oscillating water temp on 454 mag
 

I replaced my raw water impeller and housing last week. Not sure if I'm over reacting, but when I idle the boat, the temp runs at about 160 or so, then when I'm running at 3500 rpms are more the temp seems to oscillate between 170 and 190. It seems to climb to almost 190 then drop down to 170 and repeat. Does this seem to be ok? Should the temp just remain consistant? Any ideas? When I replaced the impeller I was able to find all the old pieces and was even able to rebuild each vein to make sure I found them all. Any help is appreciated. Motor is 454 mag efi 1998.

Sounds like it may be time for a new thermostat. I've had them do wacky things like that in cars before.


Darrell.

mine does the exact same thing, i put a new thermostat in it and it still acts the same. Im gonna order a colder one and see if that helps.

thermostat
 

I had this same problem and come to find out that there is a difference in the thermostat. If you or anyone before you ever changed the thermostat need to make sure the thermostat was for an open or closed cooling system. If you put a regular thermostat from the auto parts store and the system is an open cooling system that will happen. only the marina has the correct thermostat for an open cooling system and the ones from the marina are stainless steel so they wont rust. Mine is also a 1998 502mag and I run a 140 degree and it stays right around 155 running hard and little lower at idle. hope this helps. Just my 2cents

Well i feeel like a jackass, i figured there was a difference in thermostats, but i needed one asap so i got an automotive one. Guess i will order a marine thermostat.
Thanks Swan2