Is my 4 cyl 50 hp Merc running too hot???

[steve2manyboats]

I have a late 80’s Merc 4 cyl 50 hp on my bay skiff. Always loved these motors and have had a few over the years. They run very, very smoothly and tend to last forever…or until the water jackets corrode to powder.

My current 50 Merc is running flawlessly, but I noticed today that the water from the tell tale is quite hot. The stream from the tell tale is very strong, but hot. I put my IR gun on the motor and this is what I found:

Temp of side exhaust water jacket – 110 deg.
Temp of back of block by spark plugs – 150 deg.
Temp of water from tell tale 130 deg.

Bay water temp is 78 deg.

The motor seems to be running hotter than other 4 cyl Merc 50’s that I’ve owned. I haven’t installed a water pressure gauge on the motor – I usually don’t on small motors.

Is my motor running too hot or is it OK?

Advice appreciated…Steve2ManyBoats.

[mr-asa]

To preface this, I’m not that familiar with that particular motor, so you’d know better than I would. Some others on here might have more experience with that motor
My initial question is what do the plugs look like? If it’s a little lean or the timing slightly too advanced, that would cause it to run a little too hot but otherwise be a decent runner.

[steve2manyboats]

I’ve had a number for these motors and this one runs hotter than the others I’ve had. Plugs look perfect. Used a borescope and the piston wash looks good too. This motor does not have a thermostat, which is why I’m concerned that the temps I measured are so high.

Would appreciate advice/experiences folks have had with these motors..

Thanks…Steve2ManyBoas.

[billw]

I had one of those on my Whaler and have serviced lots of other ones. I don’t think those temps are too far from the norm…..yet.

That being said, there are three things those motors like to do…..eat impeller hubs and fill the water jacket with junk, especially in salt water. Also, rot through the inner exhaust cover and dump water off before reaching the top cylinder. If the impeller is two years old or older, I would definitely change it as a precaution. Also, you may then note it runs a little cooler because the new blades of the impeller will be a little more efficient.

If your motor is one that has the crank seal on top of the drive shaft and not an o-ring, order the seal with the impeller, ’cause you will find it’s probably shot, when you pull the gear case.

If your motor’s tell tale hose comes off the top of the block, you’re good. If it comes off the center of the water jacket cover, you should get a longer hose and route it to the top of the block, then down and out, with a loop at the top. That way, you’ll know you have water to the top of the block or there’d be no water coming out.

I think you are okay to run for now with a new impeller to be safe; but I would definitely keep an eye out for the temps getting any higher than that. If they do, you’ll have to go after the water jackets.

Long live American manufacturing!

[steve2manyboats]

Bill:

Thaanks so much for the helpful post. The impeller is reasonably new. I have had a few of these motors over the years. The boat is used in the midbay Chesapeake region so the water is only about half salt. I have never had the water jackets on any of these motors plug up, but I have had the inner water jackets on the exhaust side rot out twice on other motors. In each case water got into the exhaust ports, the motors ran crappy and I could tell exactly what was happening when the spark plugs from those cylinders were steam cleaned so I was able to repair before any damage was done to the rings or bores.

This motor is one of the later ones and has the tell tale hose coming from the block. It’s a very low hour motor. Probably only has 50 hours on it.

I’ll pop in a new impeller and keep an eye on it.

Thanks…Steve2ManyBoats.

[crosbyman]

make certain you have noting between the water inatkes and the impleller water entry port

I once found my hot 50 to plug up intermittently due to some idiot dropping a grommet in that cavity !!!

Joining AOMCI has priviledges 🙂

[steve2manyboats]

PROBLEM SOLVED – Turned out the impeller was weak. Popped in a new impeller and the temps went down a lot. The old impeller was only a year old, but had taken a set. Rear water jacket water temp went down 20 degrees. However, the tell tale stream didn’t change with the new impeller even though the new impeller clearly is delivering more volume. That’s what threw me – stream seemed fine but the motor was not getting enough water volume.

I didn’t have an IR gun the last time I owned a 50 Merc. But that motor never ran hot. The tell tale stream was only warm to the touch and you could hold you hand on the rear water jacket as long as you wanted. On this motor, before the new impeller, you could barely hold you hand in the tell tale stream, even though it was strong, and you could only touch the rear water jacket for a few seconds.

Since this motor seems to need a strong impeller, it’s probably going to get a water pressure gauge like all my V6s do.

Steve2ManyBoats

[billw]

It’s good to know that helped! I remember the difference when I put a new one in mine; but in my case, at least, I saw it as a difference in strength of stream. It will be interesting to see what they run for pressure. Not having a pressure control valve, (well mine didn’t, at least) it won’t be much….but it will be SOMETHING…

Long live American manufacturing!

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