Mysterious cooling problem on 9.8 mercury

I am in dire need of a real mercury expert. I have a 1972 Merc 9.8. I have gone through the whole lower unit and water pump, replaced impeller, blew all tubes out, took water jacket cover plate off and looked for debris, looked at water pump housing (it looked fine, no grooves or anything). It pumps o.k. at dead slow idle but idling up at all it shows less water coming out the telltale and at fast idle the flow is kind of an intermittent or pulsating one that seems like it's constricted. I took out the flush screw and it pumped more water out at dead slow idle than at fast idle. Going down the lake wide open the water coming out of the telltale is insufficient and hot, not warm, but hot. After a 10 minute ride at wide open, the cylinders were pretty darned hot--not overheated but enough that I'm certain if I ran it a long time I would probably burn it up. I am baffled by this and don't know what else to check. I blew a lot of air through the power head and just don't believe there is any obstructionin any of the passages. I also don't understand the good flow on idle and less flow but a harder flow on faster throttle. The power head runs and looks great and the cylinders and pistons and stuff looked as clean as a new motor (absolutely no carbon build up), just as good as the outside of the block which looks brand new also. It appears to be a very low hours engine. I'm sure this baffling problem along with the water in the gear oil is why the original owner sold me this motor! My dad said years ago he had a 20hp merc with a similar cooling (problem and it turned out to be the lower unit housing even though the dealer mechanic (a good dealer mechanic) couldn't even see anything wrong with the old housing. Any ideas? I hate to be so lengthy but this is my first post on the forum and I wanted to be specific in my description of the problem. The lower unit also leaks water into the gear oil although I replaced the prop shaft seal and o rings on the bearing carrier. Please help---- jbjennings:

Re: Mysterious cooling problem on 9.8 mercury

your gear lube is most likely getting water in from the pump base seals. this is more common than water from the prop seals. mine had a poor but not real bad tell tale flow, which did smarten up with the seals done. how does your water tube look? nothing that looks like its rotting? how were the exhaust lower leg gaskets?

Re: Mysterious cooling problem on 9.8 mercury

The tube looked fine, the whole motor actually looks excellent and the side cover bolts came off incredibly cleanly and easily for such an old motor. I don't know what the exhaust gaskets look like to tell you the truth or where they are.
Thanks for the help.
Jbjennings

Re: Mysterious cooling problem on 9.8 mercury

jbjennings said:

I am in dire need of a real mercury expert. I have a 1972 Merc 9.8. I have gone through the whole lower unit and water pump, replaced impeller, blew all tubes out, took water jacket cover plate off and looked for debris, looked at water pump housing (it looked fine, no grooves or anything). It pumps o.k. at dead slow idle but idling up at all it shows less water coming out the telltale and at fast idle the flow is kind of an intermittent or pulsating one that seems like it's constricted. I took out the flush screw and it pumped more water out at dead slow idle than at fast idle. Going down the lake wide open the water coming out of the telltale is insufficient and hot, not warm, but hot. After a 10 minute ride at wide open, the cylinders were pretty darned hot--not overheated but enough that I'm certain if I ran it a long time I would probably burn it up. I am baffled by this and don't know what else to check. I blew a lot of air through the power head and just don't believe there is any obstructionin any of the passages. I also don't understand the good flow on idle and less flow but a harder flow on faster throttle. The power head runs and looks great and the cylinders and pistons and stuff looked as clean as a new motor (absolutely no carbon build up), just as good as the outside of the block which looks brand new also. It appears to be a very low hours engine. I'm sure this baffling problem along with the water in the gear oil is why the original owner sold me this motor! My dad said years ago he had a 20hp merc with a similar cooling (problem and it turned out to be the lower unit housing even though the dealer mechanic (a good dealer mechanic) couldn't even see anything wrong with the old housing. Any ideas? I hate to be so lengthy but this is my first post on the forum and I wanted to be specific in my description of the problem. The lower unit also leaks water into the gear oil although I replaced the prop shaft seal and o rings on the bearing carrier. Please help---- jbjennings:

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Hi,
I had a similar problem on a 60hp and I replaced the impeller and same thing. I finaly replaced the whole pump (65.00) and that solved the problem. The new pump of course had new o-ring and inner seal and maybe there was a leaky gasket in the old houseing, I don't know, but it fixed it and so far there is no water in the gearcase.............Jerry

Re: Mysterious cooling problem on 9.8 mercury

Did you seal the pickup tube with sealer 1000 or simular product. When they leak it is simular to your problem, as the exhaust pressure builds up it from the increased RPM it forces the water level down inside the leg and they begin to suck exhaust in around the tube ends

Re: Mysterious cooling problem on 9.8 mercury

Thanks for the advice, men. i'm going to try that in just a minute (sealing the tube) and see if it helps. if that doesn't, i'm thinking of ordering a whole new pump housing with new lower housing seals, for sure the lower housing seals. it sure is helpful to have access to all of your knowledge! i do have one question if anyone can answer just to clear this up for me. my mercury seems to dump all of its exhaust out 2 ports on the middle section just below the back of the power head. how then, does it put exhaust pressure inside the lower leg. is this why diaric suggested the lower leg exhaust seal may be burnt-- it's leaking exhaust into the lower leg where it shouldn't? also, how do i get to that exhaust seal? also, i did notice that my tube just slid into the housing fiberglass looking receiving tube on my pump--i didn't use any sealer around it-- is this the nono you're talking about? once again--i appreciate your help and will pass on your info to a couple of others who are having similar problems as I am.
thanks again,
jb

Re: Mysterious cooling problem on 9.8 mercury

There should be a short piece of hose (1 3/4) That slides down the water tube and into the pump outlet,(around the water tube) that seals the tube in the pump. That's how the 60hp is anyway. The exaust seal should be in your lower unit just to the prop side of your water pump. (rectangluar piece of rubber that fits down in the rectangular hole) about 1 3/4 x 3" long. when the lower unit is bolted up it squeezes into this rubber and seals. There is where the exaust should go out through the prop. The holes up by the power head is a vent for any exaust that escapes in the lower leg.

