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7.4 bravo 3 overheat

Old 08-17-2006, 10:41 AM
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Default 7.4 bravo 3 overheat

I'm trying to solve a cooling problem. Twin 7.4s with Bravo 3s. Starboard motor starts to get hot when the boat is on plane and the ocillation starts about 3000 rpms. Stock gen V 7.4. Normally runs 170 on the gauge. Engine has a perko flush device mounted inline on the intake hose to the raw water pump. No evidence it is sucking air. Once over 3000 rpms the temp gauge starts to swing between 110 and 200 with the frequency increasing with speed. There are no problems with timing.

The water inlet hose from the drive was supposedly replaced when I had the engines rebuilt 2 years ago. I have removed and checked all hoses and coolers from the transom water fitting forward. No debris or blockages. I have replaced the thermostat and checked the replacement in a pan on my stove. It opens at 160 and is fully open by 180. I replaced the raw water pump with a new impeller and housing. No effect. Old impeller was scored corresponding to the ridges in the pump, but otherwise ok. No effect. The belts have been checked for condition and tension. Last night I bought an infrared thermometer. Here is what I found at idle after a short run at 3000 rpms. I didn't have anyone to ride with me to check it at speed.

Intake manifold-165
Port elbow107
Port manifold 105
Starboard Elbow127
Starboard Manifold 120

It looks like my problem is in the starboard elbow or manifold. This engine engine has always warmed up faster and run 10 degrees hotter on the gauge than my port engine. Other recent work performed on the engine includes:
rebuild of outdrive, replacement of fuel pump.

My port engine for comparison
Intake manifold-160
Port elbow107
Port manifold 105
Starboard Elbow108
Starboard Manifold 105

I have recently replaced the starboard elbow gasket after pulling it to look for corrosion as part of annual maintenance. Found no corrosion in either riser or manifold. Even though I boat in saltwater, there was no visible rust although there were rust streaks on the outflow of the elbow. Engine is almost always flushed with salt away. Water intake on drive does not appear to have any obstruction.

I am going to pull starboard elbow again tonight. What should I be looking for that I might have missed? My thoughts as I am writing this.

1. Obstruction in riser
2. bad flapper valve in y pipe
3. turbulence from depth finder transducer that shows up when the boat is on plane (above 2400rpms) The transduce is installed about 18 inches starboard of my starboard drive gimbal housing around the time the overheat showed up)

Other recent work performed on the engine by my mechanic includes:
rebuild of outdrive, replacement of fuel pump.

Any thoughts would be appreciated.
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Old 08-17-2006, 10:53 AM
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Default Re: 7.4 bravo 3 overheat

i AM NOT FAMILIAR WITH THE SYSTEM ON THAT ENGINE, BUT IF IT WERE ONE OF OURS, THE CAUSE WOULD BE LACK OF HOLES IN THE CROWN OF THE THERMOSTAT. WE DRILL 3 5/32" HOLES IN THE STOPPER OF THE THERMOSTAT TO ALLOW SOME WARM WATER TO PASS THE THERMOSTAT AND LEVEL THE TEMPERATURE. ALL THERMOSTATS ARE ROAD BASED, AND EXPECT TO SEE THE SAME TEMPERATURE, MORE OR LESS, AT BOTH ENDS. WHEN THE THERMOSTAT IS CLOSED, THE BYPASS WATER HOLDS THE THERMOSTAT SHUT TILL THE LAST MINUTE. IT FINALLY POPS OPEN AND ALL THE HOT WATER COMES OUT. THEN IT SLAMS SHUT, AND STARTS ALL OVER. HOPE THIS IS OF SOME HELP.
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Old 08-21-2006, 07:26 AM
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Default Re: 7.4 bravo 3 overheat

For anyone with similar problems. After going through the entire cooling system, I tracked the problem down to a loose fuel line fitting which caused a high speed lean condition. Tightened the fitting and problem gone.
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