Cooling Systems

All material on this website, and these sections, is ©Runner Outboards LLC and is intellectual property.  You may freely distribute this information as long as it is NOT edited, and credit is given to the author.

Disclaimer: The information provided should never replace common sense or the recommendations of the OEM.  I do not assume responsibility for the use or misuse of this information.  The information provided is based on my experience working as a full time mechanic, on hundreds of motors over time, reading a lot of manuals, education, and consulting other experienced mechanics along with a number of retired service reps I am friends with.
If I can offer any advice from experience, it would be NOT to try and fix your own motor if you don't have a good understanding of what you're doing.  You need to have the right special tools, reference materials, and most importantly, UNDERSTANDING of what is wrong and how to properly fix this issue.  Most people do more harm then good if just messing around blindly.  The reason why I can do these repairs is I've put in thousands of hours reading, fixing, and practicing.  I learn something new everyday.  I have also gone out and acquired the necessary, CORRECT tools and reference manuals to work on the motors.  These are very important to promote correct operation of the motor.  The idea is to have a reliable motor, not just one that 'kinda runs.'

Index - Click below to Jump to That Section

Internal Combustion Essentials
The Basics, What You Need To Know About Motors

Cooling System
Gearcase Components
Ignition System
Fuel System
Mechanical Components
Trailer 101

Stuck Thermostats

During the repair process and evaluating cooling system performance, there is one area that most people don't check.  That is - the thermostat.  Nearly every modern day motor over 5hp has one of these installed, and it controls the amount of water flow through the cylinders to help the motor run at the right temperature.  The way most of these works with a temperature (and pressure) 'thermo-ferm.'  Commonly, around 143°, the thermostat opens, and allows water flow.

So if your motor is cold, it stays shut, but usually allows a minimal amount of water to flow.  Once the motor warms up, it's time to keep it from overheating so it opens and allows cool water to flow by the hot cylinders, where there is essentially a wall of flame inside from the ignited fuel/air mixture.

Now the issue is that these things can fail either closed or open.  If open, the motor won't fry, but it won't run right, either.  It will smoke like hell, blugger, hesitate, fowl plugs, you get the picture.  If closed, your motor is going to overheat, potentially even at idle on a warm day.

The pictures below are of failed thermostats stuck open (last one is stuck closed).  If old enough, these things can actually break apart, or get clogged up with debris, salt, leaves, dirt, etc.  It's a good idea to replace it every couple of years, or at least have your motor gone through for an overall tune up once a year.  It may seem like an unnecessary expense, but it's actually a marginal investment (worst case, about $100 for a newer motor).  There is no way to predict when this might happen, and random bad luck could also strike you.  The reality is you can change it out every day, but that doesn't guarantee any extra protection.  They just fail.

Remember overheats are caused by no water cooling or no oil in the fuel (2-stroke).

stuck thermostat  2  shut

Clogged Thermostats

The 2nd issue you have to watch out for is debris accumulating in or around the thermostat and clogging it.  The water intake screens prevent debris larger than a couple of millimeters from entering the cooling system, but silt and sand, broken down leaves, weeds, fish scales, marine plants...they can all find their way into your cooling system, even small pieces of fishing line. 

Now the thermostat is supposed to open, roughly at 143°.  This SHOULD allow a clog to be flushed out, especially if you 'gun' the motor to increase water pressure.  However, this does not always happen.  In particular, if you have a clog and shut the motor down, the debris can dry out, harden, and then really lodge itself.  This can further complicate things because if the thermostat is fully clogged, and you shut the motor off, the water in your cooling system can slowly drain back out of the motor creating an air pocket.

So next time you go to run the motor the air pocket (if the clog is thick enough) stays around the cylinders, and you are essentially running the motor with little to no cooling action.  If you turn the motor on and see nothing coming out of the water telltale ('pee hole') or exhaust relief (usually at the top of the exhaust housing) within a few seconds, then you have a real problem and should turn the motor off right away.  If you see steam coming out, that means there is a trace of water getting through, but you should pretty much always see a steady stream of water coming from the motor.  Modern day motors come equipped with an overheat sensor, so if this alarm goes off there is your confirmed evidence.  But that is assuming your temperature sensor is working and/or your alarm is functioning correctly.

