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Author Topic:   OMC V-6 Thermostats: Maintenance you never think about!
Bigshot posted 01-16-2004 12:06 PM ET (US)   Profile for Bigshot   Send Email to Bigshot  
I just spent a small fortune rebuilding my 1994 Johnson 225 Ocean Runner this past month. I am the original owner and she only had 329 hours on her. Flushed and de-carbed like religion. Ran Berryman's or Techron periodically. ALWAYS ran OMC oil and 89-93 octane gas. My father had the boat up north for the last 6 seasons and only put 70 hours on it--not great for an outboard but lack of use was not the issue.

Well Swellmonster and I went out for a short cruise about a month ago--real short. Got her up on plane and she started losing power. Pulled off the gas and she started knocking like Ali. Got towed in and pulled the plugs. The No. 2 plug (top port piston) was covered in metal. Pulled the head and she was done.

We bored the one cylinder and honed the other five. New rings, bearings, VRO screens and fittings, Hi-perf plug wires, etc. The engine was clean as a whistle inside and flushing definately pays off. Carbs were spotless as well. Reason for piston failure you ask? Thermostats! Both thermostats were corroded and broken. Water pressure was great though (have a gauge). The port thermo was not opening all the way being broken, so the top cylinder was running slightly hotter. Over time she detonated enough to give up the ghost. We put in new STAINLESS thermostats. Final touch is we painted the block teal to match the decals; looks pretty trick. Plus when I go to sell it they will know it was rebuilt or I am the best in the world with masking tape.

Gonna trial her this weekend, hope she runs like she used to (52+mph)---after break-in of course. This would NOT have happened on a 4-stroke being the engine oil would have cooled it. 2-strokes are VERY sensitive to heat, hence why we use different oil, etc. For those of you who have removed their thermos, you might want to rethink that, running cold is not good either.

lhg posted 01-16-2004 01:02 PM ET (US)     Profile for lhg    
Nick - Sorry to hear about your problem. On my V-6 Mercs, the water pressure gauge take off, the overheat horn sender, and the cylinder heat temp sender (three separate devices) are on the highest cylinder. Wouldn't that be the case with yours also? And wouldn't at least one of those overlapping senders have given an indication of low water pressure/high operating temp?

But you are right about the smallest, insignificant part, something not normally checked or considered, that can cause trouble. My 1985 in-line 6 Mercs are currently in for a 2500 hour (seriously) check-up, to take advantage of 50% off down season labor rates, which I thought might be a wise idea. Cylinders, rings, pistons, compression (including a leak-down test), carbs all perfect and spotlessly clean with no carbon/gum (thanks to Mercury QuickClean), but they found a 50 cent oil bleeder hose with a small leak on one. This re-cycles lubricating oil to the top piston bearings. No damage, but eventually I could have thrown a rod if not found! Also found a few 75 cent plastic "restrictor" plugs missing, which affect acceleration. The $700 total charge, including new impellers, was well worth it as preventive maintenance, and a lot cheaper than a new pair of engines.

Bigshot posted 01-16-2004 02:00 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Both thermos housings have temp senders on them at the top. Water pressure is great. I guess it got "just" hot enough on that one cyl to make it detonate. Detonation over time(who knows how long) ate up the piston head. My overheat alarms work so does the S.L.O.W. feature. This is so slight it goes undetected. I was with you....no alarm, no heat.....wrong! It could heve been doing this for 6 years or 6 minutes, no way to tell.
cape_rover posted 01-16-2004 02:11 PM ET (US)     Profile for cape_rover  Send Email to cape_rover     
[For those of you who have removed their thermos, you might want to rethink that, running cold is not good either. ]

What are the issues associated with running an engine cold?

diamondjj posted 01-16-2004 02:21 PM ET (US)     Profile for diamondjj    
How often should the thermostats be replaced ?
Bigshot posted 01-16-2004 03:14 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
What is the point of warming up an engine? If thermos are removed, the engine never gets up to ideal operating temps which the manufacturer sets by deciding what thermo to use. This is usually in the mid-upper 100's(150-170?). If you don't run thermos and water is just flushing through, you may run at roughly 100 degrees. At 100 the cyl walls or ring/pistons, etc are not expanding all the way because they are cold...hence it MAY cause premature failure....may not. This failure can be from many reasons like piston slap, cold spots/cyls(where water first enters), hot spots(where water leaves last), just run like hell because it is cold and cause issues from that, etc. Would you run your car down a dragstrip when it was ice cold?

Second question.....I have no idea when they should be changed. My engine was 9 years old with 329 hours....at some point they BOTH failed. I am gonna check mine every other season or 200 hours I think.

stefan posted 01-16-2004 04:39 PM ET (US)     Profile for stefan  Send Email to stefan     
the thermo must have been removed on my 140 Johnson, 'cause it doesn't get hot, but runs like a charm.
I emailed previous owner to see if they had it removed.

