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  1988 Johnson 40-HP Overheat Alarm at Low Speed

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Author Topic:   1988 Johnson 40-HP Overheat Alarm at Low Speed
RussAdams posted 05-05-2006 04:42 PM ET (US)   Profile for RussAdams   Send Email to RussAdams  
I'm considering re-powering my 13-foot Boston Whaler boat due to the current 1988 Johnson 40-HP engine's tendency to overheat at low speeds. It seems to occur when I'm at no-wake speed with the wife. Or at just above planing speed, again with the wife aboard. I have not had this issue occur when it is just me in the boat. Before I go to the drastic step of a new motor I thought it might be worth asking here:

Could there be a simple and relatively inexpensive explanation for why the motor would be over-heating?

Thanks!--Russ

dgoodhue posted 05-05-2006 04:48 PM ET (US)     Profile for dgoodhue  Send Email to dgoodhue     
When is the last time you changed the waterpump?
Graham posted 05-06-2006 02:02 AM ET (US)     Profile for Graham  Send Email to Graham     
The only common factor in your description seems to be the wife.

Has the engine been serviced regularly? There may be a problem with water circulation. Are you on salt or fresh water? Do you normally wash the water cooling sysyem down after each trip?

Hope you find the problem.

RussAdams posted 05-06-2006 02:11 AM ET (US)     Profile for RussAdams  Send Email to RussAdams     
I bought the boat late 2004 and had a Coast Guard mechanic go through the motor. He didn't do a rebuild, just sparks, tuning, lube, and other maintenance. At that time he replaced the thermostats, and I'm pretty sure the impeller, but I'm not sure about the water pump.

Is the waterpump the same thing as the impeller?

The overheating occurred last year, just months after the work by the coastie. And occurred periodically thoughout the summer. I have not had the boat back in the water yet this year, and am trying to decide what to do about the motor.

I'm pretty new to outboards. I worked on cars a lot when I was younger, but only once on outboards.

So any suggestions to a complete newbie would be greatly appreciated!

RussAdams posted 05-06-2006 02:18 AM ET (US)     Profile for RussAdams  Send Email to RussAdams     
Hi Graham:

Yeah, I have wondered if the boat gets jealous...

And mentioning extra 'weight' is not too politically correct when my wife is aboard...

<grin>

I keep her at my pier on a saltwater creek in North Carolina. So all of her use is saltwater. And no, she does not get flushed with fresh water very often. I only flush her when I pull her out for other maintenance.

I do have water almost to the dock. Is it important enough that I should run water the rest of the way and then flush her each time she goes out?

Thanks, Russ

jimh posted 05-06-2006 07:13 AM ET (US)     Profile for jimh  Send Email to jimh     
You describe the problem as only occurring when your wife is aboard. Let's accept that as absolutely true--the engine never over heats in any other situation. The implication is:

When your wife is aboard the trim of the vessel changes due to the distribution of weight. The change in hull trim is affecting the engine cooling system. One possible interpretation: your wife sits in the forward half of the vessel. This causes the bow to be trimmed down, and this in turn raises the stern slightly. The engine water intake is affected by this and fails to provide sufficient water for cooling.

A second interpretation:

When your wife is aboard the weight of the vessel changes. This increases the load on the motor. The cooling system of the motor is operating with very marginal reserve, and the added load of more weight produces too much engine heat. The cooling system capacity is insufficient for the added load, and the engine overheats.

RussAdams posted 05-06-2006 03:25 PM ET (US)     Profile for RussAdams  Send Email to RussAdams     
Hi Jim:

I think it's part 2. Though she is not very heavy. I'm about 170 and she's CONSIDERABLY lighter (never know who's reading...)...

I really think it is probably a marginal cooling situation.

She sits on the aft seat with me, so the engine is not being tilted such that the water intakes are riding in the air.

So what types of things cause the cooling system to be marginal?

I've been reading as much as I can on impellers and waterpumps. In addition to searching here, iBoats.com has some good articles on swapping out the impeller. It sounds like something I can do.

Is there any way to get at the water channels in the motor to see if they are blocked? Or do you need to tear the engine down to do that?

Maybe it's a stupid question, but if I drop the lower unit and swap out the impeller, and remove the thermostats, can I probe around with something to see? A coat hanger or ? Or do I risk making things even worse?

Could carbon buildup in the engine cause the cooling system to have problems?

I was reading with interest the articles about SeaFoam and wondering if that could be contributing.

Thanks,

Russ

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