Water pump theory question

[wetwillie]

Wow! That’s great to know. I’m not going to be able to do anything today because of the steady rain we’re getting.
Tomorrow is supposed to be nice. I’ll get back to it then.

[melugin]

I have ’79 and ’80 25hp OMC motors which are also giving me fits cooling. Thermostat is not the problem here. Knowledgeable sources say that the upper water tube grommet may swell requiring powerhead removal to ream out restriction. Other problem could be plastic deflector also under powerhead which has deformed. Any input along these lines?
Overheats with and without good thermostat.

[fleetwin]

Melugin:
The 25hp engines of that era do not have the troublesome gearcase with the pickups too far forward. Are these salt water engines? If so, the best bet is to go through everything in the cooling system. I would definitely pull the head and look at those deflectors, these deflectors are in the cylinder cooling galleys, not under the powerhead. Pull the head cover off and replace that thermostat. And yes, remove that powerhead/ inner exhaust tube, and replace that grommet, don’t bother attempting to ream through it. The problem here is if the engine has seen salt water use, and never been properly serviced, many of these fasteners aren’t going to come out easily. So, I should have prefaced my comments by saying trying to service salt water engines of this vintage may leave you with several broken fasteners, and a corroded mess not worth repairing.
Needless to say, I would also update the water pump to the latest complete kit from BRP.

[opposedtwin]

Well, most of this discussion appears to be on the right track. But my first reaction was "is there a pin present driving the impeller"? My newest motor is a 1959 and so I’m not even sure if they still use them in 1979. So,if there is supposed to be one,you would have seen and reinstalled it already.

[dan-in-tn]

I am the one giving Melugin advice about his motors. My understanding was they were not salt motors. The plastic guide #321513 ref#114 in ME for Exhaust Hsg section is what I am referring to. If the motor has been over heated to any amount that guide will close at the top restricting water flow. The grommet #302497 should be replaced while you are there. Like Don has said many things can cause problems with this motor pumping water. Is the motor getting hot or just not indicating water out the overboard pisser? Again Don mentions that OMC overboard indicators just show the pump is pumping, not that the motor is not getting hot! You can have no indication out the pisser and still have no overheat problem.
Assuming you have an overheat problem at this point, then you have all the possibilities Don has brought up + exhaust cover inner plate being warped. That lets exhaust gases into the cooling system. It could give you the pulses you are talking about. The water deflector under the head gasket could be out of place or melted. Several other more complicated things, but we can save those till we get answers to above questions.

Dan in TN

[wetwillie]

Wow! Although I appreciate the additional info I’m certainly not very happy with what I’ve just read. Especially the possible explanation for the pulses I felt at the elbow.
To answer your question re: the engine getting hot. I allowed the head to get hot to the touch before shutting it off. Nothing ever came out the indicator, not even steam. ALL the paint on the head and block looks great. NO indication of overheating in the past.
I was unable to get close to working on the motor today. Too many other tasks with higher priority. Getting to it tomorrow looks like a possibility, at best.
FWIW, the motor has never seen salt water and looks practically new. I did find the drive key when I removed the impeller. I thought it odd that there was no hole in the driveshaft for it to go into but I recessed area instead. That would explain why the replacement I ordered includes the piece that goes in that recess. None the less, the impeller was moving with the shaft.
Please include a link to the diagram or parts list you referred to.
To be contd.

[wetwillie]

I just got back from the lake after installing a new impeller (with key) and I’m not very happy. I saw a little bit of smoke come out the indicator while the motor warmed up. I shut it off after running for a good 30 seconds. I saw ONE drop of water at the end of the indicator. That’s all.
I’m leaning towards a bad head gasket being the problem but can’t help but wonder if it could be a warped exhaust cover inner plate as suggested by Dan. Based on this theory, I figure the most logical path forward is to pull the head and then the exhaust cover and inspect the gaskets and plate before ordering any parts.
One other observation that might be noteworthy: I looked up into the exhaust cavity last night before replacing the lower unit. I was curious about the condition of the grommet the water tube slides in to. I don’t know what I was looking at on the other side of the grommet but it was light tan and appeared to be perpendicular to the opening.
All things considered, what would you (ie. anyone who has an idea) do if it were your motor?

[melugin]

I’m waiting until idle mowing season and uncomfortable boating weather to tackle my similar hot motors.

[fleetwin]

Well, I guess I would hesitate a bit before ripping that engine apart. You mention it is a fresh water motor, looks like new, and shows no signs of overheat. So, that makes me wonder just what could have happened so quickly that would cause a no pump condition. Seems like whoever owned if before wasn’t having a problem, or you surely would have seen evidence of overheating. Did you run the engine before replacing the impeller? Did you replace the impeller just for routine service, or after you discovered the engine wasn’t pumping water.
Did you use an OEM impeller, or aftermarket? Are you running the engine on a flushette, or did you back the boat into the lake?
From what you have reported, it doesn’t seem like this engine has a messed up head gasket, exhaust cover, cylinder deflectors. I guess I would remove the gearcase, then rig up some sort of hose adapter to the water tube. Gently turn the water on without the engine running to see if water flows through the engine (overboard indicator) now. If water does seem to pass through the powerhead using this garden hose adapter (very low pressure please!), then start the engine with the gearcase off and the garden hose rig supplying water to the powerhead. I would only suspect internal leakage if water stops flowing through the engine once the engine is started while using your garden hose to supply water.
If water flows through the engine nicely with the garden hose rigged up when running, then I would suspect something is messed up with your water pump. The 79 35s came with the newer style water pump with a plastic housing and SS liner, these parts will not interchange with the original style aluminum impeller housing pieces. You could always try removing the gearcase and water tube, then installing the water tube into the pump and submerging the gearcase in a bucket of water. Now spin the driveshaft with a drill to see if water comes up and out of the water tube.
Again, I would stop and recheck several simple things before ripping the powerhead/head/exhaust cover off….I can’t help but thinking we are missing something simpler here. It just seems like too much of a coincidence that this engine never overheated for the previous owner, and has developed some major internal leak/blockage after your took ownership…
Feel free to post as many pictures as you like, that might help us spot something that is getting missed/overlooked.

[wetwillie]

I have no problem with trying eliminating other possibilities before resorting to taking the head or exhaust cover off. I like your ideas A LOT and will try the garden hose to water tube adapter. I’ll run water through it, as suggested, w/o the engine running and then with it running and report back with my observations. I am curious why you stress using low pressure. Please explain.
Also, to answer your initial questions. I ran the motor before replacing the impeller and had the same problem as I have now. That is one reason I replaced the impeller (aftermarket, BRP, I think). The impeller vanes were bent but no cracks, etc. The other reason was that I felt it best to R&R it for the sake of peace of mind. I initially ran it on water muffs that were duct taped in place. It was only after reading someone’s suggestion that I immerse the lower unit in water that I tried that.
To be contd.
THANKS!!

[Author]

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