Newly rebuilt 350 Mag MPI is running good during break in, but water pressure gauge smoothly "sweeps" up and down, and temp fluctuates with it from about 130 to 175. Neither one bounces, just a smooth, constant up and down, at all ranges except idle. Any ideas?

Thanks!

What kind of cooling system do you have? I had a similar problem with a crossover system. When I replaced the thermostat it didn't have the required hole drilled in it. If the thermostat doesn't have the hole drilled in it there will be steam pockets that will form. When there is a hole drilled in it it promotes flow through the valve. Take a look at my post.

http://www.offshoreonly.com/forums/d...ross-over.html

If this is running on a hose, then I would not worry about it. Almost all of my engines have done the same thing running on the hose. They stay steady when in the lake.

Stock raw water cooling. On the lake, not the hose. Could a brand new thermostat be sticking?

It's possible but unlikely. You can boil the thermostat on the stove to verify it cracks at the correct temp.

If your running to high of a degree thermostat it will do that,,go and get a merc thermostat and see if that fixes the problem..also where is the temp sender?? if its on the back of the intake it will do what your describing..

Few years ago I put a new and inexpensive (cheapie) thermostat in place and it did the same as you describe. I got it at an auto supply store. I replaced it with a Merc t-stat. Allot mo $$ but no more fluctuation in temps. I believe both were 160* t-stats .... but it's been a while.