Here are a couple of pics of a clogged thermostat.  The motor was pumping water, and I knew this for sure because I had changed out the water pump as a standard service practice I do on all my motors, but yet no water was coming out.  Pull the thermostat cover and voila!  Here is the problem.  There were several dead leaves clogging the top and bottom of the thermostat.  The 3rd picture is of a motor that overheated badly, and you can SEE the thermostat was wide open, yet it was completely clogged with debris.  The picture is next to a new thermostat.

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Overheated Powerheads

overheated powerheadWhen the cooling system is failing or simply not keeping up with the heat produced by the motor damage is going to occur.  It depends on how bad the cooling system is performing, how hard the motor is being run (idle vs WOT), and how long the motor is forced to run in a retarded state. 

If you're lucky, the pistons, cylinders, and rings don't sustain any significant scratches or wear.  At the other end of the spectrum the motor gets seized from an overheat which usually presents itself as a sudden stop of the motor, or you're going along and the power drops off and the motor quits.  The operator tries to restart the motor and finds that they can't get the motor to turn over and the flywheel is stuck.  If you're lucky, after a few minutes the metal internals cool off enough to shrink back down (remember most things expand when heated up), and the motor starts back up.

Here is a picture of a severly overheated powerhead.  The motor had been thoroughly cooked and compression had dropped to the point where the motor wasn't worth tryint ot rehab.  Note the severe discoloration of the cylinder head (dark brown) as compared to the normal light metallic blue paint on top of the cylinders, and the melted coils on the starboard side of the cylinder head.  What had me scratching my head on this one is that this motor was equipped with an overheat alarm, so either that alarm didn't work, or the operator simply didn't shut the motor off when they heard it!

Below we see some pictures of another overheated motor, and some other areas that are badly damaged. After pulling the powerhead it was obvious that the motor would not be worth trying to rehab for a combination of reasons.  The primary one is the inner exhaust tube (black snout off bottom of motor) had fasteners that were all badly corroded due to lack of care, and all of the water system was badly damaged.  In the left picture you can see the water tube (small tube next to exhaust tube) and the plastic guide that is melted.  The right picture shows the cylinder head off, and the gasket for the water cover/thermostat cover gasket is badly burned/dried.  The coils are also melted (different than the motor above).

Looking closer at the water tube guide, you can see it has been melted and destroyed by a severe sustained overheat (left).  Newer motors (circa 1970 and newer) for OMC also utilize a protective driveshaft tube that connects the lower exhaust housing and water pump/impeller housing (center/right pictures).  What this does, is shields the impeller from exhaust impulses that are sent down through the exhaust housing and sends it out the gearcase via through-hub prop exhaust, or behind the prop exhaust.  Without this, the exhaust pulses force their way into the water pump housing and suppress water flow to the cooling system, quickly leading to an overheat situation.  The right picture shows what it is supposed to look like next to the badly melted one off the cooked motor.

The other thing that happened is above the guide 'snout' (above left picture), there is a rubber grommet used to help hold the water tube in place.  This became collapsed from heat, so even by repairing/replacing all these damaged components, this rubber grommet if left in place would restrict water flow by collapsing/constricting the water passage.  So switching out an impeller is not going to do much with motors that have not been maintained properly!


This picture shows the inner exhaust tube (upper right), the shift lever (thin pole hanging down), the driveshaft water tube orifice (bottom left), and the yellow arrow is where the copper water tube usually goes into, which is where the water pump sends water up to the powerhead through.  So there is a lot going on here to keep the motor cool and running the way it should.

Water In Cylinders

For most OMC motors, this is the result of one of a few sources (other than the motor taking a swim). 