My neighbor, who has worked on OMC motors on his own boats said you really don't need one down here in FL, but what bigshot said has me thinking. But boat was in LA. all these yrs w/out Stat, if it ain't broke, don't fix it I guess

Clark Roberts posted 01-16-2004 06:06 PM ET (US)     Profile for Clark Roberts  Send Email to Clark Roberts     
I assumed that OMC engines, especially the big ones has pressure by-pass devices so that a stuck thermostat or planning speeds force open this simple spring loaded device. It is simply a plastic stopper held in place by a spring and it opens automatically. So on the Mercs that I have from 60hp up to 200 the cooling is not thermostically controlled once rpm gets above about 2000rpm and is getting all the cooling possible from ambient temp water. I thought the OMC's were the same and had the crossover hose for pressure balance between cyl banks... I learn something everyday! Happy Whalin'.. Clark.. Spruce Creek Navy
hooter posted 01-17-2004 10:17 AM ET (US)     Profile for hooter    
The big-block engines may all have those pressure bypass valves, but Ah'm very partial to redundancy, especially when designed into the equipment by the manufacturer. That's why Ah've never bought a used boat motor of any size whatsoever without replacing the thermostats. It's cheaper than replacin' a waterpump impellor, and nobody'd have any qualms about recommendin' that proceedure. New thermostats is such a cheap peace o'mind item, Ah cain't see any reason for not doin' it. As for frequency, in our salt water environment it's on mah every-four-years list. Do it every other re-build of the water pump, and you're in bid'ness.
DaveH posted 01-17-2004 11:04 AM ET (US)     Profile for DaveH  Send Email to DaveH     
Nick:
I think this advise is very important and maybe Jimh can put something together with your guidance for part of a "recommended engine maintenance" in the reference section.

All too often, we have excellent info such as your article get forgotten and only a decent search will recover it. Please consider writing something that could submit to Jim and permanently place it in the Reference section.

Larry (lhg), your use of the Merc QuickClean and other decarb solutions would be an excellent choice as well for other who visit the site. It was one of the first things I learned from this site about engines and realized that I didn't know shite about keeping my previous outboards in proper order. Good habits get passed down from father to son and other times from mechanic to "somber engine rebuilder". With the introduction of the lean burn 2 strokes to the 4 strokes, we could really put some knowledge on the page.

kingfish posted 01-17-2004 11:15 AM ET (US)     Profile for kingfish  Send Email to kingfish     
I spent some time messing around with and trying to unscramble the real skinny on the thermos on my 1992 Evinrude 225 last summer - Swellmonster was around *my* motor at that time, too...hmmm... ;-).

Anyway on my '92, there was a bypass system that had been disconnected and when I finally got to the right OMC tech, he indicated that un-hooking the bypass was never a written service bulletin, but customers who called in or whose mechanic called in with complaints about poor idling, were directed to unhook the bypass. Apparently the bypass allowed too much water to cool at idle, at least under some conditions, and those big loopers need to run hot (140° to 160° or so) at idle in order to run properly.

The understanding I came away with is that there (at least until '92; don't know about later) is no need or use for any kind of thermo involvement above idling speeds +/-; the motor can use all the cooling it can get above those speeds. That is, the thermos need to be open. So restrictions from a thermo that is corroded would certainly be problematic for most people and the way they operate their motors. I know from experience with my 225 that the thermos can be removed entirely in emergency situations and the motor will run and cool fine at high speeds but will run rough at idle.

kingfish posted 01-17-2004 11:32 AM ET (US)     Profile for kingfish  Send Email to kingfish     
p.s.

I wonder if Nick's condition could have been caught sooner if he had both pressure *and* temperature gauges? We've discussed this before, and I think LHG has temp gauges and I have a pressure gauge and we concluded that in order to really know what was going on, one would need to have both (and I plan to have both before Spring).

But would the additional temp gauge have read water temp at the right place to have given Nick the additional data he needed? I don't know for sure...

jimh posted 01-17-2004 12:04 PM ET (US)     Profile for jimh  Send Email to jimh     
[Edited thread to improve readability.]

Nick's narrative of his OMC engine troubles and their remedy is great information. I'd love to work this into a more permanent article in the reference section, but, as Andrew Marvell once observed, "If we had but world enough, and time...."

daverdla posted 01-17-2004 01:54 PM ET (US)     Profile for daverdla  Send Email to daverdla     
I believe my 100HP Evinrude has the spring loaded bypass. I thought the thermostat's should be replaced every two years, same as the impeller. I thought I got that advice from this group. They're kind of a pain to get to but not too bad. I'm may be more concerned than most about thermostats. Having owned several MG's, Triumphs's and a Morgan, the standard drill was to install new thermostats every fall and remove them every spring.

Sorry to hear about your troubles.

Dave

lhg posted 01-17-2004 05:25 PM ET (US)     Profile for lhg    
From my OEM Mercury service manuals, I have noticed that they show photos of pistons ruined by "detonation" (pre-ignition pinging). They indicate that the primary cause, other than bad ignition timing/low octane fuel, is carbon build-up in the combustion chamber or on the piston, which raises the compression level and causes the detonation/pinging. I would assume that a high compression ratio would also cause some overheating as a secondary condition.

