Mercury 90HP 4 Stroke 2003 Impeller Change

Good morning.

I ordered an impeller kit, new housing, etc. to do a full water pump change in the lower unit. When I purchased the boat, the stream seemed a little weak, plus, I have to squeeze the muffs to get water flow.

Last Fall I changed the oil and gear lube for winterization. Now that it's Spring, I want to do impeller change. There's lots of videos that have helped out, but no one has answers to this question. Will I have to drain the lower unit gear lube before removing the lower unit? I ask, because as noted before, I have fresh lube in there already, but I also don't want it spilling out all over the driveway.

It seems to me that the gear lube would be sealed in the lower unit and not sloshing around where the shaft and water pump is. If that were the case, I'd assume there'd be bigger problems than just replacing an impeller/pump housing.

If 4's are like 2's the drive shaft comes up out of the LU and is oiled by the oil in the LU. On the bottom of the water pump lower casting is a pair (usually) of seals facing each other...one keeps the lower unit oil in the lower unit and the other keeps ambient water out of the LU oil. The water pup is connected to the water pump via the rubber impeller and the key that fits into a slot in the drive shaft right where the impeller fits on the water pup lower plate. The impeller is usually soaped up to make it slick and the top cover is pushed down on the impeller as the crankshaft is turned CW, a combination that causes the tips of the impeller blades to align in the proper direction and with the soaping, allows them to slip up under the top cover as you twist and push down.....then just bolt it down and ensure any sealing devices are back where you found them on both ends of the water tube from the water pump to the bottom of the engine block.

If the LU doesn't want to go the last 1/2 to 1" and mate up to the mid-section, the drive shaft has to be turned till the male and female splines and slots on top of the drive shaft line up and then it will go all the way in. I like to grasp the flywheel and turn the crankshaft a few degrees until it mates and slips together.

On any shifting gears or changing anything that will change any of the shift linkage including in the gear case you need to ensure that it is lined up properly when inserting. I like to use N as N is between F and R and is easier to ensure that your LU and remote shifter are on the same gear. Course if you are careful and don't mess with either while disassy and reassying then it will go back together without any guessing if you got it right.

Thank you Texasmark.

I answered my own question by cracking it open. Lower unit lubricant does not drip out and it is indeed sealed from the water pump area.

Replacing the impeller went smoothly. I put it in forward gear, removed the LU, cleaned up around everything and followed the directions from others on youtube, including adding RTV between the exhaust chamber and water pump. I put a wrench on the flywheel when I was stuck, moved the flywheel slightly to line up the splines, and it went back in. Little awkward doing it myself, but all-in-all it went pretty smooth. I took my time as well.