#1 Your lower crank seal may have failed, allowing water up through the crankcase through the exhaust housing, through the crankcase, and throughout the motor.  This requires a full teardown to replace the seal (not a short job).

#2 You have a failed cylinder head gasket - fairly easy fix.

#3 You have a failed exhaust bypass gasket or metal baffle (this one requires a near full teardown, depending on the motor and if you have access to all the bypass cover screws).  This is a somewhat rare one, but isn't all that uncommon.  There is usually a stainless steel baffle plate sandwiched between a couple of gaskets that provides a water jacket on the exhaust side of the powerhead.  Remember, this is nearly the hottest gases in the whole powerhead, being ejected down the exhaust, under water and away from your motor.  It is like holding a blow torch on a piece of metal non-stop and trying to keep it from melting.

So this is usually the 1st place water is supplied to in most motors, with the cylinders being 2nd, then the cylinder head, and then out of the motor.  When running, water passes through the motor so fast that it is going through this whole loop (pickup, through the motor, ejection) in 1 second.  Water temperatures can rise from say, 50° to 130° in 1 second.  That's how much heat the motor generates.

A failed baffle or exhaust bypass gasket will lead to very erratic motor running, if at all.  It may start and run fine temporarily (before water gets in the cylinders), then start dropping and picking back up cylinders (the water is extinguishing the explosion in the cylinders), then die, then you have to pull it over for several tries before it fires back off, then repeat.  If you start fiddling with fuel mixtures, you get that much more stumped. 

Stop fooling around - pull the plugs and check for water!  If you see water in there, you have much bigger headaches to deal with.  Just make sure you turn it over continuously if you see water in the cylinders until you see next to nothing coming out, then IMMEDIATELY fog the motor...basically drown it in fogging oil, or WD-40, or anything that inhibits/discourages rust.  If you let the motor sit overnight, it may be lost for good.

baffle holeThe pictures I have here are of 1 motor with a GIANT hole in the exhaust baffle.  This motor actually ran!  Not well, and erratically, but it ran.  Long-term veteran mechanics are still amazed by this picture, none that I've spoken to have even seen a running motor with a hole this big.  Usually all you see are pin holes that eventually rust through.  This condition can easily be accelerated by taking a motor that was run, and putting it away lying on it's side so leftover water in the cooling system is just sitting in the powerhead, instead of draining out like it should.

The other picture is of a failed bypass gasket, which is common.  The 3rd picture shows 2 motors (taken apart) side by side, one with a failed gasket, the other that was still OK.  Notice the one with the failed gasket that exhaust pulses had pushed into the water jacket, and how the water 'cleansed' the exhaust passages?  Dead giveaway.  The other one shows normal accumulation of carbon, fuel, oil, and unburned mix caking up.  This is also stuff you're supposed to clean out of the motor during a full tune up to make sure you're getting maximum performance and power output.  When the airway is clogged up, less fuel passes through, and you get less power.

Once the baffle or gasket fail water intrudes through the exhaust holes in the chambers and causes all sorts of problems.  Once these are replaced, both motors ran like clocks immediately.

   exhaust1   exhaust 3

Salt In Water Jacket

As most folks know, salt, other than to keep roads safe, is bad for metal.  In a marine environment it can wreak havoc on motors when they are not cared for by operators.  In particular, folks who don't flush their motors after each exposure to salt are really just on borrowed time. 

Now just because a motor has been used in salt does not mean the motor is going to be no good.  It does, though, mean that it will require a closer inspection and maintenence to prevent issues.  The most common issues are stuck bolts and clogged water passages (see the gearcase section for a couple of good pics). 

The pictures here are of a good working 6 & 15hp motors, but I noticed some 'hot spots' on the cylinders.  They were pumping plenty of water, and not overheating, but the unusual hot spots set off some red flags.  I pulled the cylinder head (6hp) and exhaust bypass (15hp) to inspect and I found sand and salt clogging it up.  The rest of the cooling system was OK.  5 minutes with a pick, wire brush, compressed air, replacing the gasket, and the motors were restored to normal cooling function.  Note that this type of accumulation doesn't happen in 1 season.  This happens after years of neglect and misuse.  Annual tune-ups would prevent this and are recommended, or using common sense and flushing the motor after each exposure to salt beach.

