25 hp Volvo auxiliary runs hot-question

[harpguitar]

Hi,

I have a Volvo 25hp on my 26' Westerly. I am the new owner.

The engine runs well but the temp indicator goes into the red and drops back into the green. Stays in the red about 10 secs then drops in the green for thirty seconds and cycles back up to red. Cycle keeps repeating.

I changed the thermostat but the problem persists unchanged. My question is whether this is normal or should I pursue a professional repair? Other suggestions?

Thank you.

Rick

[Rockter]

As long as it doesn't stay in the red. I trust this motor is raw water cooled? What motor is it?

That cycle is the thermostat alternatively opening and closing.

The cycling is normal on my MD17C, though it does not make it into the red.

If the motor is raw water cooled, the thermostat must be one designed to run a bit cooler... 75 degC is one guess for raw water cooling, from memory. A motor with a fresh-water cooling circuit (with heat exchanger) will run hotter.

[harpguitar]

It is the MD 2B motor. It is raw water cooled. Thank you. Is there a way to check the temp? Thank you Rockter!

Rick

[Rockter]

Harp..

Very similar to my MD17C, from 1977.

Take the thermostat out of there... it is on the front end of the exhaust manifold. Just take off the thermostat and put in a saucepan of water, and raise the temperature slowly. You will see it open clearly. It means buying a thermometer, of course.

Try to check what thermostat you bought. If it is the raw water type, I suspect it will open at about 75 degC.... but I am guessing. There should be a part number on it to confirm, and Volvo should know. Remember that the thermostat for closed cycle cooling with heat exchanger is different and is designed to run hotter... I think about 90 degC. Don't put that one in your raw water cooled motor as it tends to cause aggressive scaling.

On my motor, the temperature makes it to about 12 O'clock on the guage, then drops back, repeating all day.

Harp... just be wary it does not stay in the red.... then you really are overheating it.... then you will probably hear it bubbling on shut down (not the normal short-term intermittent gurgling as the water settles), but a clearly audible boiling in the motor that slowly subsides. I have been there with that one in the Gulf of Mexico and it was horrible.

That is not what is happening to your motor.

Be wary that the wee water entry ports in the cylinder heads do not scale up. One of mine did, and the fwd piston and cylinder head got overheated. The ports are easily cleared though. A coat hanger will do fine. The scaling is a black, biscuit-like material, but it clears with effort with the wire and with a wee hose to flush it out. Poke the scale off the corresponding manifold feed port too.

If ever you suspect that it needs done there Harp, give me a shout. It is among the easier jobs on a raw water cooled Volvo.

It's a good motor, it really is, and it's from the old cast iron school of design, but with truly shocking spare parts prices.... like off the clock.... about $2300 for the exhaust manifold on mine, back in 1997. I shudder to think what it would be today.

A last point. Don't let the motor freeze there Harp. I did. There is a crack, but it is not worsening.

Rockter.

[donradclife]

My car did this once when I installed the thermostat backwards.

[harpguitar]

Thank you again Rockter. I suspect everything you said is right on. I did get some of the scale you describe out of the thermostat housing itself. It would make sense that it is elsewhere as well. I will see what looks doable and whether it is something I can do or beyond my skills.

I will also check the temp of the thermostat...the new replacement thermostat came with the boat and I did not check the specs, but I still have all the packaging and worse comes to worse, can pull it out and check as you describe.

Your responses have been a huge help. Thank you.

Harp

[erps]

The only thing I would add is that if I were at the point of pulling out the thermostat to check it, I would probably just install the replacement one at the same time and then check the old one later to determine whether it was worth keeping for a spare. Thermostats just don't cost that much. Also it's my understanding that the thermostats in raw water cooled engines typically are set to run at a cooler temperature than their fresh water cooled cousins to help prevent salt/scale build up in the engine.

[DolphinLeap]

My MD17C just threw a rod (I think) on cylinder number 3. Motoring at 1500 rpm with the standard temp cycling from low gee to high green (w/o a thermostat), the rpm began to vary and the engine shut down as oil was coming out of the number 3 air filter, always a bad sign.

Does anyone know what repair is required?

keith

[LakeSuperiorGeezer]

Originally Posted by DolphinLeap

oil was coming out of the number 3 air filter, always a bad sign.

Does anyone know what repair is required?

keith

A valve spring broke, or the head came off a valve, then the valve punched a hole in the piston letting oil from the crankcase out the intake past the air filter. You will have to pull the engine to replace the piston. The head probably is also destroyed. Buy a used head under guarantee, and have an automotive machinest check it out. You may want to price the cost of an outboard (figure 3 to 4 horsepower per ton) or replace the engine with the same engine, but one that has a heat exchanger. Raw water cooling causes cracks and internal corrosion so do not get a used engine that never had a heat exchanger fitted.

[LakeSuperiorGeezer]

Originally Posted by harpguitar

Hi,

The engine runs well but the temp indicator goes into the red and drops back into the green. Stays in the red about 10 secs then drops in the green for thirty seconds and cycles back up to red. Cycle keeps repeating.
Rick

This is a very fast cycle of less than a minute. The metal in the engine cannot change temperature that fast so it has to be the temperature gauge, or actually the transmitter on the engine that sends an electrical signal to the temperature gauge telling the gauge how hot the transmitter senses the engine to be. Also check the temperature of the thermostat, cold for raw water cooling and high for an engine with a heat exchanger. High would be around 180 degrees.