cold running a motor words of caution!!!


New member
i found this over on the BBC website and should be words of wisdom for ALL boat motors in cold weather.
i had the exact same thing happen to a suzuki 175 i used to have.

Cold running an Optimax 11:58 AM 1/5/2005 Reply Edit

The first thing I want to talk about is cold water through a cold motor..
1) A outboard motor does not warm up very fast if at all at an idle in cold water.
2)Thermostats can only help it warm up so fast if you have 40 +/- deg. water running through your power head.
So how do you run a Opti or other outboard under these conditions?
1) Idle at least 3-4 minutes minimum.
2) Take off easy.....Do NOT hammer down from an idle while still cold. Take it easy for a ways. Increase the RPM's gradual.
3) Make sure your thermostats are working properly!
* This one is very important ... (I had this happen to me) I was running a 2004 225 Opti in 40 deg. water, motor was running at 130 deg range it picked up a pebble and stuck it in a thermostat (held it open) I had Smart Craft gauges and always have it on eng. temp in cold weather/water..
When the thermostat got stuck open it cooled the motor down to less than 80 degrees!...It would NOT go above 80 degrees even at WOT!
So what's the problem??
First you have a certain amount of clearance between the piston and cylinder wall.
example... cyl bore on a 200/225 Opti is 3.6265 in. (std.) The piston has approx. dia. of 3.6210... So you have approx. .0055 piston to cylinder wall clearance...
Now when a motor heats up it expands...So when it's up to normal operating temp. the piston and cylinder wall are both a larger diameter.
When you run a cold motor hard or up to WOT real quick what happens is the piston gets hot much faster than a cylinder wall that has cold water against the outside of it....The hot piston expands but the cylinder wall hasn't yet....What happens from there is called a Cold Seize.. The expanded piston don't have the .0055 clearance needed and it will seize or stick to the cylinder wall. If it seizes/sticks running @ 5500 RPM + it just rips the wrist pin/connecting rod right out of the piston...The connecting rod will go out of control and end up out through the side of the block...At this point the power head is destroyed...This is one of biggest failure issue of a Optimax and one that can be avoided by just following the above making sure it warms up and has time to warm up right ........Sherm

See what CAN HAPPEN when you don't let a 3L Warm up or COOL DOWN PROPERLY? 1:48 PM 4/30/2009 Reply Edit

Here's the worst one I've seen:

This appears to have started with a UNHEALTHY CASE OF "UNCONTROLLED EXPANSION"!

Guys- be sure to let them warm up to temp before you "hammer down"... and give them 60 seconds or so of idle time before shutting down!

This applies to EVERY engine startup... not just the "first start of the day"!

The above scenario will definitely put "an damper" on your livewell!

(Master Technician)- Mercury Outboards, Mercury Racing and (Certified Technician) Mercruiser Sterndrives, Greenville, SC
35 years as a "Marine Apprentice" (still learn something new every day). Mercury Parts, Smartcraft and Accessories- BBC Sponsor


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New member
I just got my thermostats fixed last week. My motor wouldn't get warm either. Mine is too darned puny to blow up though.


New member
Good advice.

It is even worse if the engine has been rebuilt and someone installed Wisecos in them as they are a forged piston and expands even faster than a stock cast piston. That is why you have to add additions piston to cylinder wall clearance for them. Many people call them SIEZCOS.


New member
I think the best advice is to just ALWAYS keep an eye on your temp gauge AND your oil gauge. At the 1st sign of a problem, shut the engine down and figure out what is wrong. I know a lot of people continue to drive their car's when the engine light comes on and they often get lucky because now the o2 sensors etc will kick on the engine light (I like gauges myself), so no "real" engine damage occurs. Of course I also know some people (my son for one) who kept driving and there was a serious cooling problem and they ruined the engine.
A boat is much easier as we have gauges so there is no 2nd guessing - shut it down or keep running it and end up replacing the motor.


New member
I thought I saw where you had one of those superduper Yammis now Beetlespin ?????puny ??
I had the same deal with my Merc last year ,engine wouldn`t get over 100 degrees on Smartcraft . Took it in and a couple of punkin seed size pieces of trash in the thermostats. I was aware of the cold seize thing and didn`t push it till I got it checked out.


New member


New member
If you own a Mercury you should have a Merc Monitor installed...... You have no idea what the temp is without it. If you have a t-stat stuck open in cold water you can idle all day and will not get to the correct temp (120).


Active member
This is not just for opti's in cold water- Any water cooled 2 stroke needs to be warmed up before taking off regardless of water temp. If its summer and you stop and fish long enough allowing the block to cool, you risk "Piston Jacking" against the cylinder wall's if you just simply start the engine and put the hammer down. Always allow the engine to warm up to temp . This is a bitch in regards to tournament fishing against the clock. Most rules state that the "life vest and kill switch be attached whenever the internal combustion engine is running". Some rules like BASS, state that so long as the engine is running in neutral, you are not required to attach the kill switch lanyard to your PFD until before the engine is shifted into gear. This allows the angler at least 2 options as follows; If you have very low batteries, you can let your engine idle while fishing to maintain a charge so you are not left stranded with a dead crank battery and, allows you to run aerator pumps to keep your catch alive. The option also allows an angler to warm up their motor while preparing the boat for travel to the next spot or weigh-in. Most elite trail anglers don't care about warming up the engine as, time savings can mean a good check or a victory or AOY points etc. They also sell their rig at the end of the season so scuffed cylinder walls are not a concern.


New member
GBHoss - 8/29/2016 5:56 AM

If you own a Mercury you should have a Merc Monitor installed...... You have no idea what the temp is without it. If you have a t-stat stuck open in cold water you can idle all day and will not get to the correct temp (120).

Agree 100000%, I've had Merc SC1000 on my last three boats. Knowing the temp is how I caught a stuck poppet valve before toasting a block.