The third picture is from a 9.5hp motor that was running hot.  The cylinder head was plugged solid and water was barely circulating.  Trying to remove the cylinder head was impossible due to salt making the fasteners stick.  In this instance, it was easier to CUT the cylinder head off, carefully avoiding damaging the actual cylinder surface.  Replace the cylinder head and the motor worked just fine.

waterjacketsalt   2  3

Marine Growth & What it Can Do

This may be slightly off topic for this section but nonetheless, involves the cooling system.  I've mentioned and shown examples of how neglect can badly effect your motor.  Well, below is a before and after of two identical 6hp engines.  The first engine was purchased by a customer who used it as sailboat motor for doing island work on a regular basis.  He didn't tip the motor up while not in use, so it sat in the salt water for several months.  The motor next to it is the replacement motor he had to purchase after just 1 season of use because the salt had caused problems with his motor.  Needless to say, he's tipping the motor up when sitting for extended periods now!

Using your motor in salt isn't the kiss of death, especially if you're only putting it in for a day or two.  After each use, put the motor in a bucket of fresh water and run it for 5 minutes to flush it.  Spray it down with the hose and give it a quick rinse down.  Using turtle wax to coat it down and spraying the whole thing down with WD-40 both internally and externally goes a long way.

Below is an excerpt from the Johnson 10th edition service manual (circa 1962) showing the effects on hull speed due to marine growth in salt water for an extended period.  Very interesting stuff.  Brackish water and fresh water also cause loss of performance, albeit, generally at a slower rate.  But if you pull your boat from the lake or camp half way through the season and scrub down the hull, you should see a significant improvement in speed.  If you see marine growth, that is just adding drag to your hull.

Water Pump Maintenence

There are a lot of things to look for when maintaining your water pump. You should check to see not just what the water output of the motor is, but the the strength of that output. It makes sense to change the pump as soon as you see water output drop off.  Generally older motors don't have overboard water indicators (pee streams).  When 100% correct, they should be pumping water like a water hose (exception would be from 1974-1976.  These were transition years). Even when cold, they should pump a ton of water.  When warm, you will see basically a garden hose of water coming out.

When evaluating the pump, check to see if the blades have a melted appearance or are worn, if it's taken a permanent form, if the rubber is hard/vulcanized, or if the rubber has separated from the metal internal ring. Of course if you see a blade broken switch it out. 

The first row of pics (and 1st pic on the 2nd row) below show the progression of impeller wear.  These are 4 different impelelrs, all at various stages of deterioration.  They all still pump water, but absolutely shouldn't be used any longer.  From left to right, less and less water is pumped.  There are other factors that can slow down water flow, the impeller isn't even the starting point, even on a brand-new motor.

The 2nd row shows the same situation with a smaller impeller but larger HP motor.  The size of the impeller doesn't necessarily translate to more water pumping.  The very last picture (on the right) shows the brass ring missing; this is what ties into the driveshaft and impeller key to rotate the pump.  Either the impeller is defective, or it is so old that it has failed.  Only operators who don't know what they're doing leave the pump around long enough to let that happen.  And the very last picture (single bottom), is an example of when an impeller was left in so long, it exploded into pieces!  Let it go this long and try to start it, and you run the risk of the pieces getting stuck in the water jacket, and you will be into it for a huge repair bill trying to figure out where that piece is, and how to unclog it.

It should be noted, that visually you may not be able to identify a failed or weak impeller. There are situations where the impeller appears new with little or no wear, the brass core is secure, but the rubber is so weak that it flexes and cannot keep up pumping water when much over idle.  So looking at the water output at trolling speed keeps the motor cool and pumping a lot of water, but when you throttle up water stops pumping due to the fins/paddles folding over and unable to overcome exhaust pulses. 

