First Impeller Job, 1987 OMC Cobra Standard

First time R&R'ing an impeller. (OMC did make it simple on the Cobra). A 3/8" six point socket and rachet, and six screws later ,and a couple minutes, the impeller was out. See photos.

The photo with the impeller still in the housing shows most of the blades going one way, and then (3) of them pointing the other way.

Is this normal for a worn out impeller?

The OMC manual says to coat the blades with Hi-Vis Gearcase Lube and then turn the new impeller CCW while installing it. I would think that would cause the blades to all face the same direction.

The impeller close-up shows how pitted and worn the old one is.

The impeller plate looks a bit worn, but it can be turned over if the other side is still unused, according to the manual.

Re: First Impeller Job, 1987 OMC Cobra Standard

All the blades are supposed to be in the same direction.

Re: First Impeller Job, 1987 OMC Cobra Standard

Thanks for the reply.

Re: First Impeller Job, 1987 OMC Cobra Standard

I will be getting my boat from storage this week & changing the impeller will be the first job to get done. I have never changed it & it's a 1990 OMC Cobra. I wonder if it might look like yours? Anyhow did you find that the metal liner looked OK? I don't want to buy the whole kit if all I need is the impeller. What about the gasket, is it reuseable? Thanks, Brad

Re: First Impeller Job, 1987 OMC Cobra Standard

The gasket (see photo) is in pretty good shape because whoever worked on it last, thinly coated both sides with some blue RTV silicone sealer. The cover came right off after the three bolts were removed. There was RTV sealer on all the bolt threads, even the (3) cover bolts.

Since this boat is not going into the water soon (still checking it out), I'm going to recoat the gasket and put it back together with the new impeller.

I bought an impeller today, tax and all, $20.61. It was $15 cheaper than the Mercury impeller I had put in last year. I saw an impeller with housing liner, gasket, etc. for about $42 on iboats. That's not too bad.

The liner in mine looks okay. I'm going to reuse it.

I checked the OMC manual, and as long as yours is not a King Cobra, it should be the same.

With the open side of the impeller housing facing you, coat the impeller with some Hi-Vis GL-5 lower end gear lube, and rotate it into the liner turning it CW (edit: It's clockwise).

Edit: Below is a shot of the new (left) and the old (right). At least the old one was still in one piece.

Re: First Impeller Job, 1987 OMC Cobra Standard

Thanks for the reply, the information & pics are very helpfull.

Re: First Impeller Job, 1987 OMC Cobra Standard

I had earlier posted the new impeller was to be turned CCW into housing liner....WRONG. According to the OMC manual, it is to be rotated CW. (Section 11-4, Item 12)

Thanks for the correction, Four Winns.

I honestly don't know if I read it wrong or accidentally hit the 'C' twice, but that post got edited.

Below is a photo of the new impeller installed in the liner. I lightly coated the impeller with gear lube, then twisted it in CW.

The gasket is in good condition, so it got cleaned with a paper towel, slightly damped with rubbing alcohol, then dried with a clean paper towel. A thin coat of blue RTV gasket sealer was applied to the gasket and the threads of all the bolts.

The impeller was rotated CW until the small end of the drive slot was aligned with small end of the drive. The housing went right on and seated properly.

The hose was put back on the barb fitting (a little sealer applied there, also).

Housing bolts were torqued to about 8-10 foot lbs. Cover was re-installed and bolts torqued again to 8-10 ft. lbs..

I'm going to try it on the muffs after I let the sealer cure.

Re: First Impeller Job, 1987 OMC Cobra Standard

Tested the new impeller on the muffs. Noticeably more water coming out the exhaust, not a whole lot, but better. Saw no leaks coming from the housing.

Re: First Impeller Job, 1987 OMC Cobra Standard

Having run the engine on muffs, and putting it in forward and reverse and revving it up to about 2200 rpm, I decided to pull the impeller to see what it looked like.

There's something wrong. See the photo. There are some of the blades backwards! Yet it was installed in the correct direction. How can this be?

Is it possible the engine is rotating backwards when it's shut off? This engine, 2.3L has good compression: 150 on one cylinder and 155 on three cylinders.

I relubed and reinstalled the impeller, blades in correct direction. I'm going to try it again in a couple of days and then pull the impeller to see if any blades are backwards.

Outside of the engine going in reverse a few degrees, the wear plate may need to be flipped. It is glazed and maybe that is causing the blades to reverse????? The glaze is apparent in one of the photos above.

Anyone have some thoughts on this?

Re: First Impeller Job, 1987 OMC Cobra Standard

First of all, you shouldn't be revving the engine up to 2200 rpm's running on muffs. That impeller looks fine to me. If it ain't broke, don't fix it.

Re: First Impeller Job, 1987 OMC Cobra Standard

Possible that your engine rotated slightly backwards when you turned it off. That might account for the blades looking like they do. Start the motor and then have someone turn it off while you look at the front of the engine. That will tell you your answer.

