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Old 07-30-2019, 12:31 AM   #1
rwc
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Default Overheat issue

After three years, I finally managed to get my propeller replaced with the suitable pitch. I haven't posted in a while because I was feeling guilty about my lack of care in getting the right prop on, so reintroduction: 2004 Maxum 2500SE, 5.7L MCM Mercruiser with 2 bbl carb and Bravo 3 drive.

I put the boat back in the water with a great deal of excitement to test out the new prop and after 30 seconds it stalled in a strong wind, resulting in many dramatics and me vaulting off the front onto a dock finger to catch the boat. Not so great. After much stalling, fussing and replacing the fuel filter, I decided to rebuild the carburetor. That was a good plan as it was filthy. So now the engine is running well, the propellers are great, the performance is awesome (4600RPM at light and 4400 heavy condition, very quick to get up on plane) - but the engine overheats (sort of).

Prior to this latest bout of maintenance, once warm, the engine would run at exactly 175 degrees, rock steady. Now it rises to between 175 and 185 when on plane. It doesn't seem to go beyond the low 180's most of the time, but unfortunately, the high temp alarm goes off at 180, which is really disconcerting. It also will now cool to below 175 when not on plane and very quickly cool with reduced RPM (down to about 170), which it didn't do before.

Can anyone help me with diagnosing what changed and what I should try? Much appreciated.
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Old 07-30-2019, 01:26 AM   #2
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Should have mentioned I replaced the raw water impeller first because it was two years old and due anyway and I hoped that was the problem. Sadly nothing changed and the old one was intact.

At the same time, I backflushed the system with fresh water from a hose from port elbow inlet to raw water pump outlet to see if anything would come out. Nothing. I have no idea if that would work, but seemed like a good improvised method of dislodging potential blockage.
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Old 07-30-2019, 01:54 AM   #3
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I would replace the thermostat based on the fluctuations. Get a IR temp sensor to take readings of the thermostat housing and exhaust manifolds to get a better feel of overall component temps.
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Old 07-30-2019, 02:17 AM   #4
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Thanks mmjwr! You are the best.

I suspected that was the next step but probably would have agonized over it for days without advice.

Is the IR temp sensor one of those pointable thermometer things? I saw the other thread where the guy had the FLIR camera image but my only experience with those cost upwards of $80k... can I rent the sensor? Are there cheap yet serviceable versions?
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Old 07-30-2019, 03:07 AM   #5
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Found the IR temp guns. They look quite reasonably priced!
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Old 07-30-2019, 11:20 AM   #6
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Quote:
Originally Posted by rwc View Post
Found the IR temp guns. They look quite reasonably priced!
Yup that's what I use, FLIR is nice but overkill for the majority of us.
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Old 07-30-2019, 03:09 PM   #7
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I have the exact same boat but with the 6.2L Merc. I love it. Nice boat
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Old 07-30-2019, 03:53 PM   #8
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Quote:
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I have the exact same boat but with the 6.2L Merc. I love it. Nice boat
Is the 6.2L carburetated or injected? Is it also 2004 or is it a refit? If refit, did you do it and was it hard?

I love the boat and it's just the right size for us, but engine has 860 hours on it, is very rusty (previous owner must have had an elbow failure), and my wife hates carburetors. She really wants to refit a newer, injected engine.
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Old 07-30-2019, 04:31 PM   #9
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2004 2500SE 6.2l injected.

Fresh water boat with no rust and the engine has 540 hours on it. Stock motor.

I agree it's just about the right size. I like the fact I can stand up in the galley but it's not too big to prevent us from getting into some of the nicer canyons on our lake.
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Old 08-08-2019, 02:29 AM   #10
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Hi folks,

So I replaced the thermostat, changed the coolant, backflushed the raw water system, and inspected the heat exchanger. Nothing notable came out of the system and the old thermostat looked fine.

I haven't bought the temp gun yet to get a feel for temperatures on things, but will do this soon.

No progress so far. The temperature still holds steady at +/- 5 degrees of 175 with the alarm sounding briefly at ~+5 then shutting on/off as I change RPM. Temp doesn't rise above about ~180 on the dial. I now know that the thermostat is/should be opening at 160, so I'm figuring I must have a blockage or faulty sensors.

Next thoughts are that either I have a faulty temperature sender or heavily corroded elbows. I'm hoping you can help me with a few questions.

Can I change the temperature sensor without draining coolant again? Do I have to drain coolant to inspect the elbows? Do I have to /should I replace the gaskets on the elbows (they are the metal ones) if I remove them to inspect? How do I tell if the elbows are too corroded? I know they will have some corrosion as the water in the pipe connected to the elbow was rusty when I removed it and it was only sitting a week or so prior to that.

Any help in next steps or possible diagnosis would be much appreciated. Thanks.
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Old 08-08-2019, 02:35 AM   #11
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Also, when I drained the coolant it looked pretty dirty. Is there something I should have done to flush the system? I just removed the port side block plug, drained it, until no more came out, then replaced the plug and refilled with new coolant/water at the heat exchanger.

Should I have flushed with water? Removed both plugs? Is there an easier way to refill (I refilled half, ran the engine until the thermostat opened and coolant drained into the engine, shut down and refilled the other half - this seemed like it might have been a bit of a hack method).
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Old 08-08-2019, 11:34 AM   #12
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Can I change the temperature sensor without draining coolant again?
If the Heatexchanger is higher than the sensor you'll need to drain it down to the sensor level. The engine block can stay filled.

Do I have to drain coolant to inspect the elbows?
Are the exhaust manifolds raw water or closed cooled? If closed you'll need to drain the system down most of the way. If raw water drain them completely is easiest.

Do I have to /should I replace the gaskets on the elbows (they are the metal ones) if I remove them to inspect?

YES!

How do I tell if the elbows are too corroded?
If the wall to exhaust passage is thin and or the port where the raw water injects into the exhaust is badly corroded. Post some pictures.
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