This is further complicated because when you are going fast in your boat, the last thing you want to do is take your eyes off of looking straight ahead, for safety of course.  So while you may observe the water pump working at slow speeds, you have to ask a passenger to monitor performance at wide open throttle, or possibly look back quickly.  This isn't so easy in an aluminum boat with a tiller!  When you're up on plane, turn your head, hold the tiller, and lean one way or the other to look at the back of your motor, that can be dangerous!  And for motors with a telltale, that doesn't necessarily mean water is circulating through the powerhead!
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The #1 reason why impeller wear out is running the motor when it isn't submerged in water.  The moisture lubricates the rubber blades, reducing friction and heat.  Without this, they are damaged as fast as just 5 seconds.  Remember a motor at idle runs about 600RPM (or more).  That's 60 turns in 6 seconds.

Check the impeller housing too for a lot of wear (1st picture). This is an original housing off of a motor from the 60's. It was still pumping water, but was clearly on it's way out.  A quick inspection showed the obvious problem.  In severe situations where a motor is being run completely dry (say, running in your driveway and cooling system is completely dry), you can have a burned impeller (pictured below).  In these situations the rubber melts, burns, and the globs of fried rubber gets lodged into the impeller housing and possibly up the cooling system the next time the motor is run.  This can turn into a big problem, because now you have to pull the whole motor apart to get the water clog out.

burned impeller

Wear isn't the only thing to pay attention too.  Newer housings usually have a stainless steel cup on the inside to minimize housing wear (the housing is aluminum).  It's not a bad idea to remove this cup when you change impellers to insure there is no sand or other build up of debris.  The picture below (2nd pic) shows salt build up to the point where the internal metal cup has crushed down on the internal orifice diameter. This happens due to extreme neglect. This person probably shouldn't own a motor again, frankly!

 housing 1   housing crushed

The exterior of the impeller housing is not impervious to damage either.  Here is a picture of one with the water tube grommet removed.  You can see salt/debris build up wherever it was allowed to sit.  Notice the badly salted screw orifice as well.  In these cases, the screw head breaks off, and with luck you can pry the impeller housing off the gearcase housing.  The screw studs are usually left, or may break off, then you have to drill out the old stud and retap the gearcase housing, generally with 1 size screw larger.


If exposed to salt and not maintained/flushed regularly, the screws can get stuck, and will need to be drilled out/retapped (if possible). These pictures are a common thing seen in poorly maintained motors which were exposed to salt.

housing1  housing 2

Depending on how neglected a motor has been, you can see the various stages of salt build up.  Below is a picture of a gearcase screw (long one, about 4 inches) that I was just barely able to free up.  The threads are clean where screwed into the orifice, however the threads that were NOT screwed in and resided in the hole where the screw went into had salt filled into them.  What happenes here, is that the salt keeps building up each time the motor is used and it dries, and eventually packs itself into the clearance between the screw and the hole it goes into. 

If this is let go long enough it will hard pack in and cement itself, filling the tiny clearance.  Next it basically welds the stud into the orifice.  When you go to remove this generally the screw/bolt head breaks off, and sometimes the two pieces fastened together are welded too.  Heat helps, but after a certain point nothing works short of boring out the orifice and screw hole and retapping.  It will literally make a 10 second job take an hour to put back together properly.  All due to neglect and not FLUSHING after exposure!

Worn Impeller Plates

Below the impeller is a SS plate that sits on top of the lower gearcase housing.  Generally this plate serves a purpose of helping mate the impeller housing to the gearcase housing, while holding the driveshaft seals down to a degree, providing the drivehshat another cradle to hold it in place, direct water into the impeller housing void, and help keep exhaust pulses out of the cooling system/pickup.

Now this plate typically gets changed out when a full water pump kit is installed, but many times just the impeller gets changed and the impeller housing is kept too.  This is OK to a point, but when you start seeing a lot of wear on either of these items it is worth it to replace. 

