Mercruiser 3.0L Overheated What to Look Out For?

[frostystorm]

I was winterizing my boat and the first thing on my check list was to replace the fuel filter since it's never ever been done (it needed it).

I guess on the muffs there wasn't enough water coming in, misaligned muffs, or something else, that I wasn't getting enough water to the impeller while attempting to crank an engine with no fuel it just wasn't getting enough water into the lower unit and toasted the impeller. So I am in for that job... sigh

But I didn't notice this until the engine hit 200 degrees and there was quite a bit of steam coming from the idle relief ports. I would say it ran at 200 for probably 30 seconds as I went to the garage to get a bunch of stuff I needed.

I shut it down, finished winterizing (pumped oil, new oil and filter, checked the lower unit for water in the gear lube), and I did compression across all 4 once it cooled down, 145 PSI across all 4, cylinder 2 was 120 the first time I checked it but I re-checked it because it's under the shift linkage so I just think I wasn't tight enough.

Besides compression which is fine, anything else I should keep an eye on? Anything I should look for while I have the lower unit off this winter for a new impeller?

 

[Scott Danforth]

first, the drive should be coming off every year for the annual drive service.

next, re-test the compression.

if your engine got to 200F, you toasted the flapper and burned up the other rubber bits in the exhaust, like the two rubber hoses. also, the lower shift cable normally runs past the exhaust pipe. with any overheat, the lower shift cable sheath gets melted.

 

[alldodge]

Most gauges are not accurate, in case yours was it may have caused an issue, time will tell

You could let it go or dive in further and find out

140 to 120 is not great but not terrible

 

[Chris1956]

Gee, check the stuff Scott mentioned, replace the impeller and test the motor. We had 195* thermostats in our cars in the 70's, so if it really was just 200*, it may be fine.

When you replace the impeller, replace the wearplatee and two gaskets. Inspect the pump base for melting, and the pump cover for scratches in the SS cup. Replace all bad components.

Flappers keep water from rising in the exhaust pipe under fast deceleration. That should only happen if you hit something, and then the drive should kick up to prevent the water entering anyway.

 

[frostystorm]

@Scott Danforth My exhaust flappers are still there, I pulled the rubber booties on my riser, it's still in decent shape, so I am guessing everything downpipe of it is fine.

@Chris1956 Yeah the 3.0L Iron block I am not worried about, they'll run longer with basically no compression than most engines will run, and hell getting water in a cylinder is cleaning and reseating the valves before the new gasket goes on haha. And I am at 145 across all 4 which is right where the 3.0 should be at it's compression. The car engine I am okay with, it's all the boat stuff that's ??? to me.

 

[Chris1956]

Although I do not know if the 3.0l was ever used in a car, it is the same basic design as many that were used in cars, like the 4.3L V6 which is the v6 version of the venerable small block chevy 283 CID V8 used in the '56 and newer Chevys. They are pretty tough motors due to their cast iron blocks and heads, and their primitive (by today's standards) and robust design.

Marine versions of these engines run cold thermostats to keep corrosion down. Cars can run at higher, more efficient temps, with their antifreeze coolant systems. Your motor ran just a bit hotter than a car version of that motor would. It is likely no damage occurred to the engine.

 

[Scott Danforth]

The 3.0 is a fork truck motor (never in a vehicle). It's a bored and stroked version of the 4-cylinder used in the 1962 Chev nova

 

[Scott06]

I was winterizing my boat and the first thing on my check list was to replace the fuel filter since it's never ever been done (it needed it).

I guess on the muffs there wasn't enough water coming in, misaligned muffs, or something else, that I wasn't getting enough water to the impeller while attempting to crank an engine with no fuel it just wasn't getting enough water into the lower unit and toasted the impeller. So I am in for that job... sigh

But I didn't notice this until the engine hit 200 degrees and there was quite a bit of steam coming from the idle relief ports. I would say it ran at 200 for probably 30 seconds as I went to the garage to get a bunch of stuff I needed.

I shut it down, finished winterizing (pumped oil, new oil and filter, checked the lower unit for water in the gear lube), and I did compression across all 4 once it cooled down, 145 PSI across all 4, cylinder 2 was 120 the first time I checked it but I re-checked it because it's under the shift linkage so I just think I wasn't tight enough.

Besides compression which is fine, anything else I should keep an eye on? Anything I should look for while I have the lower unit off this winter for a new impeller?

200 is not that hot for that time period you are probably fine.

If your compression is still good just keep an eye out for milk in the oil that would be evidence of water intrusion.

When you do the impeller use OEM merc parts as they last longer. Not sure if this is a Gen one or Gen 2 drive. If gen one would get the housing as well. Gen 2 with SS housing just clean up with emery cloth unless it is scored significantly.

While drive is off pressure test it for leaks. Inspect U-joints bellows gimbal bearing and shift cable. Did the lube come out of the drive clean ?