: Lower unit leak

Do a vacuum and pressure check on the lower end before tearing into it.

GPSDuckHunter · Jan 23, 2011

Re: Lower unit leak

What their trying to tell you is that it is perfectly normal for oil to go through powerhead from fuel and not be burned. This oil gets expelled through exhaust out the outer portion of the bearing carrier. Here are two things to consider. Open the bottom screw on lower unit for putting oil in. Let a little drain. Does water come out first? Is the oil milky colored? If so then you have water coming in lower unit. If not then probably exhaust oil. Also look at the color of the oil you are concerned about. Is it dark or light? If darker then better chance that it is oil from exhaust.

gonzz · Jan 23, 2011

Re: Lower unit leak

I have water in my oil when i drain it. I just serviced the oil and trimmed the engine up and in 2 days there was oil in the section i labeled above without running the engine. I am thinking prop shaft seals. I'll try and make a pressure tested this week and pressurize it and see.

Mi duckdown · Jan 23, 2011

Re: Lower unit leak

Do you have an OEM MERC manual? PSI is only like 3 PSI. You can't use a compressor. You need a hand pump/vacuum/pressure gauge. You have to drop the lower end. You could have water intrusion from water pump seal,shift shaft seals,or prop seals.???

gonzz · Jan 23, 2011

Re: Lower unit leak

I have the Merc service manual. You can pressurize it with 10psi.

Mi duckdown · Jan 23, 2011

Re: Lower unit leak

OK. I just didn't want someone without a manual to pump 50 PSI.

gonzz · Jan 23, 2011

Re: Lower unit leak

Mi duckdown said:
OK. I just didn't want someone without a manual to pump 50 PSI.

LoL.. I'm a jet eng mech. That's the last thing I want to do is cause more damage..

Mi duckdown · Jan 23, 2011

Re: Lower unit leak

Funny.
But you should drop the lower unit when you do this. while your at it, changed the impeller, which you will have to do to check that seal.
Not sure what year Merc changed drive shafts to stainless steal. But your in that year range. They got grooved at the pump seal. the old steal shafts are notorious for leaking at that point.

gonzz · Jan 24, 2011

Re: Lower unit leak

Thanks for the info. I am going to remove the lower unit today and order the seal kit and impeller.

gonzz · Jan 24, 2011

Re: Lower unit leak

Took off the lower unit and inspected the water pump area. The last guy who changed my water pump didn't install one of the gaskets (water pump base to face plate gasket) according to my manual a gasket should be on both sides of that metal face plate and only one on top is installed. Is this correct? The water pump had a few cracks but it's getting changed anyways.

RadarRick · Jan 24, 2011

Re: Lower unit leak

Yes, the diagram shows there should be 2 gaskets - above & below the plate.

Re housing cracks - interesting, my Merc's housing was cracked too. I can't figure out how that would happen. Maybe stress cracks from the vibration?
How is the plate? Watch for grooving - they will decrease the pump's output. Mine was grooved quite badly - I ran into a sandy bottom in an estuary.

gonzz · Jan 24, 2011

Re: Lower unit leak

So now I will order the lower unit seal kit and impeller. Where should I buy this from? Thanks again.

gonzz · Jan 25, 2011

Re: Lower unit leak

Orderer my lower unit seal kit and impeller this morning. I made a homemade pressure tester to check for the leak. I was checking if the carrier spanner was tight and it wasn't so I tightened up some more. (The last guy to work on it didn't tighten it all the way). So I got it nice and tight and put 10 psi on the lower unit. It held 10psi for 15 minutes with no bubbles. Did I fix this by tightening the spanner? The oil was leaking in that area prior. I am going to re-install the lower unit and service it up and see what happens. I will post the results. Thanks for all the help on this.

merc850 · Jan 25, 2011

Re: Lower unit leak

I can tell from your picture that the locking tab on the washer under the cover nut hasn't been bent down to lock the bearing carrier nut. If you can undo the BC nut it might be a good idea to undo the nut, pull out the bearing carrier (the part with the seals in it) - the manual will tell you how. Then you can check the propshaft for grooves and the gears and bearings for rust. And when reassembling put Permatex sealant on the BC O-ring and front part, this helps to stop it from corroding in place (you never know when you might have to open it up again).

gonzz · Jan 25, 2011

Re: Lower unit leak

It was bent on the top left side (can't see it in the pic). Will try and find a puller for the carrier. Thanks again

saumon · Jan 25, 2011

Re: Lower unit leak

Pressure test make sure that oil won't go out. To be sure that everything is ok, you'll also need to do the opposite i.e. a vaccum test to make sure that water doesn't get in. There's always two seal back to back and maybe only one is bad...

Here's a couple of homemade testers for both pressure/vacuum: http://forums.iboats.com/showthread.php?p=1573303#post1573303

Moody Blue · Jan 26, 2011

Re: Lower unit leak

Gonzz, that carrier nut needs to be torqued to 210 ft/Ib. That's going to take a lot of elbow grease, not just "nice and tight"

Quit possible that was the source of your oil leak.

Lower unit leak

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