Below is a picture of a badly pitted and worn SS plate.  While the cooling system still functioned on the motor, it was struggling to keep up.  This came from a 30hp more modern OMC motor.  The overboard water indicator had intermittent stoppages, where the spray would not be strong but be a fine mist.  The water pump was brand new, and other aspects of the cooling system had been already renewed (i.e. thermostat, o-rings, etc).  Exhaust pulses were able to find their way into the cooling system via these pitted spots, it only takes a 1/2 mm gap to start to suppress the cooling system's function.  At idle, the problem would show itself.  Run the motor up in RPM and it was able to pump fast enough to stay ahead of the exhaust pressure.


Impeller Keys

Impellers (water pumps) are secured to the driveshaft, in most cases, by some sort of key or wedge.  Older motors utilize a small orifice hole with a SS peg.  Now depending on the impeller, this may just be a peg, or one that looks like the letter 'T.'  Newer motors utlize a plastic asymetrical wedge that forces the impeller to be installed only in one direction. 

Below is a picture of two different impeller keys.  The center one is from a motor where the last person installed the wrong key.  You can see it was worn down, and how the last person was able to get the driveshaft to sit at the right height into the gearcase is still a mistery.  I suspect they may have forced the impeller housing down onto the SS plate, and used the screws to tighten everything down.  Surely, trying to turn the driveshaft by hand would have yielded a scraping noise and a lot of resistance. 

The picture to the right is the impeller from the same motor.  Notice the gouges on the blades.  This is very unusual and a result of the wrong key being installed by an unknowing repair person.  This probably happened due to metal shards breaking away from the impeller key or uneven surfaces as the impeller spun around.  It wouldn't take long for the rubber to be torn apart in this type of situation.

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Here you see how the T shaped key was gouging not only the impeller, but also the impeller SS plate.  There is a concave wallowing of the hole, and the plate was convex.  This is not right.  It should be perfectly flat.  All of these components required replacement to restore the water pump to working condition.  It's amazing the motor had been operated at all with this configuration - the prior operator was on borrowed time, at best!

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Running Motors Dry (Out of The Water)

I've mentioned it in this section and others several times, how detrimental it is to run a motor out of the water dry.  Well, here are some pictures showing just what it can do.  Back in the 60's, there was a service bulletin that Evinrude published showing the stages of impeller wear based on time being run out of water.  Unfortunately, I'm still trying to dig it up, but will post as soon as I relocate it.  Below is a picture of a badly worn impeller from a customer who had brought in a 70hp motor for service.  He actually was running this motor weekly, and it was keeping up (staying cool), but clearly this entire assembly was badly damaged and needed replacement. 

Picture #1 shows a 'shadetree' job of sealing the impeller housing to the SS plate using gasket sealant.  Well, in most installations there is either a thin rubber gasket or a fiber gasket, or both used, not silicone used.  This is a situation where the person who changed the impeller bought just the impeller, not the actual components needed to do the job right.

Picture #2 shows the impeller housing cup with the melted rubber from the impeller.  This is a clear indicator of the motor being run out of the water for 10-20 seconds, possibly minutes.  The rubber impeller heated up and melted due to friction against the impeller cup without water lubrication.

Picture #3 shows the SS impeller plate and how it's been worn down both with melted rubber from the impeller and grooves cut in either from sand or simply the inner metal impeller core rubbing.

Picture #4 is the most dramatic reveal, the impeller itself.  You can see it is badly melted and taken a set position.

This process starts in as little as 5 seconds being run dry.  Now, if the impeller is lubricated with water, this slows it down quite a bit.  If you're ever in bad waves and the prop comes out the water, it's possible to run the motor at high RPM's for 2-3 seconds before it goes back into the water without any damage happening.  This is normal usage.  It's starting a motor in the driveway without the gearcase being submerged properly that leads to quick damage.

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