Wait a second---hold the phones.

So now I'm thinking the problem has to do with oil clog/line/delivery/pump. So I double checked the oil level in the both the reserve tank (the bigger one inside the boat) and the on-engine tank, and both levels are fine.

So I start it up. Then, just for kicks, I wanted to see if I could trip the "low oil" alarm. So right after it started, I pulled the cap on top of the on-engine oil tank and took it completely out. I made sure the float sensor on the rod from inside the on-engine oil tank was showing low. And sure enough, the buzzer goes off. But it's the same buzzer sound (constant, not intermittent) that I thought was the "overheat" alarm! I was always under the impression that the constant buzzer meant overheat, while the intermittent buzzer meant low oil. But I guess I was wrong on this engine. I think this whole time, I've been getting then low oil alarm.

So after pulling the cap and getting the alarm, I also activated the "emergency" oil switch that is on the back of the engine; it's just a gray lever switch, like a light switch, that has a label next to it that says something like 'Use this lever to supply oil to the main tank when necessary.' I held this up for just a few seconds to get some oil in there. Then shut the engine off.

Then re-started. This time, everything worked perfectly. Engine started right up, idled just fine, pea stream like a champ, and I felt the top of the back of the engine heads to "feel" for how warm they were----and it was definitely not overheating; unlike these past few days, the metal was not as hot and was bearable to the touch. I ran the engine like this for about 10 minutes straight with no problems.

So now I am thinking this must be some kind of oil clog/line/delivery/pump issue. Why? Because the block felt cooler after I fed some oil into it.

I am going to go back and out and try again here in a little bit.

I might have actually fixed the problem if the float sensor inside the on-engine oil tank was stuck, so that could be possible. Future tests should confirm that.

But it's also possible that the oil is not being properly pumped into the engine. So my question: how do I check the oil pump to make sure it is actually pumping like it should?

Pull the boat mounted reservoir, inline filter in oil line. bottom outside of tank. #25 http://www.boats.net/parts/search/Yamaha/Outboard/2004/Z150TXRC/OIL%20TANK/parts.html

Did the second test of the day, and it performed flawlessly. Started up and ran for approximately 10 minutes, perfect pea stream, not overheating, reved up, no issues.

Gary, I will check that part just for safe-keeping. Will report back.

Ok, got an update.....

I put on the muffs this afternoon and fired her up. Ran flawlessly for approximately 10 minutes. While it was running I felt the block in several places for overheating, and it did not feel too hot to touch (like it did before). Also, the telltale had a good pea stream the entire test. I am going to assume that changing the impeller, thermostats and poppet-valve helped. Later today, I will check the operation of the oil line, as per Gary's suggestion. Will report back.

Glad to hear you are making progress. These motors aren't hard to take care of, and now you know when stuff was done and that it was done right.

After doing all the above work, I decided to go ahead and clean the carbs. I know, kind of a rookie move and a bit risky. So I removed the carbs and took them apart. [If you are going to do this yourself, be sure to take your time, take lots of pictures/videos, and clean each part thoroughly with carb cleaner. I am warning you---there are a lot of little parts, screws, etc.)

The middle and top carbs were almost perfectly clean except for a few dirt/dust bits on the choke valves; the bottom carb was the dirtiest as I found a small bit of debris in the bowl and one of the tiny jet screws appeared to be almost completely blocked. Then I cleaned them thoroughly with carb cleaner spray and blew them out with an air compressor.

After re-installing them, my first test run had a very high idle. After further inspection, I noticed the butterfly valves on the middle carbs were slightly open. So I removed this one and adjusted the butterfly valve slightly by tightening the spring. The spring felt much looser than the top and bottom carbs, which I figured contributed to the problem. Then tested again and all was well. I also turned down slightly the idle adjustment screw to make sure it idled just right.

I took before and after videos. I will try to post them shortly...

Here's the before and after videos for the carb cleaning:

Before:


After: