Mercruiser 454 Magnum Overheating

[lmorefield41]

Just snagged this boat from my good friend. 1985 23' Wellcraft Nova XL with a 454 Magnum 7.4L.

Issue:
While Idling, the temp goes well over 200* (gauge stops at 240*) and the alarm goes off. When at WOT (only 4000 RPMs), the temp comes down to approximately 160*-180*. Between 2000 RPMs and 3000 RPMs, the temp will sometimes go over 200* and other times it will stay between 160* and 180*.

What I've done so far:
Hooked muffs up to the drive in the driveway and found that no water is exiting the exhaust (through hull), which appears to be a water flow issue. I revved the engine up to 2000 RPMs and still no water came out of the exhaust. I didn't rev the engine too long as I don't want complaints of the loud noise from my neighbors. The manifolds and the elbows became very hot. Too hot to touch. Cleaned out the oil cooler which had an impeller vane (old impeller, 2012 I believe) as well as pieces broken off drive gasket in there. I also back flushed the oil cooler. I checked each hose for blockage involved in cooling the engine. I found that there appeared to be rust in one of the manifolds when I took the valve on the bottom off. I also took the belt that goes around the crank, the alternator, and the water pump, to see if I could freely turn the water pump, and I'm able to move it freely.

What I plan on doing:
Changing the impeller (housing, gaskets, etc). Removing the manifolds and elbows and inspecting to see if they can be cleaned or if they need replaced. Removing the drive and inspecting, back flush again with the drive off. Inspect for Zebra Muscles (terrible in Lake Erie which is where the boat was docked during the 2016 season.

I also plan on getting back after it on Thursday as Tuesday and Wednesday are pretty busy for me. I'm hoping to get some recommendations before I start removing things.

My goal was to use the boat the rest of this season and pull the engine to rebuild it over the winter as well as restore the upholstery. So far, this overheating problem looks like I may be pulling the engine for a complete overhaul and missing this season. I tried attaching photos of the serials as well as a video of no water coming out of the exhaust with the muffs on but it's saying my file(s) are too large. Thanks for the help in advance guys!

 

[shaw520]

You shouldn't let temps go over 200, Also, without a cooling source, the rapid increase in temp is very hard on all components. Flush system thoroughly and replace impeller.

 

[tpenfield]

I would say that you have a good plan. Do the stuff in the engine bay first and see if that remedies the situation, before you go to the extent of removing the outdrive, etc.

 

[ottawamerc]

I'm having a somewhat similar issue with a used 7.4 I put into my SeaRay this year. My temps were all over the place on its first water trials, so I tried firstly to pull off the hose at the thermostat from the sea water pump and briefly fired up the engine (while still in the water) to test for flow there. It showed me that the pump was flowing copious amounts of water and the oil cooler was not plugged. I trailerd the boat home and pulled the thermostat housing and I believe I found the issue there. The housing was badly obstructed and lots of debris was in the thermostat its self. I sand blasted the housing and let it soak in CLR over night. I bought a new thermostat kit at the dealer and a mechanic told me to not install the thermostat right away but to assemble the housing and run the engine to flush it out. I did that and ran it on muffs and wow the exiting water was visibly brown!
I reinstalled the thermostat after awhile and ran it again and it warmed up to 160* and stayed steady. I have not water tested yet so I'm not sure 100% it's fixed but I'm sure it is better. Just an idea for you to check too. BTW mine is a similar vintage 454 (late 80's)