Been awhile sice I had a Cobra but I don't think you need or want all that sealant on there. the O-Ring is all you need and should probably be a new one when you replace the impeller. Maybe someone else will chime in on this.

Re: First Impeller Job, 1987 OMC Cobra Standard

Thanks for the replies.

The sealant is Permatex enviro-safe RTV. It's the first time I've used it.
I'll try it without sealant next time.

So, if the engne is backing up a bit, and a few of the blades get reversed, then maybe that means when it's running, the blades are all in correct orientation(??).

I bought a locked up lower end last year (Force 150 HP) to take apart. When I removed the impeller housing, a couple of the blades were pointing the wrong way, also. Maybe this is normal for raw water impellers??

Re: First Impeller Job, 1987 OMC Cobra Standard

Hi David,
I ended up ordering the complete impeller kit except the housing, which I will reuse the original. I was told you need new gaskets, especially the o-ring when doing this job. It's recommended to use Permatex Avaition Gasket Sealer on all the gaskets, including the o-ring. I wonder if your impeller is sucking air thru the housing to cause your problem? I don't know, but the sealer you used is the wrong type, and it dosen't look sealed all the way around anyhow. I bought the Permatex Avaition from Mcmaster Carr for $5+ shipping. My impeller kit should come in the mail tomorrow, and i'll install it over the weekend and try it on muffs and report the results. Regards, Brad

Re: First Impeller Job, 1987 OMC Cobra Standard

Aviation Gasket Sealer is really sticky messy stuff to put on and take off. It does work, but was intended for high temperature applications, which an impeller housing is not. It will work, though, but it's a mess to get off anything.

I doubt if my housing is sucking air. If it did that, it would also be leaking water, which it is not. The blue sealer looks uneven because the missing part stayed with the plate.

I'm looking forward to your feedback on your installation.

Re: First Impeller Job, 1987 OMC Cobra Standard

David, I just looked again at your last pic of the impeller, and it looks to me that the o-ring was pinched under the liner when you put it together. I have read it can be a little difficult to keep the o-ring in place when reinstalling the housing. They say to apply the Permatex Avaition to the o-ring and wait a few minutes, it gets tacky, but never dries, then install the housing. Regards, Brad

Re: First Impeller Job, 1987 OMC Cobra Standard

I got my impeller kit in the mail yesterday & installed it last night. I ordered the kit with everything but the housing, but when I opened the box it also had the new housing with it. I bought this kit on e-bay from Masonmarinenet for $29 shipped to my mailbox. It is the GLM brand. David, I see what you mean about the avaition sealer being a sticky mess to work with, but if you apply it & wait 10 minutes, it gets tacky & is easy to work with at that point. It all went together well with no leaks while running on the muffs. The housing was cold to the touch after running for 5 minutes, & I could feel it turning inside. I am confident it's working ok, so I'm not going to remove the housing because I don't want to break the o-ring seal. I will watch the temp gauge closely the first time out. I honestly don't know why your impeller looked that way, but both times you took off the housing it looked very much the same. If I were you, I just might try it in the water & watch the gauge. Maybe the water pressure from the muffs have something to do with some of the impeller fins turning the wrong way. Also when I first removed the housing, it only had sealer on the o-ring, not the rest of the base. So when I put it back together, I used a tiny brush, like from a kids paint set, & brushed the sealer in the o-ring groove & wait a few minutes, then on the o-ring. I didn't want to get any sealer on the impeller or liner. Good luck with yours, Brad

Re: First Impeller Job, 1987 OMC Cobra Standard

When I install items like this with an o-ring that wants to fall out, I just use some silicone grease to hold the o-ring in place. Then you don't have to worry about that silly sealer and the mess it makes.

Re: First Impeller Job, 1987 OMC Cobra Standard

Have someone bump the engine so that the narrow end of the water pump shaft is facing downward. If u do that, u should be able to push the impeller housing on without twisting it into place. Therefore, the seal won't move. But, I would still use Permatex on the seal. And make sure that the vent hole on the impeller housing isn't clogged.

Re: First Impeller Job, 1987 OMC Cobra Standard

I lined up the impeller slot with the pump shaft and slipped it right on. The housing was not rotated at all. The housing bolts went straight in. It's critical that the housing is not rotated when installing it.

Good point about the vent hole. Haven't checked that yet. I'm going to be running it this weekend on the muffs (boat is not seaworthy due to rotted transom). I'll check that. I'm also going to watch to see if the engine backs up a few degrees when it's shut off.

Re: First Impeller Job, 1987 OMC Cobra Standard

I'm curious if it really matters if you're seeing some blades backwards? Won't they orient properly when spinning naturally regardless once they get to their more fully extended state on the cycle? I would think the pressure of the water plus the friction of the sidewall plus the flexibility of the fins would all combine to make them naturally bend back trailing the direction of the shaft spin. Please correct me if I'm under the wrong assumption.