I have a 2001 Bayliner with 3.0l Mercruiser. It has an aftermarket Seakamp heat exchanger. The boat ran flawlessly until last summer when it would start to overheat when I’d slow down to no wake speed or throw it in neutral. The temperature will continue to rise until I either idle up to a couple thousand rpm or get her up on plane real quick and then the temp will drop right down to 160 degrees. The boat doesn’t overheat at speed, just when I’m idling into harbor or have to drive real slow. Other than the overheating it runs flawlessly.
I have put a new impeller in it, the one that was replaced was in perfect condition. I have also put a new circulation pump on the engine to circulate the freshwater side of the system. I’ve pulled the exchanger to see if it was clogged and there are no obstructions or corrosion inside. I’m stumped!
Anyone have any suggestions for me before I take it in to a shop? I live in W Seattle and was also wondering which shops people would recommend. I’d be willing to take it anywhere from Tacoma to Everett.
I would appreciate any help offered here.

Thanks, Dave

I would highly reccomend Jim Haury marine. 206 241 5016. I have used him for 10+ years and have recieved excellent service. They are just south of White Center. 3 rivers marine is another excellent shop but further away from you in Woodinville. Good luck!

The pick up screens in the outdrive are not clogged? Thermostat? Thats a tough one.

Do you have or know anybody that has one of them infra-red remote temperature sensors? It may help you isolate the problem area, or at the least verifyy the temperature versus what the gauge is tellin you.

The pick up is clean, new thermostat put in and the old one tested fine. I don't have a way to verify the temp, other than the fact that it runs at 160 until idle and then heats up, get going again and she cools right back to 160.
Running with the engine cover off I've noticed this. When I slow down to idle and it starts to overheat some of the coolant gets pushed back out of the engine thru the overflow tube and into the overflow reservoir. I talked to a marine tech about that and he thinks it might be the head gasket causing gases to build up inside which would push the coolant out. Does that sound reasonable?
I'm still stumped and willing to hear some more ideas.
Thanks

I would bet money the risers are pluged and need to be replaced. When the rust chunks plug them up almost no coolant get's by and there is the heat problem. Pull a riser off, 4 bolts and look at them.

Could be the head gasket. Also have you checked the sending unit for temp? You could have a bad temp unit, a short or a bad temp gauge.

wntrrun: one thing you just described made me think of another experience I've recently had with my pick-up......I hate to show my ineptitude to the 10%'ers on the board, but,


Deep Breath.................

My company truck's temp guage was all over the place for about two years...running normal going down the road, spiking up to high non-consistently. Loosing water out my overflow tube. Took it into the shop.....I put the bloody thermostat in backwards!!! Make sure the copper element is towards the block!!!!

I just mention it because it happened to me, my previous thermostat was either in backwards and I copied the mistake or I crackered the whole job in my driveway on a dark cold stormy night.......yours is probably right

Base upon the speeds you mentioned, my first thoughts were a plugged cooling system or the head gasket. If everything is working good on the water system, I would guess the head gasket. Going slow, water is going slow and has a chance to heat up. When you speed up, the water speeds up and doesn't have time to heat up. It stays cooler so the engine stays cooler. Pull the spark plugs and check your cylinder compression and see what color the spark plug electrodes are.

Head gaskets are not expensive, but they can be a bear to change. Especially if you check everything out and do the other little things that could be done "as long as you have it apart".

Good Luck with it.

Steve

If head gasket's leaking, you should be getting a little bit of water in your oil. Many shops have a device which can measure dissolved gas concentrations in your coolant an hopefully tell you if you have a gasket leak. Might also check to make sure the hoses aren't sucking in air anywhere and bleed the system a couple times. A couple air bubbles in the coolant lines can cause some weird problems. Also shutoff all accesory lines from the coolant system (water heater, air heater) try to get the engine as isolated as possible, this will make sure you don't have a leak in your water heater or somewhere else you don't expect. You can get a ballpark guess of what temp you're running at by just putting a meat thermometer in the radiator. Not a very exacting method, but it's a cheap way to find out if your temp gauge is way off.

good luck
david

Thanks for all the suggestions. I'm putting a new manifold and riser on this weekend. Even if they aren't the problem it was time to replace them.

Thanks again, Dave