Mercruiser 5.7 260 EFI - Water Temp Fluctuations

[sarnone]

Hello, I have twin 1998 5.7 260 EFIs 0L067794/0L067XXX in my Formula with raw water cooling. I would like to ask your opinion as to why I have temperature fluctuations while underway at certain points. I would like to know if it is an indication of any potential problems.
When I am under load at constant rpm, my gauges read between 175-180. When I come down off plane, they will tick up possibly 5 degrees and then right back down to "normal" 175-180, and for this reason I always come off plane slower instead of whacking it back to idle. I do not know specifically what the thermostat temperature is rated at. Also, I always have one motor that warms up before the other one.
1. Are these symptoms of a problem?
2. Should I replace thermostats, (haven't done this in 5 years)?
3. Should I replace circulating pump, (never had this replaced)?
4. I know I should probably replace my impellers next year for good measure.

Thanks
Steve

 

[Lou C]

The stats can get sticky after some time esp with raw water cooling. When you have one that takes forever to warm up till you run it on plane that’s usually a piece of rust or sand that’s keeping the stat from closing. I seem to have to change them every 3 seasons or so…

 

[Scott06]

Hello, I have twin 1998 5.7 260 EFIs 0L067794/0L067XXX in my Formula with raw water cooling. I would like to ask your opinion as to why I have temperature fluctuations while underway at certain points. I would like to know if it is an indication of any potential problems.
When I am under load at constant rpm, my gauges read between 175-180. When I come down off plane, they will tick up possibly 5 degrees and then right back down to "normal" 175-180, and for this reason I always come off plane slower instead of whacking it back to idle. I do not know specifically what the thermostat temperature is rated at. Also, I always have one motor that warms up before the other one.
1. Are these symptoms of a problem?
2. Should I replace thermostats, (haven't done this in 5 years)?
3. Should I replace circulating pump, (never had this replaced)?
4. I know I should probably replace my impellers next year for good measure.

Thanks
Steve

None of this is really concerning assuming u are running a 160 thermostat if you are running a 140 and these are accurate temps the something is amiss


5 degrees coming off plane is not an issue

If you don’t know when last replaced do the impellers and use OEM merc parts the gen 2 impeller kit with base plates is like $60… they stated change I think is like every 3 years but in fresh water I changed mine after 6 yr/300 hrs it was fine…

Would also do your thermostats and inspect the housing for corrosion. If you have the T with check balls feeding the elbows separately those call stay open sending too much water to the elbows bypassing the engine

Also check that your ps coolers are clear

I would not do the circ pump based on what you have presented at this time