Overheating Raw-Water Cooled Volvo

[danielb]

Hi,

I have a little MD5A 7.5hp (late 70's) in my 26 foot keeler, which has recently started overheating after owning for 6+ years (incl. partial rebuild).
Very quick recent history:
The boat had been out of the water for a year, during which time I ran the engine 3-4 times to keep everything coated in oil etc (Cooled with water hose). Still starts perfectly, on one or two turns (clearly very good compression).
After relaunch, but before going further than the mooring, I replaced the thermostat housing, cooled exhaust elbow, and oil (heavier grade - 20W50 + Moreys). The thermostat housing and exhaust elbow both are custom, but I paid close attention to not changing or restricting flow. I also pulled off and refitted the head, as I suspected a fuel leak from the injector nozzle sleeve.

Under idle, the engine sits at a healthy (for raw cooled!) 60-65 degrees, if left alone. Under moderate - high load, it climbs as high as 95degrees celcius (bad), or 80-85 degrees at lowish load. Once it's gone high, it doesn't really want to come below 80 again, even if left at fast idle in neutral for 5-10min (don't want to do that too much anyway).

I have checked the water inlet and poked through it - no obstruction. Replaced the impellor (old one was fine). Replaced the thermostat (old and new tested fine in hot water - to specs). checked the head for obstruction - clean as a whistle. Taken out the block drain-cock (lowest point of cooling circuit), a little bit of black crud came out with poking, but it was not blocked at all.
Run through a descaling liquid in a closed loop for about 20-30 min or so. checked block for hot spots with thermo laser - nothing.

The whole time the exhaust seems to still be spitting out plenty of water.
Engine has minimal smoke, little bit of greyish smoke when cold, virtually gone when warm - not worse than previously to my memory.

What else can I check?

Is there any scenario where the head gasket could be at fault? eg water in to cylinder on downstroke, or gas in to the water channels? Doubtfull as it starts so very easily.

[ashdec87]

Im new to sailing, but know a little about engines.

Hows the power of the engine? I'm thinking your engine might be running lean if all else seems ok. Oil being circulated ok? pump, filter?

[perchance]

Check your raw water impeller first as it may have deteriorated from sitting on the hard and taken a "set" on the blades or hardened to the point where it has begun to break apart.

[s/v Jedi]

1. Measure intake water temperature
2. Measure exhaust water temperature
3. Measure exhaust water volume (catch in bucket while timing)

cheers,
Nick.

[danielb]

Originally Posted by ashdec87

Im new to sailing, but know a little about engines.

Hows the power of the engine? I'm thinking your engine might be running lean if all else seems ok. Oil being circulated ok? pump, filter?

Thanks for the ideas. Actually power is pretty good. Though I have reason to suspect the injection angle is not right. That said, it has worked without overheating with current injection angle. Oil pressure is healthy 40psi, and drops off a little when warm.

[danielb]

Originally Posted by perchance

Check your raw water impeller first as it may have deteriorated from sitting on the hard and taken a "set" on the blades or hardened to the point where it has begun to break apart.

As per my original post, I've already replaced the impellor (which was still in good condition). Thanks anyway.

[danielb]

Originally Posted by s/v Jedi

1. Measure intake water temperature
2. Measure exhaust water temperature
3. Measure exhaust water volume (catch in bucket while timing)

cheers,
Nick.

Intake water temp is about 20 degrees.
I'm guessing exhaust water temp will be about 80 degrees? I have a pretty new Pricol sensor fitted in the head, and have used a Infrared camera/laser thermometer which appears to agree with the pricol very closely.

Measuring the water volume is inevitable I guess - hope I can find the original water flow specs for different RPM. Does depth below water effect flow rate much?

[Viking Sailor]

I have a raw water cooled 1974 MD17C that has had cooling problems before. I suggest that you remove the thermostat and run the engine. There should be a very large drop in operating temp. If the temp stays high then it could be an injector problem, hot gases getting pass the rings, valves, or head gasket. Hopefully, the temp does drop and all that is needed is to switch to a lower temp thermostat.

Best of luck,

Paul

[Play Actor]

You mentioned a new head gasket. It's possible that the head gasket was wrong, or installed the wrong way and is blocking some coolant passages, unless you checked it carefully when you fitted it. Same possibility with the exhaust manifold gasket. It's happened to me before when I rebuilt automobile engines.

A coolant flow rate check probably won't show anything if an improper gasket is your problem, but it's still a good thing to do, and fairly easy -- just takes a bucket and a watch with a second hand.

[never monday]

Originally Posted by danielb

As per my original post, I've already replaced the impellor (which was still in good condition). Thanks anyway.

what was the condition of the pump body? was there wear on the cover? what did you use for a gasket? add any sealant?

[danielb]

Originally Posted by Viking Sailor

I have a raw water cooled 1974 MD17C that has had cooling problems before. I suggest that you remove the thermostat and run the engine. There should be a very large drop in operating temp. If the temp stays high then it could be an injector problem, hot gases getting pass the rings, valves, or head gasket. Hopefully, the temp does drop and all that is needed is to switch to a lower temp thermostat.

Best of luck,

Paul

Hi, Thanks Paul. Yeah removed the thermostat outright, and the engine never got anywhere near 60 degrees even - so that's probably good right? I don't think there are cooler thermostats, this is already a 60 degree one. Besides that seems to skirt around whatever is causing the issue.

[danielb]

Originally Posted by Play Actor

You mentioned a new head gasket. It's possible that the head gasket was wrong, or installed the wrong way and is blocking some coolant passages, unless you checked it carefully when you fitted it. Same possibility with the exhaust manifold gasket. It's happened to me before when I rebuilt automobile engines.

A coolant flow rate check probably won't show anything if an improper gasket is your problem, but it's still a good thing to do, and fairly easy -- just takes a bucket and a watch with a second hand.

Hi, It's definitely the right gasket, and I'm pretty sure it'd be hard to install wrong way up, but good thought - I'll check it out. Exhaust gasket is a single hole with two stud holes either side, therefore impossible to mount wrong way round, as it's symetrical and can be used either way. Thanks.

[danielb]

Originally Posted by never monday

what was the condition of the pump body? was there wear on the cover? what did you use for a gasket? add any sealant?

Good condition, minimal wear. Thanks.

[Viking Sailor]

danielb,

Drill a 3/16 inch bypass hole in your old (working) thermostat and try again. This is what I did as a temporary (~2 year) fix. The bypass hole is necessary to allow the thermostat to sense the true water temperature. I ended up finding a lower temp thermostat that already had a bypass hole. However, I can't find the info right at the moment. I think I went from an 80 degree thermostat down to a 60 degree one. So, temp wise you should be good.

Paul

[danielb]

Originally Posted by Viking Sailor

danielb,

Drill a 3/16 inch bypass hole in your old (working) thermostat and try again. This is what I did as a temporary (~2 year) fix. The bypass hole is necessary to allow the thermostat to sense the true water temperature. I ended up finding a lower temp thermostat that already had a bypass hole. However, I can't find the info right at the moment. I think I went from an 80 degree thermostat down to a 60 degree one. So, temp wise you should be good.

Paul

Good point about the bypass holes - may enlarge them a little bit and see what happens.
Still, need to find what the real issue is, could be causing other harm that hasn't come apparent yet.....