Overheating 7.4

[JamieH]

While waiting at idle for guests to arrive, I noticed the temp of the starboard engine creeping up. Never had any temp issues with either engine in 3 years and 150 hours of use. Up the revs on starboard and temp dropped suggesting a raw water blockage. Stopped and checked the strainer, all clear. Guests arrive and off we go, at cruising speed temp is dead normal. Upon docking, temp rises quickly, alarm goes off, shut down the starboard, dock with one engine in a crosswind and down current, no fun. Open the hatch and notice the starboard coolant reservoir is dry.Wait for everything to cool down, and add 1/2 gallon of coolant. Take her back out and temp is perfect for 15 minutes of idle and 30 minutes of steady cruise. Now I am scratching my head. Impellers are 2 months old, installed by my trusted mechanic. Before I call him in, wanted opinions from this wonderful knowledgeable forum. Could it be as simple as a stuck thermostat, or does the loss of coolant suggest a leak, gasket or manifold issue? Thanks for any guidance. Jamie

 

[FireIsland1]

It could be as simple as a new defective pump. the first item the water sees is the trans oil cooler. If this is blocked you will have big problems at idle. After that the water goes to the fuel cooler and then the heat exchanger. I had this problem once, I pulled hoses while the engine was at idle and found the restriction. Have a garden hose handy it can get very wet, very fast

 

[Alegria]

Well having just lived through a similar situation, I would say thermostat first. The only red flag I see is loss of coolant. In my case I know where it went as I wet vac'ed up the 1/2 gallon in the bilge. In my case it was the port engine and that trapped it, in your case it being the starboard engine did it leak into the lower sump and get pumped over???

 

[fc3]

Pretty obvious on a V-drive 330 if you have bad raw water flow. The exhaust system gets noisy. If you didn't notice the exhaust noise getting worse, then you probably had good raw water flow. Thus it's more reasonable to suspect the thermostat.

Best regards,
Frank

 

[tobnpr]

Loss of a 1/2 gallon of coolant means there must be a leak- somewhere; unless perhaps you just drained/replaced the coolant. When I do this I find I always have to add a bit after a while, I guess maybe because of trapped air in the system that works its way out.

When you opened the hatch did you get a whiff of that "coolant" smell?

Check your oil, be sure it's not milky/water contaminated. I'd then pressure-check the closed side of the cooling system. I had a similar problem of disappearing coolant- I bought the pressure test kit, and pumped it up. Sure enough, I could hear the drip of coolant hitting the bilge and tracked it to a loose hose clamp. A few turns of a screwdriver and problem soved. It wouldn't leak until it was under pressure.

 

[fc3]

Nah. A 7.4 can blow a gallon or more of coolant out the pressure cap if it's not closed completely. Ask me how I know.

OK, I'll tell you. I forgot to check the cap when the boat was new following some service work that the dealer had performed. I checked the oil, the transmissions, and some other stuff, but forgot to check that that coolant caps were secure. Shortly after startup, one of the engines overheated. Shutdown and checked it. Lots of coolant everywhere. Brought her back to the dock on one engine. I forgot how much coolant I had to add to fill the system, but it was about a gallon or so.

I flushed and refilled the system a little while after that. Don't trust overheated or air exposed OAT coolants.

Best regards,
Frank

 

[mwph]

Trying to learn so I gotta ask............OAT coolants?

 

[fc3]

Organic Acid Technology. Dexcool and others of it's type. Google it. They're good if you maintain them perfectly. But if you screw-up, OAT coolants will screw you back.

 

[mwph]

Im off to google world!

As always - Thanks.

 

[JamieH]

Thanks guys for the great advice. My guy is checking on it next week. I'll let you know what he finds.