running OB on muffs = overheating, any ideas?

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Capt. John Deering

Well-known member
Mar 27, 2006
Reaction score
Cape Coral, Fl.
i have a 2001 200 ox-66 i use to be able to run on muffs, no more. i changed the thermostats, pulled lower end to check water pump, it looks new. i also use the muffs that provide dual cooling, ie. the muffs are split and water goes into each pcik-up point.

any ideas? can you guys run your motors on muffs?

might not be getting enough back-pressure....but it use to?
Well, you certaintly hit on the things I was going to suggest, (1) to use dual-feed muffs and (2) about the backpressure. To tell the truth IMHO the only diagnosis that can be done on the muffs is that it starts and stays running ... but only on muffs! I have seen OBs that ran 'great' on muffs but once put into the water and/or put into gear - they would stall out ... due to the backpressure and/or other issues.

I'd be more inclined to believe that something changed in the once of many variables. It could be the source of your water supply, e.g., hose, diameter, pressure from the city, etc., but note that the water route in an OB is complicated. Your new impeller might also be pumping 'differently', not with less force mind you, but maybe even with more force such that the hose water hits the deflector pins in the powerhead unlike raw water pumped solely by the impeller.

It also could be where you parked the boat from the hose (uphill or downhill), the length of the hose, and even the up/down position of the OB itself (that and ANY of the other variables mentioned could change how the water stream hit the deflector pins or routes through the powerleg or powerhead.

Now what did your analog temp guage do during this or do you just have warning lights ;? ? I'm not implying that you have a salt accumulation or blockage brewing inside there, rather let me put it this way. Whenever anyone ever tells me that their OB runs 'hot' on muffs ... the first thing I'd ask is "How does it run in the water?"

If all was well there (meaning when run in the water), I personally wouldn't lose sleep over it. I put so little value into running 150hp and up OBs on muffs that I will not, do not, fog them or do anything else on muffs. All I do in the Spring putting my V6 OB on the muffs is to start it to burn off most of the fogging oil before I hit the ramp, plus making sure she cranks, lights, and sputters to life ;) .

Regarding OB temperatures: FWIW I bought one of those non-contact laser "temp guns" that you simply point at a target for and pull the trigged to read the surface temp. I bought one for $35 from Harbor Freight after I did my recent OB engine rebuild and you can bet your boopy that I will check it at the start, middle, and end of each season! This is akin to the way a good diesel motor owner keeps an engine log. The gauge just lets me use quantitative data :) .


FYI - I moved this post to the "Power" forum ...
Could wasps have made a nest up in the water intake area of the lower unit? I've seen that happen before. For some reason, the glue they use to make those nests is water resistant and can hang in there long after you would think it would disolve.

The only other obvious answer is that your pressure control valve is stuck open. This would cause you to overheat at low RPMs. In any case, its very easy to check before you take any more extreme measures.

-- Tom
2001 OX66 225's were notorious for having factory-installed pressure control valves with orifices that were too small and debris would get lodged in them and keep them open. It was just enough too set off the overheat alarm at idle and be annoying, but not enough to damage the motor. 200 or 300 RPM gain would usually quiet the alarm. Yamaha changed the design, and when you (I) ordered a replacement, you (I) got a better-operating valve and the problem ended. There are two of them, they are inexpensive and easy to change out. Up in the remote area of Canada where we keep our 2520, with a marina new to Yamaha, it took over 2 years, a new water pump and thermostats before we tracked the problem down, and then it was with the help of the guys on the Yamaha parts forum. I'm sure other motors and other years besides our 2001 OX66 225 were affected by these valves.
The very first fishing trip I went on after buying my Parker, I was spring trolling south in the Bay on an outgoing tide. Due to the tide, the motor was running in gear, at idle, and only turning 600 rpm's.

After about 20 minutes of trolling, a horn began sounding...
Being new to the boat, I couldn't figure out what that noise was, and finally realized it was an overheat alarm. :shock:

The motor was shut down and raised to see if I'd picked up a plastic bag (none was found), so my next step was to call my dealer where the motor had just received it's spring commissioning service.

