1988 Mercury 90 horsepower water pump replacement

[Schnidly]

Dear Forum,

Can someone give me specific directions for replacing my water pump and related gaskets and seals on my 1988 Merc. 90hp motor? The serial number is: OB302485 if this helps. I am not sure if I should have the motor in forward or reverse or neutral when removing the lower unit containing the water pump? Please advise and what about sliding the lower unit back up into the motor afterwards, any special instructions?

THANKS SO MUCH!

Rick S.

 

[Texasmark]

Re: 1988 Mercury 90 horsepower water pump replacement

Different people will tell you different things, as do some manuals. The reason for F gear is so that you can use the prop to move the drive shaft slightly so that it's spline and the spline on the bottom of the crank can line up and mate....the last 1" in getting the LU to seat with the mid section.

The shift lever on the engine or remote has to be in F gear also. You can tell if the LU is in F by the prop...it ratchets CW and locks up with the driveshaft rotating the prop CCW.

Other thing is ensure the water tube fits in it's hole at the bottom of the powerhead...little hard to see up there but it is a must for the cooling to work, or some just stick it up there initially and guide it into the water pump outlet hole.

Kits will usually be for numerous models so identify what you take out and only put back what you take out. Position of impeller blades supposedly doesn't matter, but I always put some soapy water on them and turned the prop CCW in F gear while installing the cover over the impeller.....and don't forget to put the key in the driveshaft before you slide the impeller down or it won't be locked to the shaft and obviously won't work.

Mark

 

[Schnidly]

Re: 1988 Mercury 90 horsepower water pump replacement

Thanks so much Mark! From what you said I need to have the shifter on the control forward. Should I do this before I unbolt the lower unit and remove it or should I wait until I have the lower unit removed to shift the forward control? I am trying to teach myself to do more of my own repairs instead of always having to rely upon a shop or my neighbor who has no clue about boating repairs, haha. Any more advice is greatly appreciated Mark or anyone on here.

Thanks again,

Rick

 

[Sunfish12]

Re: 1988 Mercury 90 horsepower water pump replacement

HI, I have the same engine. Put it in forward before you drop the lower unit. It's fairly easy as long as the lower unit seperates easily. When putting it back together I used a small pair of needle nose vise-grips on the shift shaft where the lower unit meets the mid section to line the splines back up. As soon as they're lined up remove the visegrip and push the lower unit the rest of the way up.

 

[Chris1956]

Re: 1988 Mercury 90 horsepower water pump replacement

The other posters are recommending a method that has some issues. The gearcase can easily get out of sync with control. My way is much better.

Set the control to neutral. Remove the wrap cowl and top cowl. Trim the motor all the way up and lock it up. Remove the prop to maker the gearcase lighter. Remove the bolt holding the zinc anode. Remove the balance of the nuts, 1 in front, two on top of AV plate, two under AV plate and hidden bolt under AV plate. Now remove the water pump cover and inspect the cover insert for scratches. If you find some, replace the cover. Clean the driveshaft with emery if necessary. Replace stainless steel wear plate, and both gaskets. Install new impeller and key. Lubricate impeller and pump cover insert. Place cover over impeller and turn driveshaft CW to seat cover. Install bolts. Install slinger and water pipe extension. Now grease drive and shift shaft splines. Lift gearcase and shove it into midsection, placing copper water tube into white plastic extension. Turn flywheel by hand to mesh driveshaft and crank splines. Wiggle gearcase using nuts and fingers only to seat gearcase. Use needlenose plier to mesh shift shafts. Tighten nuts and bolt securely.

 

[Texasmark]

Re: 1988 Mercury 90 horsepower water pump replacement

The other posters are recommending a method that has some issues. The gearcase can easily get out of sync with control. My way is much better.

Set the control to neutral. Remove the wrap cowl and top cowl. Trim the motor all the way up and lock it up. Remove the prop to maker the gearcase lighter. Remove the bolt holding the zinc anode. Remove the balance of the nuts, 1 in front, two on top of AV plate, two under AV plate and hidden bolt under AV plate. Now remove the water pump cover and inspect the cover insert for scratches. If you find some, replace the cover. Clean the driveshaft with emery if necessary. Replace stainless steel wear plate, and both gaskets. Install new impeller and key. Lubricate impeller and pump cover insert. Place cover over impeller and turn driveshaft CW to seat cover. Install bolts. Install slinger and water pipe extension. Now grease drive and shift shaft splines. Lift gearcase and shove it into midsection, placing copper water tube into white plastic extension. Turn flywheel by hand to mesh driveshaft and crank splines. Wiggle gearcase using nuts and fingers only to seat gearcase. Use needlenose plier to mesh shift shafts. Tighten nuts and bolt securely.

Couple of variants here Chris for me. Having attempted both N and F, the F provides me access to the prop on the larger engines and is much more convenient for me than turning the flywheel in a N situation, but both have worked for me.

On the grease, I grease the shafts also, but we are cautioned on here and in the SM's that too much grease can get lodged up between the crank and drive shaft and prevent you from completing the assy. So in due respect to the advise of others and my own desires, grease the shafts then wipe the grease off just leaving a thin film.

We have the same differences on installing the flywheels (non-related maintenance action)....I grease it and everyone else doesn't....but mine comes off easily and the grease is a very thin layer.

