Merc 120HP overheating or not? How to tell?

[Fordiesel69]

You may have remembered my impeller topic, and with the #2 impeller now installed, it seems to be able to drive for 45 min without red lining (all the time I had to test). However the needle is right in between green and red like any minute it will overheat.

I am running 87 octane and the sticker says 93, and I can hear a very faint ping which is also makign me question is it really needing 93 or is it overheating?

The riser elbow is warm and you can keep you hand on it forever. The intake/exhaust manifold is also warm, but warm enough you can only hold your hand on it for about 15-20 sec. The block and head are too hot to hold onto.

There is a little molded rubber elbow going from the manifold to the t-stat housing that is always fairly cool like there is little flow.

How can I further diagnose if it is really overheating?

The t-stat in a pot of water will crack open at 140 degrees and be fully open at 160. Is this normal? It is a new one and it was stuck wide open when I picked up the boat which made the engine never heat up at all. I had to push it closed and now it seems to work fine.

 

[Don S]

Re: Merc 120HP overheating or not? How to tell?

Get one of those IR temp guns and check the engine.
If you can hold your hand on it forever (anyplace except right at the exhaust ports of the head), it's not overheating.

 

[Alumarine]

Re: Merc 120HP overheating or not? How to tell?

I believe you're ok on the fuel.

That octane rating of 93 is a different type of measurement than what the pumps say. Regular grade should be fine.

I'm sure someone has a proper explanation.

 

[cr2k]

Re: Merc 120HP overheating or not? How to tell?

I highly disagree on the fuel. Back in the day the octane rating method was different. What we have as 93 octane is more like mid grade from the past.

If you are hearing pinging you need to: 1. Reduce your timing 2-4 degrees. or 2. Use better gas.

Looking at pricing for say a 20 gallon tank...Usually the difference is $.10/gal between grades. so for a jump to 93 from 87 it would be a whole $4 per tank.
Now I would say that is cheep insurance that your engine is not self destructing from the inside.

Also today's gas has a very short shelf life. If you are leaving gas in the boat for longer than 3 months add a stabilizer to it.

 

[Fordiesel69]

Re: Merc 120HP overheating or not? How to tell?

Of course I could figure it out, but if I wanted to RETARD my timing, I beleive I would turn the distributor the same way as the rotor rotation, correct?

 

[Fordiesel69]

Re: Merc 120HP overheating or not? How to tell?

Well I installed a aftermarket temp guage where the old sending unit was, and also installed a new thermostat. The other one was only a week old but must have been faulty. The guage stayed at 140 and the water was very cold. I think I heard it was 53. I did not hear any ping this time either.

The riser and manifold were nice and cool. It must have been a bad t-stat causing the problem.

I will pull the dist over the winter and check the advance components, and simply go thru it. Who knows how long its been.