[Fr. Frank]

Merc V6 thermostats begin to open at 143 degrees, but the telltale stream is mixed with water exiting the exhaust housing as well as from the thermostats. Depending on how much of each is in the telltale, it can get pretty hot. My telltale nearly always steams, even in summer, but my cylinder heads are both still sitting at about 170-180 degrees on an infrared thermometer. If I run WOT for a long time, they will creep up to about 185-190 degrees. The DNE temp on 87-98 Merc 2 stroke V6's is 225 degrees according to the mechanic who built mine. Later models have a DNE temp of 205 degrees, especially the Optimax.

I actually ran aftermarket 165 degree thermostats for a while. I found I had more power, but less economy, and shorter plug life.

I currently run stock thermostats, which are supposed to begin opening at 143 degrees and be fully open at 145. However, both mine actually begin to open at about 125, and are fully open by 135 degrees.

To maintain good water flow, put in a new impeller every 12 months or 100 hours of use, whichever comes first. I change my impeller every January, and use a whole water pump rebuild kit at least every 3 years.