windsurfnut

Had an impeller issue yesterday, overheated the motor (5.7 efi merc) shut it down, let it cool and limped back in. Temp hit ~200 degrees when the alarm went off. Smoke was in the engine compartment. mostly from a blower hose that was on one of the exhaust downpipes. When limping in the temp hovered around the 185 mark, no alarm sounding. How hot is too hot with these engines and what damage should I look for. I replaced the impeller and water is flowing good now. I'm going to change the oil and get that fresh, but other than that, what other damage was likely?

It seemed stumbly at idle today, temp gauge is all over the place, you can see the thermostat open and close, water is cold still in the lake. Boat use to stay at 175 degrees all day long, but that may have been the impeller going last year. Is it normal to see the temp go from 100 degrees to 150-175 and back down again so quickly? Again it is may and the water is only 59 degrees. How hot would I have had to get the motor to cause piston/ring etc damage? Thanks again, Jeff


also had a hard time starting after sitting for 2 hours after a good run. Engine was stumbling at idle I'm on last years gas still, so I'm not ruling that out, but my boat has always started within 5 sec of turning the key. again it was stumbly then

sleeper_dave

those motors can get really damn hot and not have a problem.

if it only hit 200 degrees you're fine. they run close to 200 degrees during normal operation in cars.

windsurfnut

There was Zero water running through it, only the antifreeze that was left over from winterization. Water had not made it as far as the thermostat housing.

What about the erratic temp readings?

sleeper_dave

We overheated my friends 350 last year. Maxed out the temp gauge, burned the boots on the exhaust, melted the water pump housing. The only thing keeping it from running right (after fixing the water pump) was a melted (sticking) shifter cable that had been sitting against the lower y-pipe when it overheated. When he shifted into gear the ignition would cut out (alpha drives have an ignition cutout that momentarily cuts ignition when you shift... if the cable sticks, it's not momentary anymore and the motor dies).

I would say just change the oil before you run it again, if it got really hot. Other than that, just fix the water pump and fire it up.

If it started to burn the boots between the exhaust risers and the y-pipe, replace them. If they don't leak now, they'll crack and leak. Later in the season my friend had one go out, because it was burnt and cracked from the overheating.

As for the erratic temp readings, that is probably something wrong. Maybe a thermostat acting up, maybe a water flow issue. Swap the thermostat and see if that makes a difference.

windsurfnut

Thanks for the help. Can the boots be done in the water, or are they too low and allow water in?

windsurfnut

What kind of temps do piston/ring or head gasket damage?

Darkspoilerzx

I would check tht the thermostat is in the right spot and the spacer is init....that may make the temps fluctuate like that

sleeper_dave

Depends on the boat, for the boots. My friend was able to do those boots in the water. On my boat, water would come rushing in if i tried to replace them in the water.

As for temps to do head gasket / piston damage, hard to say. It's going to depend on a lot of things. Aluminum heads would be a lot more susceptible to damage due to overheating. If you overheat when you're running real hard and get detonation as a result, you could get piston / ring damage. If you're idling, it would have to get a lot hotter to do damage. Too many things come into play.

Wobble

It's a real crapshoot with those motors, if the exhaust rubber didnt burn you may be ok, best bet would be a compression check.

Unfortunately the temp guage does not indicate temp very well when no water is flowing. I saw 3 smallblocks that blew head gaskets last year with very mild seeming overheats.

jmherbert

That makes 4 with the addition of mine. Stupid brand new mercruiser circ pump.