Home › Forum › Ask A Member › Mercury Mark 6 Water pump Woes
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jeff-register.
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April 17, 2015 at 11:55 pm #1244
My newly refurbished Mecury Mark 6’s water pump
wont pump a drop. Removed lower unit and put it
in a 5 gal bucket full of water. Hooked up 1/2"
drill, spun drive shaft and no water shooting out
the pump.
Tore lower unit apart and found nothing obviously wrong.
As I removed impeller, I notice that the rubber was very
loose on brass hub, and it actually popped off as I was
prying up the impeller with two little screw drivers.
Ordered a new OEM Merc impeller, but my uncle gave me
a new impeller he had (might be 30 years old, but had
no cracked, and was pliable. I installed it, did the bucket
test again, and still no water is pumping.
Any ideas? All passages on the suction and discharge side
of the pump are open.
Thanks, BuccanneerPrepare to be boarded!
April 18, 2015 at 9:16 am #14203Not really sure how you’d do a bucket test on a Mark 6 pump…The top cover would have to be on tight; and that’s held down by the upper drive shaft housing holding spring tension on the pump cover. Anyway, assuming the obvious is okay, sometimes Mercs get corrosion build up between the stainless water pump cup and the housing that holds it, warping the stainless cup. That causes a poor fit for the impeller and they won’t pump. The top wear plate can’t be warped, either. You could also make sure the top cover fits well, that its spring tension tabs are good and that there is at least some of the seal left in the top cover, so it won’t allow exhaust gas to be sucked into the pump. Make sure the cover’s water tube grommet is good and that the water tube is engaging the grommet and not warped. The impeller should move freely up and down on the shaft so that it can seek its own mid point in the pump housing. And make sure when you barrel test, that the tank water level is at least up to where the impeller is. Mark 6 pumps are not the best set up; so everything has to be just right. As stated in your other thread, there is no tell tale per se; only mist blowing out the four little holes with the exhaust relief. A properly cooling Mark 6 should allow you to hold your thumb on top of the block indefinitely.
Long live American manufacturing!
April 18, 2015 at 1:02 pm #14211Bill, thanks for your reply. When I tore apart my two Mark 6’s,
using the best parts to make one good one, I noticed that
on one unit the spring tension clip on the pump discharge cap
was missing. When putting everything back together, I used
the cap that had the clip, and installed the new seal in it.
When I tried to bolts the upper housing to the mid section
where the pump is, it would not mate together all the way,
and had about a 1/16" gap. Could not figure out why, so I ended
up removing the clip on the cap, and then the housings bolted
together tight. It appears that the cover plate would be tight
without the clip, but I’m glad you mentioned the clip, as that’s
one thing I can re-hash.
When I tested the pump in the bucket, all of the lower unit was
bolted together, and the water was over the pump area.
What’s a fast idle speed for an outboard? I wonder if my
heavy duty 1/2" drill spins fast enough for testing?
Thanks, BuccaneerPrepare to be boarded!
April 18, 2015 at 7:53 pm #14225Had the water pump apart four times today, comparing any differences
between the "parts" unit and the one resurrected, and tested in the
bucket each time. Success on the last time.
Noticed that on the mid section of the lower unit (that looks like a shoe)
where the water pump goes, there’s a 1/16" difference in the depth
of the step machined where the water pump discharge cap sits it.
This makes more room for the discharge cap with the spring tension
clips, to sandwich between the upper leg of the lower unit.
Not sure if this was a change "mid-year" as both units are 1956, or
a factory goof? The tension clip is a double layer clip. Tried adding
one layer on the good Mark 6 discharge cap, but it still wouldn’t pump,
even though it appears that the cap would be seated well on the pump housing.
Then planned down the bottom of the discharge cap a hair to get the ridges
out of it, and added the the other layer of the tension clip. For some
reason it all the lower unit pieced mated flush, and it pumped!
BTW, my old 1/2" drill turns 500 RPMS, and that was fast enough.
I added another foot height to the discharge tube with plastic tubing
to make sure it was capable of pumping higher thru the power head.
Will wait until the new OEM Merc impeller arrives before I put the lower unit
back on. There’s hope!Prepare to be boarded!
April 18, 2015 at 9:03 pm #14229Sounds good. All the shimming, measuring and planing down of things, that’s just regular Mercury Mechanics 101. Ha ha. Seriously, Mark 6s run great but they are right up there near the top for poor serviceability. They are one big reason a lot of old time mechanics hated Mercurys. Just imagine yours, only having lived in salt water. You get the idea.
Some people think the beer opener found on many Mark 6 drive shaft housings was for the owner. They were actually for the mechanics.
Long live American manufacturing!
April 18, 2015 at 9:13 pm #14230Bill, funny about the beer opener for the mechanics, but I think you’re right!
Prepare to be boarded!
April 22, 2015 at 7:37 pm #14512Bolted the lower unit back on. Took a while this time to get everything
line up. Guess I got luck last Fall. Runs good, and pumps water now.
I’m a happy camper. 🙂Prepare to be boarded!
April 22, 2015 at 8:33 pm #14514My Merc 5 had many water related problems too! Little crap that would go on & on. Good for you & me both to have EVERYTHING right before it pumped, It will now pump for a long time now!! Did you find a vane in the water line tube too?
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