HTH,

Mark

 

[Chris1956]

Re: 1988 Mercury 90 horsepower water pump replacement

Mark, I am just an average guy, and I can reach the flywheel with my left hand, while pushing the gearcase into position with my right hand. I always remove the prop to lighten the weight of the gearcase. On the mercruisers pushing the prop CCW holds the shifter in the proper position for installation, so yo must have the prop installed. The Merc OB gearcases (at least on the '88 and older motors) don't have that feature, and frequently the shift shaft can get out of synch a variable amount. That is why I use neutral.

I have never had an issue installing the driveshaft spline after greasing it. Ditto on the shift shaft spline. I usually grease the entire driveshaft as I run in saltwater.

 

[Texasmark]

Re: 1988 Mercury 90 horsepower water pump replacement

I usually grease the entire driveshaft as I run in saltwater.

Not hijacking the thread but back before I was a Merc fan a friend was and we were down in Biloxi, MS. on the Gulf Coast at the Merc shop where they were putting in a new impeller in his 90hp tower. OMC used SS shafts at the time and Merc apparently carbon steel and I couldn't believe the rust and scale on that shaft. Glad Merc finally experienced the error of their ways and went to SS like everyone else.

Mark

 

[Chris1956]

Re: 1988 Mercury 90 horsepower water pump replacement

My '77 Merc 1500 had a Stainless steel driveshaft. I am not sure how much earlier the SS went.

My '58 Johnny had a ss driveshaft. Of course that motor had all ss bolts, alum block and brass fittings. No external steel components at all.

 

[Texasmark]

Re: 1988 Mercury 90 horsepower water pump replacement

My '77 Merc 1500 had a Stainless steel driveshaft. I am not sure how much earlier the SS went.

My '58 Johnny had a ss driveshaft. Of course that motor had all ss bolts, alum block and brass fittings. No external steel components at all.

Time line was '65. Couldn't believe my eyes.

 

[Schnidly]

Re: 1988 Mercury 90 horsepower water pump replacement

Dear forum, I was talking with a friend of mine who last installed my water pump and he mentioned that he leaves the control shifter in NEUTRAL and says just be careful NOT to move the shifter in the lower unit which he said is easy to avoid moving since it is away from the main long drive shaft. He said the only shaft you will need to move is the long drive shaft when pushing the water pump impeller cover down over the impeller itself. He said to move the drive shaft CW and push the cover down. Does this sound right? I mean to just be careful NOT to move the shifter shaft on the lower unit and to ONLY rotate the long drive shaft.

 

[Texasmark]

Re: 1988 Mercury 90 horsepower water pump replacement

Dear forum, I was talking with a friend of mine who last installed my water pump and he mentioned that he leaves the control shifter in NEUTRAL and says just be careful NOT to move the shifter in the lower unit which he said is easy to avoid moving since it is away from the main long drive shaft. He said the only shaft you will need to move is the long drive shaft when pushing the water pump impeller cover down over the impeller itself. He said to move the drive shaft CW and push the cover down. Does this sound right? I mean to just be careful NOT to move the shifter shaft on the lower unit and to ONLY rotate the long drive shaft.

Not being arguementative: The shift shaft is relatively easy to connect to the LU splined shift shaft..right out there in front and a pair of pliers can accomplish the mate. Only requirement there is to ensure that the shifting mechanism (remote shift lever) and the lower unit are in the same gear. Otherwise when you get her back together, shifting just won't work like you would expect it to....like you select F and it is in N sort of thing.

On moving the drive shaft or not makes no difference how you do it. The name of the game is to get the two splines aligned. You either do it via the prop in F or via the flywheel in N. As soon as you get it right the last 1" of separation between the mid section and the LU will close and you can bolt her up.....but remember the water line and ensure it is connected on both ends.

That's it.

Mark

 

[Schnidly]

Re: 1988 Mercury 90 horsepower water pump replacement

Thank you Mark, thats what a slow old guy like me needed to hear, when in Neutral use the flywheel and when in Forward use the prop. I hope by some sort of Divine Guidance when I do mate the lower unit with the upper half they just fit like a glove and I dont have to move any shaft!

 

[Texasmark]

Re: 1988 Mercury 90 horsepower water pump replacement

I hope by some sort of Divine Guidance when I do mate the lower unit with the upper half they just fit like a glove and I dont have to move any shaft!

That's not going to happen. Ha! Murphy's Law attests to that.

It's not that big of a deal. I think there are 6 splines on the shaft so you won't have to turn more than an 1/8th of a turn (more or less) to get them aligned.

The process is: With slight pressure on the lower unit (however you manage that) engage the shifter first (long nose pliers on the shaft in the mid section works) ensuring that the Remote control is in the same gear as the lower unit, then moving closer up to mating, the water tube, then at the last inch, rotate what you have to slowly with the slight pressure mentioned and when it's right it will jump into engagement. Put in the hardware and you are good to go.

And don't forget to put your trim tab back where it was when you disassembled it. If you don't your steering will be a problem till you figure out where you had it...boat will pull to port or starboard when you release the steering wheel. Whichever way the bow (front) of the boat wants to go, turn the rear of the tab in that direction however much it takes till you get a centerline tiller with your hand off the wheel.

Mark