Yamaha F250B Overheat at Idle

[shotgun16]

Would appreciate some input / opinions on an issue I am having with my port engine.
Engines are Yamaha F250B whose production date was 8 / 2008 and purchased new as leftover on 8 / 2012. The engines have 400 hrs, always maintained, flushed etc, etc. The engines are still under warranty .
Took her offshore for the first time this season two weeks ago, and ran fine, till we were drifting around for awhile with the engines idling and the port overheat alarm goes off. Shut her down, then start her up again, no alarm for awhile but eventually goes off again. When alarms goes off says engine overheat but temp gauge right down the middle. Runs fine at all other speeds. Next day at the dock start them up and after awhile same thing happens again.
Background- New thermostats and water pumps (complete kit with housing) were installed in the spring before launch. (I replace my tstats every two years and pumps every season)
Take her in to service and they hook up to computer while idling. Confirm that the block is indeed overheating (no faulty switch / sensor). Temp slowly climbs to 200, and alarms goes off, but if you give it a rev in neutral temp comes right back down. First thing done is to swap out the t-stats just in case the new ones were defective, no go. Next hauled the boat and dropped the LU to thoroughly check the water pump assembly- no issues with impeller, housing, gaskets, seals, etc. everything looks good. Change the PCV valve, put her back in and still happening.
Next thing up is to scope the exhaust stack, so now I am getting pretty upset. I didn't think the corrosion issues affected this late of a model Yamaha especially with less than 5 seasons and 400 hrs on her. Will have results next week. Anyone hear of these engines having corrosion issues too? If not corrosion, anyone have any ideas of what else could be causing this idle overheat? Never had any issues with this engine before.

[TownsendsFJR1300]

Sounds like you covered most everything.

The "quick rev" and the subsequent lowering of the engine temp indicates (obviously), not enough water getting/ staying in the powerhead..

Just for S&G's, did you check the LU pick up screens for any debris?

[Jason2tpa]

I didn't think the corrosion issues affected this late of a model Yamaha especially with less than 5 seasons and 400 hrs on her. Will have results next week. Anyone hear of these engines having corrosion issues too?.

Saltwater corrosion is non biased. It pays no attention to manufacturer, model, or year. I'm not implying corrosion is your issue here, just stating the facts. Im not experienced with running twins so I can't tell ya if it's abnormal for one engine to have corrosion when the other does not. Perhaps someone who runs twins can comment about that. But it wouldn't surprise if it does happen. See my first statement as to why.

Is there a noticeable difference of T/T stream between the 2 engines? If you're uncertain, run both on flush. Hold an empty water bottle at the same time under each stream. Theoretically they should both fill up at the same time, so long as both engines are receiving the same amount of water pressure.

[rejesterd]

Did they actually measure the temperature with another instrument, or did they just look at the YDS output on a computer? If the latter, then it still could be an issue with the cooling water temperature sensor (as opposed to the thermostat).

And I know the thermostats are new, but did you actually test them and see if they open at the expected temperature? Maybe the ones you installed are just defective. Unlikely, but what you describe (overheat at idle, fine at other speeds) sounds like a sticky valve. Thermostat or poppet valve maybe.

[shotgun16]

Saltwater corrosion is non biased. It pays no attention to manufacturer, model, or year. I'm not implying corrosion is your issue here, just stating the facts. Im not experienced with running twins so I can't tell ya if it's abnormal for one engine to have corrosion when the other does not. Perhaps someone who runs twins can comment about that. But it wouldn't surprise if it does happen. See my first statement as to why.

Is there a noticeable difference of T/T stream between the 2 engines? If you're uncertain, run both on flush. Hold an empty water bottle at the same time under each stream. Theoretically they should both fill up at the same time, so long as both engines are receiving the same amount of water pressure.

The stream from the problem engine might be a little weaker than the other when idling, tough to tell visually, what I have found interesting is that on the port engine I can't get a stream out of the weep hole when flushing even with the engine tilted up (in years past I could), as opposed to starboard engine where I can get stream with engine tilted. I have tried to clean the weep hole tube with wire, but it doesn't make a difference.