I explained to Harvey what had happened, and he told me that big-block OX66 Yamahas have a tendency to overheat if the engine rpm's are under 800 because at that speed the motor doesn't pump enough water to maintain proper cooling when run for long periods at that speed.

Harvey told me that they set idle in-gear, to the point right where the tach winks from 700 to 800 rpm while in their test tank... and that I should stop at their dock so a tech could bump mine up a bit, as 600 in-gear was too low.

Armed with that information I spent the rest of the season trying to keep the motor at 800 or above while in gear, but that wasn't always possible, especially when fishing the creeks for perch and pickerel.

The next year when I took my boat for its spring service, I mentioned that I was having some cooling issues, so that became an item on my maintenance ticket.
When the techs put my boat in the tank, they were able to replicate my problem, so they started looking for the "why"...

What they found were considerable salt deposits in the cooling passages that required cleaning out, as well as the replacement of the t-stats, poppet valves, and a water pump kit. I'm guessing that the previous owner had never given the motor a fresh water flush, much less a Salt-Away treatment.

Since that de-salting service by the dealer, I always give my motor a fresh water flush at least once a week using the Yamaha provided flush port at the base of the motor cowl. This past season I also started giving the motor a Salt-Away treatment about once a month in addition to the fresh water flush.

Since incorporating these maintenance procedures, I've only experienced one temperature alarm, and that was due to a plastic bag being picked up while running slowly in the Severn River. My overheat alarms while fishing the creeks have been completely eliminated.

Take this information for what it is worth, but it could be that you have been getting salt buildup over time which is just now making itself evident.
Of course, it's equally possible that I'm totally wrong. :roll:
UPDATE: changed the water pre. relief valve, aka poppit valve to the new style, but re-used the rubber grommit. my passage way was clear, unlike tom s's link. went to ramp, tested the motor in the water, no overheating or alarms, maybe as dale says don't worry about it.

ps: that poppit valve was a pain to change.

i also installed the new valve with the mushroom head facing the motor and the fins in the spring...hope thats right.
There must be some differences in the 2001 OX66 motors between the 200 HP and 225 HP that I wasn't aware of - the pressure relief valve on our 225 was easy to change out. Sorry yours was difficult, but I hope your overheating problem is solved. :?
thanks kingfish.........mine is under the cdi stuff look at link above. someone sugggested i make a dual hose assembly. basically, garden hose to "Y" branch to the muffs and the other to the washdown gizmo on the underside of the port side cowling. i think that makes sense. my house pressure is 60 psi. what do you guys think?
As I understand it, the flush port allows the water to enter the motor above the poppets and t-stats allowing you to flush the block after use. It cannot be used while the motor is running because the water pump and impeller would be running dry, as they are located in the foot.

Now... could you do it with two hoses? That would be a question best posed to Harvey or some other Yamaha tech, as I have absolutely no idea.

Since my boat sits in a wet slip, I have never needed to run the motor on a hose. I only use the flush port to give the motor a freshwater flush while sitting in the slip with the motor up, and off.
problem solved, i.e. running motor in driveway. i rigged up a dual feed hose system, with on branch going to the muffs and the other going to the cowling under port side cover, i used all quick disconnect fitting which makes connecting it al breeze. i also used the muffs that are single feed round cup. i tried the more $ ones but they are rectangular and feed from both sides...started her up no warning buzzzers and she runs cool.
Congratulations Capt!
Gotta love it when a plan comes together. :wink:
I remove the plastic grates where the water intake is when I use the muffs, being that it lets more water in with less restriction.
These large V6 OB's require a lot of water pressure and flow. It's not uncommon to read about them overheating while on the muffs.
before i removed those grates as i posted above my engine used to run hot on muffs, with them removed I have not had a problem, also bungy cord the muffs tight to the lower unit might help. Good luck!