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Re: 1GM10 cooling problem.... again!
Sounds like carbon/scale build-up in the exhaust elbow. Drain the cooling system. Then take the hose off of the "out" side of the pump and blow into it. If there is more than slightest resistance the mixing/exhaust elbow is probably clogged right where the cooling water enters (usually via a much smaller elbow fitting). The same thing happened to my 2gm20 a while back. I changed it since it also had about 10 or 15 years of saltwater corrosion. Since your boat is in fresh water, you may be able to just clean it out.
It could also be a stray bit of the old impeller. If so it's probably stuck in the smaller elbow fitting. Either way you'll have to take off the mixing elbow and clean it out, or just replace it.
Never forget them. Do something to prevent it from happening again.
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Last edited by SlowButSteady; 06-25-2013 at 06:01 PM.