Re: Mysterious cooling problem on 9.8 mercury

your reply was EXTREMELY helpful. there must be some issue with that exhaust seal because the two vent holes when i first got the motor let out a lot more exhaust than i thought it should and also it doesn't bubble up exhaust from the water like i thought. just another quick question before i go out and try to tackle it. how does the exhaust get to the prop? why doesn't it take the path of least resistance and go out the 2 upper vents like it's doing right now rather than going through the prop through the water... as you can see, i'm not an outboard mechanic but am capable of learning. also, i couldn't find that seal on the tube and will try to locate it or make one-- i bet it is the problem too.
thanks thanks thanks,
jb

Re: Mysterious cooling problem on 9.8 mercury

UPDATE:

i dropped LU again, checked tube where it fits into water pump---grommet inside pump outlet-- it was fine and sealed good. sunny out today--checked tube for cracks or splits--it was fine also. lower water pump seal definitely bad judging by the milky stuff below it on bearing. i did notice that the lower aluminum pump insert where the impeller sits had a tiny ding in it at the edge of the shaft opening-- it appeared to be where the impeller key had dinged it by someone punching down on it and there is a very tiny opening i believe near the shaft. could this be a problem? i do see where the exhaust goes down around the prop . the pickup tube grommet on the pump housing looks good also-nice and tight--is this the exhaust seal you're talking about? anyway, i'm to the point of puchasing a new housing and lower seal. this thing really has me baffled. i'm starting to like my johnsons more and more-- i can fix them in a jiffy if they screw up which is rare. i may have to rebel and chalk a mercury up as a pain in the butt. i am however intent on fixing this darn thing as it has become a challenge that i am determined to meet
thanks again,
jb:redface::redface:

Re: Mysterious cooling problem on 9.8 mercury

How about it Bob? Does this motor have an exaust tube coming down to the lower unit? I don't have any experience with this motor. I had a 7.5 years ago but only put an impeller in it. Had a 150 xr4, never had a problem with it. Rebuilt this 60hp and learned alot about it from this forum. Does the 9 use the lower leg for the exaust?. Curious.................Jerry

Re: Mysterious cooling problem on 9.8 mercury

This motor does not have an exhaust tube. The exhaust manifold dumps into a plenum on the plate which the powerhead bolts to; this leads directly to the middle of the exhaust tower (center section aka "leg"). Only some of the exhaust comes out the relief ports at the back of the motor, below the cowling. The majority of exhaust goes right down the middle and exits via the prop. When you're sitting still in the water, backpressure will force more exhaust (and water along with it) into the exhaust relief passages and out the ports at the back of the motor.

In one post you spoke of "side cover bolts" which I would take as being the exhaust manifold cover bolts on the port side of the engine; in another post you referred to the "water jacket cover plate" which I would construe to be the back cover over the spark plugs (the cylinder block cover plate).

So, did you remove both or just one. It makes a diff where you're looking for debris. I've seen blocks so silted up on the inside that water wouldn't circulate and you couldn't tell unless you took the cover off the back.

Another problem these motors can have is a squeezed-shut (or almost squeezed-shut) upper water tube grommet. The powerhead must be pulled to replace this.

Since you say that you do get a fair amount of water flowing at idle I wouldn't be so inclined to think the upper grommet is bad, but it's something to consider.

You might try (with the lower unit removed) runing water up the copper water supply tube. This'll tell you a lot about water circulation thru the motor.

If you're getting excellent flow thru the powerhead I doubt the problem would be in the upper end.

It also sounds like your water pump cartridge is damaged; I'd renew it and also the pump plate which goes over the top of the cartridge. If the pump plate or the bottom of the cartridge are scored or have a lot of wear, you won't get good pumping action.

Another thing you might try is to immerse the lower unit in a bucket of water and spin the driveshaft clockwise with a drill motor. If you see just a dribble coming out of the pump discharge, you know where the problem resides. Conversely, if there's a decent amount of flow coming out, and it'll spray all over if you partially block the discharge with a finger, the pump's probably OK.

Anyway, just a few things to try and look at, but I wouldn't be replacing the lower unit just yet!

BTW if you do end up replacing it for some reason such as rusted driveshaft, get a later-model unit with stainless-steel driveshaft and you'll never have that problem again.

If I recall Merc started using stainless in these around '76 or so.

HTH & G'luck..........ed

Re: Mysterious cooling problem on 9.8 mercury

Ed,
thanks for the detailed instructions, i haven't pulled the plate around the spark plugs yet-- am going to do that right this minute, will report the result..also, i am going to order a whole new water pump housing and water pump base gasket.
will post results--thanks again