5.7 merc/bravo2 temp creep - Maxum Boat Owners Club

geoffm70 · 08-08-2022, 09:30 PM

Have a strange one for a '94 3200SCR. Temp on one engine (5.7 Merc carb/bravo2) that creeps up to 195F when on plane and at 3400rpm or more. Alarm doesn't go off, everything runs and works fine, when we come off plane temp comes right back down to 170/160 where the other engine always is. I've replaced impeller, (tiny nick in one blade) cleaned heat exchanger and new exhaust. Ran on plane for an hour at 3600 and it still does it. Temp never went over 195F, never an alarm. Is this anything to worry about?

mmwjr · 08-08-2022, 10:33 PM

Could be Bravoitis. This is where the transom hose retainers get clogged with growth. The retainers are plastic insert that hold the hose in place. Very common problem. When was the transom assembly last rebuilt?

geoffm70 · 08-08-2022, 10:55 PM

due to a log strike, had both legs removed, this leg with the heat issue was completely rebiult.

mmwjr · 08-09-2022, 01:05 AM

Quote:

Originally Posted by geoffm70

due to a log strike, had both legs removed, this leg with the heat issue was completely rebiult.

I’m taking about the transom assembly that the leg bolts to. If this was rebuilt as well I would perform a water flow test by removing the raw water hose to goes to the heat exchanger and verify the water volume per the Mercruiser manual. When was the antifreeze and thermostat last replaced?

geoffm70 · 08-09-2022, 02:25 AM

This wasn't touched, seemed fine but maybe not. Change antifreeze and termostate wasn't change in March wheb the exhaust risers were replaced.

shrew · 08-09-2022, 07:02 PM

The issue Mike is describing is the passage through the aluminum transom assembly. The transom assembly itself would corrode and effectively squeeze the intake hose passing though it, restricting incoming raw water cooling flow.

The other place to look is whether you have closed cooling and are loosing antifreeze from the heat exchanger. The closed cooling anti-freeze side is much higher pressure than the raw water cooling side. When pin-holes eventually develop in the heat exchanger, antifreezer can escape into the raw water and get discharged with the exhaust discharge. As the AF levels lower, their ability to cool decreases.

The last place to look is the exchange manifolds and risers. Even in a closed cooling engine, these are usually raw water cooled. If you're in saltwater, these need to be replaced about every 5 years.

geoffm70 · 05-12-2023, 09:35 PM

I have temp creep again in my newer engine (180hrs on 5.7 Merc, carb/bravo2).
Last August ('22, my 2nd with this boat) started to get high temp readings (190 to 195) on what was then a 165hr engine. Replaced the 142degree thermostat with a 160, new impeller, heat exchanger was removed and cleaned at a part shop and then put a new seacock in with a direct 1" hose for water intake. Also done previously (March '22), new risers, exhaust manifolds and bellows. Heat alarm was tested and seems to be working fine; at around 200 it goes off.

1st trip this year, for more than 20min at cruise (3600rpm) and the creep started again. Ran her for almost a full hour at 3600, alarm never sounded and the gauge never when higher than 195ish. Throttled down to 1700 and engine started to cool immediately, all the way down to 155/160. Running back that day, she did the same thing again to 195. Then yesterday, again everything is fine, for about 20min, then she creeps to 195 in about 10min. Tested with a heat gun at the thermostat and its reading between 190 and topped out at 199.

This is bad right? I've attached a picture of the culprit with the seacock.

mmwjr · 05-13-2023, 01:04 AM

Quote:

Originally Posted by geoffm70

I have temp creep again in my newer engine (180hrs on 5.7 Merc, carb/bravo2).
Last August ('22, my 2nd with this boat) started to get high temp readings (190 to 195) on what was then a 165hr engine. Replaced the 142degree thermostat with a 160, new impeller, heat exchanger was removed and cleaned at a part shop and then put a new seacock in with a direct 1" hose for water intake. Also done previously (March '22), new risers, exhaust manifolds and bellows. Heat alarm was tested and seems to be working fine; at around 200 it goes off.

1st trip this year, for more than 20min at cruise (3600rpm) and the creep started again. Ran her for almost a full hour at 3600, alarm never sounded and the gauge never when higher than 195ish. Throttled down to 1700 and engine started to cool immediately, all the way down to 155/160. Running back that day, she did the same thing again to 195. Then yesterday, again everything is fine, for about 20min, then she creeps to 195 in about 10min. Tested with a heat gun at the thermostat and its reading between 190 and topped out at 199.

This is bad right? I've attached a picture of the culprit with the seacock.

What thermostat is in the engine? It should be 160 *F. My 5.7 with closed cooling never gets above 175*F and it uses the Bravo water pick up.

geoffm70 · 05-13-2023, 01:15 AM

I changed the thermostat from 142 to 160. Beginning to really worry about what I thought was a newish engine.

mmwjr · 05-13-2023, 01:00 PM

Quote:

Originally Posted by geoffm70

I changed the thermostat from 142 to 160. Beginning to really worry about what I thought was a newish engine.

160 is the correct thermostat for closed cooling system you have. I would be concerned as well. Have you verified that the coolant level is correct in the heat exchanger?

geoffm70 · 05-13-2023, 02:52 PM

Going to get a coat hanger and go through the tubes today. Fingers and ties crossed I find something.

geoffm70 · 06-01-2023, 11:27 PM

Last summer we put in a seacock for direct raw water supply to my impeller because the boat was overheating on plane.

Well that didn’t really fix it, temps went from 200 to abt 180, but now, once on plane and over 3000 rpm, temps quickly jump to 195 again.

Come off plane, cools right down to 160. Stay off plane, 155 to 170 all day long. I’ve used a heat gun to test these temps. Risers never got too hot to touch.

New:
(engine has less than 200hrs as of Jun’23 )
Seacock and direct raw water to impeller (july’22)
T-stat, 142 degrees (May’23)
Impeller and impeller housing with no missing pieces from old one (May ’23)
Water circulation pump (May ’23)
Heat exchanger has been removed, cleaned and tested at a rad shop, no issues. (May’23)
Risers and manifolds (June’22)
New alternator (Sept ’22)
Timing is perfect (May 31, ’23)
Fresh 50/50 GM antifreeze (May ’23)
Compression in cylinders is good to excellent (ditto)

What am I missing?

mmwjr · 06-02-2023, 02:41 AM

I would do a water flow test per the Mercruiser manual. This is for the raw water side to verify there is proper flow rate per rpm. Possible blockage in the power steering cooler.

geoffm70 · 06-02-2023, 02:55 AM

We've had some incredible weather here in May and I've been battling this for almost 3 weeks now. Power steering cooler, noted, also next.

geoffm70 · 06-02-2023, 05:58 PM

nothing in there, looks almost new, seems like its just a pass through anyway?

mmwjr · 06-02-2023, 07:49 PM

Quote:

Originally Posted by geoffm70

nothing in there, looks almost new, seems like its just a pass through anyway?

Mainly a pass through but it is a heat exchanger to remove heat from the PS fluid. So I would perform the flow test to verify the proper water flow is getting to the heat exchanger. Then pull hoses off exhaust to check water flow to them. If you have a full closed cooling system you only need to pull the riser hoses.

geoffm70 · 06-12-2023, 05:10 PM

That only took almost 4 weeks, but i replumbed to a 'half closed' cooling system. There didn't appear to be anything wrong with the water flow, but this new system works, done!