As the title says, my 1996 Mariner 25 seems to have a prostate problem.

Patient History: Purchased last fall, less than 100hrs on it. Rebuilt the carb, replaced the water pump, changed the lower unit oil, plugs, and compression tested it- 120psi on both cylinders.

Chief Complaint: After a cold start, the cooling system telltale does not produce a stream of water until the engine has warmed up sufficiently. Once the thermostat opens, the stream from the telltale is inconsistent. It will achieve a full strong stream for a duration of 6-7 seconds before tapering off to a trickle then stopping alltogether for a period of about 2-3 seconds and then repeat the process. Water temp is too hot to touch to start and then tapers to bearable.

I also own a 1988 Mariner 20. This motor does not have a thermostat so there is no required period for the motor to come up to operating temperature and it produces a full stream constantly.

Suspected etielogy: my best guess is a bad thermostat that is opening and then closing over and over and over. It seems to be exactly cyclical from the time the motor reaches op temp and the exact same process of the thermostat opening. Second guess is that an impeller blade is cracked causing cavitation/aeration and a loss of suction.

Thoughts from the panel?

Thermostat. I drill the thermostats that I install just outside of the valve. http://www.boats.net/parts/search/Merc2/Mariner/25%20%282%20CYL.%29/0P325500%20%26%20Up/Cylinder%20Block%20and%20Covers/parts.html

Thanks GP, I have a couple additional questions- when you say drill just outside of the valve, could you be a little more specific? What I think you're saying is that by drilling out something in the thermostat, you are bypassing or preventing the thermostat from closing, therfore water is cycled through the powerhead constantly? I've heard of this before, but could you elaborate a little? I see a benefit to bypassing the t-stat for a couple reasons- no failure of cooling system due to failure of the t-stat and immediate confirmation of cooling system integrity. Because I see such benefit in both those things, I wonder why they incorporated a t-stat to begin with? Better cold starts? Better for extremely cold water operation? Are there any consequences of thermostat bypass- ie. longer warmup period?

Additionally, do I need to replace my current t-stat or by "drilling it just outside of the valve" mean there's no need to replace?

Thanks again for your input!

Main purpose of drillin a very small hole their is to prevent an air lock. You need to replace the thermostat. 2 cycles are designed to operate at a 160-175 degrees optimum water temp for internal parts to expand to correct clearances.

Called the service department at BPS to check if they had the proper T-stat and they did. They did not, however, have the entire kit which includes the rubber o-ring and gasket in addition to the T-stat. Was a little bummed thinking I might be waiting on some parts all week, but the gal working down there was thinking on her toes and pulled an O-ring kit for the powerhead which lo and behold, included the correct o-ring which she threw in for free.
T-stat Cover-

Cover Removed-

Old Thermostat Removed-

Old and New Thermostat-

New Thermostat Installed-


Re-installed the cover and the gasket was still usable- may order a replacement just for peace of mind. Put the muffs on and fired her up!

Etiology confirmed. Success! I'm a little hesitant to just go and start drilling things i'm not sure of GP, and I still don't understand where you were talking about. Maybe you can describe it with the help of the pictures above?

Here's a video of my motor running, theres rumors about auditioning for the next flo-max commercial! Thanks to Gary Purdy for the advice!

Your welcome. The drilled holes are something I have done for years. I would drill the new thermostat right between the 2 rectangle retainer clips 10 oclock in the picture of the old and the new thermostat.