5.7 Osi engine temps stumping me

[The Revenge]

Port engine: Idle out it warms up normally. On plane runs at 175. Come off plane it creeps up to 190-200. Then on plane goes back to 175. Had a service call done on it over the weekend because after plane it now started to over heat. We changed the thermostat, impeller brand new. Took out for test drive. Come off plane boat over heated. Shut it down revved to about 1500 for a few seconds. Temp came down drastically. Drove the boat another 5 hours and the temp wouldn't go above 140. Then at idle it drops down to just over 100. Temps are accurate with gun. I've been told now it's running to cold and we may have bent the new thermostat when it over heated. Also checked block pressure while running to see if it was sucking air at outdrive. XDP outdrives.

Starboard engine: Warms up normally at idle. Runs at 175 on plane and now it's starting to creep up to 180-190 when back down to idle. Get back on plane it drops to 175. Impeller new, going to change this thermostat also.

Anyone have an idea why the engines want to creep up to 190-200 after coming off plane?
Would a thermostat stuck open on the port engine cause it to run 40-50 degrees cooler now. 100-110 at idle and 140 on plane?
Thanks

 

[Don S]

Re: 5.7 Osi engine temps stumping me

Do you run in salt water? What's the condition of the exhaust manifolds and risers?
If they start plugging up, the flow of cooling water through the engine decreases, when coming off plane you have a lot of heat built up through every part of the engine, but not enough flow of cooling water at lower rpm. The pump puts out enough flow, the manifolds and risers just restrict the amount of outgoing water.

 

[The Revenge]

Re: 5.7 Osi engine temps stumping me

Salt water only once or twice a year. Engines are 2006 with 200 hr on them. Boat stays on trailer also. Are the risers the portion of the exhaust that rises from on top of what would be the normal exhaust manifold? Or are they the big tubes connected to them via a rubber connector with hose clamps into the metal down tubes? Thanks

 

[The Revenge]

Re: 5.7 Osi engine temps stumping me

I'm looking at the engine manual now. I see exactly what the risers are. Wasn't sure if that included the big down tubes to the transom. If I change the risers do I need to replace the down tubes also? Do they get blocked? There pretty large to get blocked.

 

[DriftR]

Re: 5.7 Osi engine temps stumping me

I have the 5.7 Osi single with the composite DXP duoprop. I too had an overheat problem from the get go. Like you, I changed out the thermostat twice as well as the impeller twice. My system is the closed cooling system. I finally found the problem and have had no problems since. It was the heater system that ran off the engine block. My heater was located in the cuddy 20' from the motor. The 5.7 was pushing that water thru the heater system and it did not have enough power to do so. I installed a small in line pump near the cuddy heater. Also installed a valve near the engine block. I keep that valve in the off position during warm months. If I need heat up in the cuddy I open the valve and flip a switch on the helm to activate the small pump to help push water thru the cuddy heater system. I fought that overheat problem for a long time. Gallons upon gallons of antifreeze and countless calls to VP. Luckily this solved my problem. I run offshore from May thru Sept off the central Oregon Coast. It is not a place where one needs overheating problems......