1978 mercruiser 160 overheating

tony dz

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Jun 5, 2006
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My 1978 mercuriser 160 inline six overheats when I run less than 1500 rpms (about 5mph), but runs fine above 1500 rpms. I've already replaced water pump/impeller and thermostat.

I'm new to this boat.. is this normal or is there somthing else I should be checking?
 

Bt Doctur

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Aug 29, 2004
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Re: 1978 mercruiser 160 overheating

clogged manifold or riser.
 

Boatist

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Apr 22, 2002
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Re: 1978 mercruiser 160 overheating

I do not think it is a clogged manifold or riser. When they are clogged usually overheats at speed and is cooler at low speed.

Pull the hose coming outdrive the outdrive to the engine and see if it has good water flow. You could have a water leak at the impeller on on the way to the engine.
 
Joined
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Re: 1978 mercruiser 160 overheating

....Sounds like the water pump impeller is shot....the one in the engine....they rust out and wont flow at low RPM.
 

tony dz

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Re: 1978 mercruiser 160 overheating

I haven't been able to follow-up on this yet.. any other suggestions on what it could be or where I should start?
 

Don S

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Re: 1978 mercruiser 160 overheating

Every piece in the cooling system is suspect in an overheating situation. Either plugged with something or sucking air if o the intake side.
What you have to do is start eliminating things. Like the obvious problems first. Raw water pump impeller, and risers/manifolds plugging up. (99% of most overheats)
Along with the cooling system, you could have an engine siezing up, or even a drive siezing up causing it to overheat.
It's been a month, start doing some checking. There are also detailed test procedures in the OEM manual.
 

tony dz

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Re: 1978 mercruiser 160 overheating

Thanks Don... Can you explain how to check the risers/manifolds (or point me to a link if it's online)? I do not have a manual yet (yes I need one), but would like to check some things this weekend if possible.

Also, does the fact that it only happens at low RPM's narrow it down at all?
 

Bondo

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Re: 1978 mercruiser 160 overheating

tony said:
Also, does the fact that it only happens at low RPM's narrow it down at all?

Ayuh,............... It Narrows it down to the Cooling System.............

Normally Seizure of the Engine, or Drive starts to Really Bother at High RPMs....................................

When you Replaced the Waterpump Impeller,..??....
Did you Find ALL of the little broken pieces from the Old 1,..????
 

tony dz

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Jun 5, 2006
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Re: 1978 mercruiser 160 overheating

actually the old one was still in good shape... there wasn't any sign of any broken/missing pieces pieces... I was surprised how good it looked, but figured it was still worthwhile replacing
 

Bt Doctur

Supreme Mariner
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Aug 29, 2004
Messages
19,093
Re: 1978 mercruiser 160 overheating

Water pump base gasket/ hose between gimbal and transom cracked/split/collapsed internally/kinked/water pocket cover/copper coupling tube.
 
D

DJ

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Re: 1978 mercruiser 160 overheating

Someone mentioned the circulation pump on the engine. That advice was ignored.
 
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