So my question to Nick is are you sure the REAL CAUSE of the failure was detonation as opposed to simple overheating/cooling problems? You did indicate that the cylinders were clean.

whalersman posted 01-17-2004 05:59 PM ET (US)     Profile for whalersman  Send Email to whalersman     
I just had my Thermostats changed out as they were stuck open. The engine was running too cold at idle..

Also, lhg brings up another point about "bad ignition timing/low octane fuel, is carbon build-up in the combustion chamber or on the piston, which raises the compression level and causes the detonation/pinging" which brings me to what my mechanic told me after looking at my engine.

I have a 1985 Evinrude 150 that was designed to run on the middle grade of 93 Leaded octane fuel .. The engine always sounded like it was pinging more then it should even though I was using that grade fuel but Unleaded. My mechanic said there is a outboard bulletin out that states you can put in "thicker" head gaskets to reduce the amount of compression in the engine and then use the lower grade 86AKI Unleaded fuel. Outboard Bulletin #2155.

I ordered a pair of this head gaskets and installed them myself.. Very simple. I cannot believe how quiet the engine runs now. It is unbelievably quiet compared to what it was.. Almost sounds like a 4 stroke when cruising as I have no problems having a converstaion with other people on board. I can now also run the lower 86AKI Unleaded fuel which costs about .10 cents less a gallon.. 60 gallons equals a $6.00 savings per tank.

I may not have the total 150 hp as it used to produce, but I surely can't tell the difference... Also, the newer engines aren't included in any of the above..

For What It's Worth,
Joe

John from Madison CT posted 01-18-2004 08:44 AM ET (US)     Profile for John from Madison CT  Send Email to John from Madison CT     
Thermostat are considered a part of preventative maintenance, just like a water pump.

Remember, they are in direct contact with Saltwater (if you boat in SW) and need to operate properly for your engine to work well.


John

lhg posted 01-18-2004 10:39 PM ET (US)     Profile for lhg    
Further to the subject of "detonation", I also read that it's resultant piston damage can be caused by over-propping, and running the engine full throttle at less than recommended RPM range. This situation advances the timing too much for the given rpm's, and causes the detonation.
NFORKFISHEAD posted 01-19-2004 09:14 AM ET (US)     Profile for NFORKFISHEAD  Send Email to NFORKFISHEAD     
Along the lines of what John from Madison said... My mechanic who is a Mercury dealer recommends changing water pumps and thermostats every 2 years. Maybe this is overkill but when you think of the cost of replacement or rebuilding, this is a cheap maintenance solution.
Knockerjoe posted 01-19-2004 09:39 AM ET (US)     Profile for Knockerjoe  Send Email to Knockerjoe     
Thanks bigshot for the info as I also own a 1994 200 Johnson Ocean pro. I replaced mine right after I purchased it a little over a year ago. Remember to use Genuine parts as aftermarket thermostats may not be stainless.
Termostats on most engines are very easy to change and just take a few minutes. Don't forget to get new gaskets when installing.
Running a motor without thermostats is a bad idea. Just like a human an engine needs to run at constant temprature. Without thermostats you will build up carbon deposits very quickly which can contribute to engine failure.
Bigshot posted 01-21-2004 04:05 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Ok....a few things to say. No a temp gauge would not have helped. I do have a water pressure gauge. My boat is proped right but I do not run WOT but for maybe 30 seconds every 10 hours. Timing is correct and rarely goes out on 2 strokes so I doubt it was that.

Detonation was the reason for its demise but whether or not it was the T-stats is anyone's guess. Upon reassembly on Friday the only thing that came up wierd was when we rebuilt the carbs we set them to factory starting point...say 1.5 turns out. When we ran the engine all the carbs were pretty much in spec at that setting EXCEPT for the one that detonated. That one took almost 3/4 turn to stop the poping. We think that the carb was defective from the factory and NEVER adjusted right. This in itself might not have been enough to burn a piston being it ran for 10 years but add the added heat of the t-stats being bad and you have a lethal cocktail. She has almost 3 hours on it and all seems to be well. She runs at 153 degrees on all cyls. I nailed it once and she came up easily to 5500 and I pulled back so all looks good. Gonna pull the plugs today and inspect. At 10 hours or so I am gonna pull the heads and take a look at the cyls to be sure all is good. Will keep ya posted.

lhg posted 01-21-2004 05:35 PM ET (US)     Profile for lhg    
I think there have been other comments on this site that a bad carb/jets, running too lean, can trash a piston real fast. Maybe that's what happened?
Bigshot posted 01-22-2004 11:20 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Carbs were clean as a whistle and set the same since 1994, had to be a combo of things.
Steve Leone posted 01-27-2004 01:06 AM ET (US)     Profile for Steve Leone  Send Email to Steve Leone     
Those 225 v-6 models are notorious for that problem. I had a pair that took me to the moon before we found out it was a combination of the thermostats plus partially clogged cooling galleries. A heat crayon is a good tool for dianosis on those problem blocks. Steve
Bigshot posted 01-30-2004 01:50 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Have a laser temp gauge. All even now at 153 degrees when hot. I think I have 140-146(not sure which) thermos in it.

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