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tank vs muffs

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westpoint

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I have an 1986 Evinrude 90HP crossflow that I just changed the thermostats and water pump on, but she still overheated while on muffs. I'm the original owner and only used in fresh water. I ordered exh. and cyl. head gaskets and water deflectors and intend to clean everything up. While waiting for the parts I decided to use an IR thermometer to see which cylinders were overheating, but this time she didn't overheat, and I ran her a good ten minutes. Each cyl temp ranged from 135 - 140F, cycling up and down I guess as the thermostats opened and closed. My water pressure gauge read about 4 PSI. So I figured perhaps the small bleed hole in the thermostat housing may have been clogged. I even re-routed the tell-tale hose over the engine block to help prevent air from entering the system through the tell-tale. All seemed great.


The next day before going to the ramp, I decided to do one more test on the muffs. Now she overheated again, and to my surprise, all cylinders were overheating. So then I started questioning my water pump rebuild (I used Sierra parts, next time I'll try BRP). To test pump I instead ran her in a tank with fresh water replenishing the tank as she ran. This time no overheat, every cyl. was in normal range. Water pressure cycled from 4 to 6 PSI, I guess as the thermostats opened/closed. Put the boat in the lake and ran fine all day with no overheating.


I still intend to pull the cyl heads and exh cover, clean things up and replace the water deflectors. But my question is, why is she overheating on the muffs? I've ran this engine for 27 years, which means I probably ran it on muffs at least 54 times without overheating. Could there still be something wrong with the pump? Perhaps it sucks in air on the muffs but not while submerged.
 
i keep rereading this and it sounds like you are ok...but i ran the same motor for 18 years before i got a new one in 2011 and if i had to do it over i would install a temp gauge....cheaper than a rebuild or 10k for a new one...
 
On a flushette, the water must be turned on full force. Also, flushette's have a habit of vibrating off center of the water intake.

Possible portion/piece of old broken impeller stuck in water tube... intermittent blockage?

Best to drop lower unit and recheck rather than to gamble on the overheat problem.
 
Having it submerged in a big tank or in the lake is the only way to trouble shoot an over heat problem. I never trouble shot an over heat problem on muffs.
 
Thanks guys. I do have a temp gauge, and the sender is hooked up to cyl #2. That why after it overheated again after changing the pump and the stats I wanted to get a reading on each cyl with the IR therm. But then it didn't overheat. Next day it did overheat, and I found it was all cylinders. That's why I did the tank test, where it stayed cool, as it did out on the lake. I do know the tank is the only real way to do good test. Next spring I will pull the heads and exh cover, clean every passage, replace the deflectors, and do the water pump again. I know there's no broken impeller pieces in there because I always did my own work. I guess it's like dental flossing- if you don't do it every now then things will eventually build up.

Question regarding the temp gauge- would it work if I also hooked up another sender on cyl #1 and ran the circuit in parallel? This way if either passes current, you'd know it. I'm sure it would change what is normal on the gauge, but once you know what the new normal is, you could tell if either cyl bank is overheating.

Thanks again, Jim
 
True, but with only one sender the other bank could have a problem and I'd never see it in time. I've seen some car forums talking about this, or using a dual needle gauge to see both. I figure something must work, because the overheat senders control one buzzer.
 
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