[shotgun16]

Did they actually measure the temperature with another instrument, or did they just look at the YDS output on a computer? If the latter, then it still could be an issue with the cooling water temperature sensor (as opposed to the thermostat).

And I know the thermostats are new, but did you actually test them and see if they open at the expected temperature? Maybe the ones you installed are just defective. Unlikely, but what you describe (overheat at idle, fine at other speeds) sounds like a sticky valve. Thermostat or poppet valve maybe.

The temp is coming from the YDS, so I guess the sensor could still be bad, but with the scenario indicated in the live temperature readout, seems unlikely to me. The thermostats were new when I put the boat in, so they swapped out those new ones (just in case they were defective), with another new set, and the engine still overheat. The poppet / PCV was replaced with new one and problem persists.

[TownsendsFJR1300]

Basic question, especially regarding the tell tale.

When they replaced the impeller, was it missing ANY PIECES, or was it all there...

Also, maybe coincidental, but look / remove, check water lines (one at a time if need be) till you find the blockage with the tell tale. What ever is clogging that may very well be clogging / deflecting water flow

[shotgun16]

Basic question, especially regarding the tell tale.

When they replaced the impeller, was it missing ANY PIECES, or was it all there...

Also, maybe coincidental, but look / remove, check water lines (one at a time if need be) till you find the blockage with the tell tale. What ever is clogging that may very well be clogging / deflecting water flow

As far as the impeller, to be honest, I didn't inspect upon replacement so I can say if it had any wear. Since I have the pumps done every year to avoid pump problems, I am not super vigilant about inspecting the old one, unless I have some sort of event during the season. It has been in my mind that something may be restricting water flow in cooling passages, and that could be connected to lack of flow in tell tale on flush. If scope comes up clean, I will revisit this with mechanic, as when I originally brought blockage up as possibility, he felt more that if there was a water flow restriction, you would see it at higher rpms as well

[pstephens46]

Have the head or exhaust anodes been changed recently? Sometime they can break apart on removal. Chunks blocking water flow. Haven't heard that occurring much with Yamaha though. Just a wild guess.

[shotgun16]

Have the head or exhaust anodes been changed recently? Sometime they can break apart on removal. Chunks blocking water flow. Haven't heard that occurring much with Yamaha though. Just a wild guess.

I change the head anodes every year, only thing I notice on the anodes is a little scale build up, always come out in one piece.

[rejesterd]

The temp is coming from the YDS, so I guess the sensor could still be bad, but with the scenario indicated in the live temperature readout, seems unlikely to me. The thermostats were new when I put the boat in, so they swapped out those new ones (just in case they were defective), with another new set, and the engine still overheat. The poppet / PCV was replaced with new one and problem persists.

Agreed, it's unlikely that those newer parts are defective. I just know that if it's not a sensor/electrical issue, then it's a physical blockage due to a clog in the cooling system or corrosion. I would just be very surprised if it is due to salt water if you've been flushing it regularly. Good luck.

[99yam40]

I thought dry exhaust stack corrosion had nothing to do with salt water wetted areas

[pstephens46]

Yes, Rodbolt has mentioned many times flushing has no bearing on that particular corrosion issue. If that is the issue.

[fairdeal]

I thought dry exhaust stack corrosion had nothing to do with salt water wetted areas

Yes, Rodbolt has mentioned many times flushing has no bearing on that particular corrosion issue. If that is the issue.

True that the surfaces are not touched either by the pumped cooling water,
or by any normal flushing.

But certainly when an F200/225 is on the boat, in the water - but not running -
the interior of the muffler must be wetted -
without any exhaust pressure forcing it out, the water "seeks its own level" back through the propeller and up into the interior of the exhaust -
rising easily as high as the bottom of the oil pan, and potentially higher -
say, when anchored in heavy swells

the green is the normal water line:

[dfcanuck]

If your Poppit (PCV) valve is stuck open, it could cause your symptoms by dumping excess water out the exhaust at idle (when it shouldn't).