F300 Overheats at high power settings but not at low to mid. What thinks you?

[fairdeal]

4.2 Liter 4-strokes have no water pressure control valve

[Keithbaja]

Thanks fairdeal. I've only seen one out here in Monterey bay. Never worked on one. Not popular around here

[panasonic]

61S5K9kjclS._AC_SL1500_.jpg

This would probably make a lot more sense. I could get away with using a lot less water and a lot less chemical, whichever I decide. Maybe even a simple drywall bucket, or a smaller 10-20 gallon tote. Plug this in and walk away. Let it run all day. 60 bucks.

https://www.amazon.com/MEDAS-Portabl...xpY2s9dHJ1ZQ==

I have a pump just like that for filling a 250 gallon fresh water tank from a brook next to our cabin...works great and never a problem with it for probably 20 years now.

I wonder where that piece of oring ended up in your cooling system. Have you tried back flushing from the thermostat housing and catching the water coming out of the water tube..LU removed of course.

Last time I did this was on a friends f70 which destroyed an impeller...it took three times before we got all the pieces out and the engine would not overheat at higher power settings.

[Keithbaja]

I recommend all the time to salt water boat owners. Use saltaway. Let run ten mins. or so (never walk away) turn motor off and leave the tight fitting muffs on. Let sit for a day or two with the saltaway in the motor. DO NOT flush with fresh water.. Very important !! U must have a watertight flush muffs. Which are like everything else is getting harder to find. I keep finding part #20 missing on overheat motors. Easy mistake to make. I did it myself, once, many YRs ago. If this gearcase has a upper/ forward water pickup. (U must check) besides the the usual side pickups,, check for blockages. If this gearcase has it/Them, close with duct tape...

[99yam40]

here are some questions for all.

why are there 2 #20s shown in this parts break down?
one on the pipe leaving pump housing and one where the drive shaft goes thru the water pump housing?
and why are there 2 oil seals #15 under that one #20 where the shaft goes thru?
#31 should be the oil seals under the water pump sealing the lower unit gear case
Yamaha Outboard 300HP OEM Parts Diagram for Lower Casing Drive 1 | Boats.net

[oldmako69]

The 20's - One is for the driveshaft and the other is for the water tube.

The 15's - There is an upper and a lower. They're about 3/4 of an inch apart.

One of my lower unit drive shafts shows scoring right where those two seals live. So at some point, the engine with the scoring injected sand. So it's probably a safe assumption that both have ingested sand. And what's even worse, these engines have water intakes on the bottom of the lower unit "bullet". I read somewhere that Yam put them there to augment the volume of water available. Apparently, the 300s were overheating in development. I've also seen YT vids where guys are chasing down O-Heat symptoms like mine find a lot of sand clogging something. I hope that simply flushing my engine for a few hours with Barnacle Buster will cure this but skeptical. I am also not anxious to start messing around with the engine's internals.

Regarding the missing two-inch piece of O-Ring #17, I would think that pressurized water would push it out and away from the impeller, but I don't know.

Here's what hit me at 0330 this AM. Late last year I noticed that one of my engines shucked its trim-tab zinc. It's AWOL. There's probably no way that missing zinc is causing this, but I sure do wish that it was. IF that 3-inch hole was actually pulling water IN, perhaps it might be "pressurizing the coolant path" to a point where it's causing the loss of water flow. Think of where it is, right on top of the prop. Perhaps at high power settings, enough water being slung off the prop is being forced uphill to cause this. I'll replace that ASAP. For starters, I can't recall which engine is missing the zinc. GAWD I hope its the overheater!

I want to run this thing, not wrench on it.

[99yam40]

The 20's - One is for the driveshaft and the other is for the water tube. I see that, but I have not noticed that cap or seals on the drive shaft on other motors

The 15's - There is an upper and a lower. They're about 3/4 of an inch apart. that shows those 2 oil seals on top of the impeller and there should never be oil around those seals

One of my lower unit drive shafts shows scoring right where those two seals live. So at some point, the engine with the scoring injected sand. So it's probably a safe assumption that both have ingested sand. And what's even worse, these engines have water intakes on the bottom of the lower unit "bullet". I read somewhere that Yam put them there to augment the volume of water available. Apparently, the 300s were overheating in development. I've also seen YT vids where guys are chasing down O-Heat symptoms like mine find a lot of sand clogging something. I hope that simply flushing my engine for a few hours with Barnacle Buster will cure this but skeptical. I am also not anxious to start messing around with the engine's internals.

