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1979 omc 800 sterndrive impeller replacement question

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benjamindwight

New member
I have a 1979 mark twain with an OMC 800 sterndrive V-8. It recently started overheating on me when I'm half to full throttle. It seams to cool down when just idle. I thought I would start by checking the impeller, but can't seam to find where it is on the 79 model. (I just got this boat about 2 months ago, so I have no idea where anything is really). Maybe it's not even the impeller and I should be looking into something else. Thermostat, water pump, ect. Any advice is appriciated. If anybody has good pictures and part names listed that would be awesome too, as my boat didn't come with a manual and I haven't found one yet. Thanks in advance.
 
you should definitely change the impeller, however I doubt that's the current problem. Impellers tend to give more issue at idle.

It sounds more like your impeller is sucking exhaust. (more rpm, more exhaust pressure in the sterndrive enroute to the prop outlet)
If you grease the swivel bearing the impeller will usually stop sucking exhaust gases. (need a small tipped grease gun, fitting is through the hole above the chin of the drive, front side of exhaust housing)
give it 4 or 5 good shots at least

All the good pics and locations names etc are in the factory service manual - I wouldn't try replacing your impeller without one. Should be able to find one on eBay. Otherwise ken cook publications is where you get a new one here:
http://www.outboardbooks.com/index.cfm/model/1/308/

search the listing for your model
 
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Thanks for the quick reply. I will try to track down a service manual for sure. I'm sure E-bay will have something on there. I will get that greased and try it out to see if that helps. Thanks again.
 
Yeah, get the factory Ken Cook manual if you can afford it, Seloc manual will also give you enough information to replace the impeller/pull the drive. I have heard to avoid the Clymer manual. I've been looking for the Ken cook manual on Ebay for the '79 for a very long time and have never seen it there. So good luck with that. Best bet is to just shell out the $60 at hystat's link.

The impeller is located in the upper gear case housing. Take a look at this diagram:

convert-1.png


It is part #31. The best way to access everything is to remove the complete drive unit and then remove the upper from the drive unit to service. This is not an easy task for a first timer as the shift cable removal process is rather complex and confusing the first go-around. Once you do it the first time you realize how easy it actually is. You will need to build a drive stand. Here is mine:

2012-08-22_07-36-18_461.jpg


Here is a good write up of how to remove the drive: http://support.crowleymarine.com/entries/113061-removal-of-omc-stringer-complete-outdrive

I just cant stress how important it is to take care of that shift cable as a replacement will cost you $600. Hot soapy water in a spray bottle to lube that cable as you push/pull it through the intermediate and upper. Have a buddy help you with this process. It is possible to do it alone, but it takes a while. I did it alone my first time and it took me 6 hours but I also had to build an outdrive stand.

Also, if you do change out the impeller, go ahead and order a complete upper reseal kit. You can get them for $40 shipped from boat part headquarters and you might as well do it while youre in there.

Thermostats and water temp. sending units are cheap, around $30 shipped for both. If you're not sure as to when they were changed last or if your sender is accurate then just replace them. You can test the accuracy of your sending unit by using an IR thermometer pointed right at the sending unit to see if that temp. matches whats on your gauge.
 
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OK guys. I got the zerk greased and tried it out last weekend. It still overheated.:( I drove for prob. five minutes at 3/4 throttle and it got hot. I idled down and within 20-30 seconds it cooled right back down??????? Anything else you can think of that I could checkout? Thanks a lot. I'm sure that zerk needed greased too, so that was a good sugestion!!!!
 
benjamindwight said:
OK guys. I got the zerk greased and tried it out last weekend. It still overheated.:( I drove for prob. five minutes at 3/4 throttle and it got hot. I idled down and within 20-30 seconds it cooled right back down??????? Anything else you can think of that I could checkout? Thanks a lot. I'm sure that zerk needed greased too, so that was a good sugestion!!!!
Click to expand...

Time to pull the drive...
 
if impeller doesn't solve it, I suppose I would start with the exhaust manifolds- remove the risers and look for excess scale and debris clogging the passages.

and/or I might still replace a section of the hose between the drive and thermostat housing with clear tubing and watch for bubbles of exh. gases
 
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