Regarding the missing two-inch piece of O-Ring #17, I would think that pressurized water would push it out and away from the impeller, but I don't know.

Here's what hit me at 0330 this AM. Late last year I noticed that one of my engines shucked its trim-tab zinc. It's AWOL. There's probably no way that missing zinc is causing this, but I sure do wish that it was. IF that 3-inch hole was actually pulling water IN, perhaps it might be "pressurizing the coolant path" to a point where it's causing the loss of water flow. I'll replace that ASAP.

I want to run this thing, not wrench on it.

you say one of your motors is missing the zinc, is it the one that is overheating?
either way I would think it is a good idea to put a new one on there

checking to see if there is a path open for air to be sucked in maybe something to check on.
Seems like I remember some motors trim tabs were used to direct extra cooling water at hubs to keep them from overheating

[oldmako69]

Yes, one is missing. I noticed it last winter when I did the water pumps. Hopefully, the missing zinc is the culprit. But sitting here today I can't recall which engine is missing the zinc. I ordered a new zinc last winter. Just have not installed it yet.

Agree with your other observations.

[oldmako69]

The overheater has its cavitation plate zinc, so scratch that SWAG.

[99yam40]

maybe Boscoe can find out what those oil seals are for above the water pump impellor in the pump housing on drive shaft

[HMBJack]

I haven't read if you inspected your 4 Thermostats yet.

Perhaps the previous Owner never flushed your engines. Maybe salt crystal build up is waiting to be discovered.

Or perhaps that section of gasket that broke off went vertical and fouled a Thermostat. Just sayin...

[oldmako69]

Two thermo's on my engines. They are in fine shape and appear nearly new.

Update...

The whole flush thing with the pump was a total waste of time. That pump is gutless and will not push the water with enough pressure to do anything. None of that expensive juice got into the upper reaches of the powerhead. The impeller in that 50 dollar pump is smaller than the end of my pecker, and that's really, really small. I think the flush idea has merit, but not without a significantly larger and more powerful pump. You really need a small catch pan (LU OFF) to do this. That engine is simply too large to stick into a trash can. You'd need 30 gallons of solvent and that would be a nonstarter when that crap is 70 bucks a gallon. Plus, the exhaust blowing everything around, and the water ultimately overheating, etc. Nope, nope, nope.

On a hunch, I decided to dig into and replace my STBD water pump. I went to the local YAM store and bought the kit, including a new WP housing. While there I learned a few things. Here's the biggy...

If you recall I was bitching about the O-ring seal that lives on the bottom of the housing. The one that seals against the metal plate. I mentioned that the "groove" in the housing does not go all the way around the pump housing circumference. What is supposed to happen is this, once you stick the metal "insert cartridge" (part 16 in the diagram) into the WP housing, you flip the housing over and install the O ring (part 17). In the location where the groove is "missing" the metal insert stands ever so slightly proud of the housing and this creates a stop (or a seat) for the O ring. On my OLD water pump, this was not the case! Either the housing was ever so slightly off, or the insert was. The parts guy at Norfolk Marine pointed this out and told me that I am not the first guy to get screwed by this. He claims that there are a ton of counterfeit YAM parts available online and likely either my cap or insert (or both) were crap.

So, a new pump has been installed. Hope to get over to the yot today and put the Port LU back on the Port engine and install the Stbd LU on the Stbd engine and run it. I had swapped the P LU to the S side just to try and diagnose the issue. The P LU pumped water just fine through the S head.

This still does not explain why it overheated in the first place after a full season's use, but it just might prevent it from happening again. Fingers crossed.

[oldmako69]

Fixed.


fill

[mjm-1957]

Good for you, what a nightmare. I am a mechanic in a GMC Buick dealer and have been bitten by aftermarket bootleg shit parts before also.