1978 Johnson 140hp Overheating?

[AguaSki]

In AZ we don't really have a winter season, so the boat can be used all year. I was at the lake a couple weeks ago and my motor was running fine. Yesterday I had just launched and was idling out past the no wake zone when my overheat alarm sounded. I immediately shut-off the engine. For the next 10-15 minutes the alarm would sound if I moved the key into the on position without cranking the motor. After about 15 minutes I could move the key to on and crank the motor, but then I could not get the motor to start. I could smell fuel (flooded), but it was like there was no spark. Fortunately my brother-in-law was there with his boat, so he towed me back to the ramp. I put the boat on the trailer, and then I went back out on the water in my brother-in-law's boat. After about 3 hours we came back to the ramp and I decided to dunk my boat/trailer again. With the engine totally cold, the motor started right up like it normally does. It was getting late, so I just pulled the boat/trailer out without another test run. I did not want to have to be towed in again.

I usually change out my impeller in even number years (2006, 2008, 2010, etc.) Since the impeller was changed last year, I was not planning on changing it again until next spring. The current impeller is 10 months old, last summers high gas prices limited me to about 30 hours of running time. Do you think my impeller is bad already? I do have a pressure gauge on the dash, but I was busy repositioning gear and kids when the alarm sounded, so I was not looking at the gauge. When I ran the motor on the trailer later in the day, a small amount of pressure did register on the gauge, but it is normal to have low pressure at low RPM. When I raised the warm up lever to increase RPM, I could also see the water pressure raise a bit.

Given the symptoms I have described, I am interested in thoughts on where I should start. I also have a Clymer manual, but I have found it to be useless. As luck would have it, I purchased a factory manual last week, but the UPS man has not delivered it yet. The tracking number I have says it should be delivered tomorrow. I am anxious to get my hands on the new manual now that I have a real need.

 

[ezeke]

Re: 1978 Johnson 140hp Overheating?

Start with assuring that the water intake screens are in good condition and then replace the entire water pump; the new pumps are better.

Chances are that the thermostat system may need rebuilding. You can get a complete rebuild kit from iboats that has everything including the gaskets part number 18-3673.

If you still overheat, one or more of the four water deflectors may have failed.

BTW, Good shop practice requires replacing the cylinder head gaskets after an engine overheat.

 

[Benny1963]

Re: 1978 Johnson 140hp Overheating?

i would drop lower just to look ,have you replaced the entire assembly.
are have ya just changed impellars ,some times the base plate and cup can wear ,also givin the yerr of motor if it has never had heads off ya may have water diverters outta place or gone bad ,also themos and poppetts need to be checked , but you my flush it out and check impellar .and overheat can cause scuffiing and damage quick to and older motor .i would maint .the whole cooling system,new thermos and poppetts ,head gasket and diverters ,
full brp waterpump kit ,poewerhead parts are high,good luck,
check your link and sink on carbs if its dumping fuel and loading up give them
a good soak and rebuild ,the 140 are great motors .im looking for one know

 

[AguaSki]

Re: 1978 Johnson 140hp Overheating?

Last spring I did the entire water pump assembly and not just the impeller. I bought the motor in 2005 from reputable local J/E Dealer after a complete powerhead rebuild. I had to break the motor in as if it was new. I don't know if the diverters were looked at during the rebuild in 2005. I also don't know if the thermostats were looked at. Late last summer I did a complete overhaul of the fuel system including the carb, fuel pump, tank drain and cleaning, new fuel lines, and added a 10 micron inline water separating filter. My fuel system should be fine.

 

[ezeke]

Re: 1978 Johnson 140hp Overheating?

Your comments in the opening thread:

"..overheat alarm sounded.."

"...For the next 10-15 minutes the alarm would sound if I moved the key into the on position without cranking the motor..."

"...With the engine totally cold, the motor started right up..."

These are classic overheat conditions with the alarm system working correctly. The overheat switch stayed closed until the temperature returned to normal.

It would be prudent to examine the entire cooling system before running the motor again.

 

[AguaSki]

Re: 1978 Johnson 140hp Overheating?

Agreed. An examination of the entire cooling system is in order. The engine was running at about 1,200 RPM (no wake zone speed), and I shut the engine down within seconds of the alarm sounding. Any chance that engine damage was done? I am hopeful that the alarm sounds with sufficient lead time before damage starts occurring. If the alarm is built to sound with a little lead time, then I am confident I got the motor shut down quickly enough. I will do a compression check tomorrow. Out of curiosity, how does the overheat switch referenced by ezeke work?

 

[Rocky_Road]

Re: 1978 Johnson 140hp Overheating?

Out of curiosity, how does the overheat switch referenced by ezeke work?

Pretty foolproof...and simple!

The 'switch' is normally open...until it reaches a preset temperature. Then...it 'shorts', and completes the circuit to your audible alarm. It stays in the 'closed' position, until the monitored temperature drops, and the switch returns to it's open state.

These can be tested on your stovetop, with a pan of water, a candy thermonitor, and a continuity tester (most volt meters)...but it is a pain in the arse. And you have to know the threshold rating of your particular switch....

 

[ezeke]

Re: 1978 Johnson 140hp Overheating?

?The temperature switches on the 1978 V4 recreational motors close the circuit at 211? F. +/- 6? and reopen at 175? F. +/- 7?.

The switches were very well made and rarely fail.

211 F.= 99 C.
175 F.= 79 C.

?See OMC 1978 V4 service manual #406519 or 506753, pg 5-7.

 

[Rocky_Road]

Re: 1978 Johnson 140hp Overheating?

?The temperature switches on the 1978 V4 recreational motors close the circuit at 211? F. +/- 6? and reopen at 175? F. +/- 7?.

The switches were very well made and rarely fail.

211 F.= 99 C.
175 F.= 79 C.

?See OMC 1978 V4 service manual #406519 or 506753, pg 5-7.

Wow...that's scary, 211 degrees (+/- 6)!

My inline 4 (3.0L) triggers at around 190. I know that the V engines are designed for more temperature...but at what point has damage occured?

Can't you go to an auto parts house...and 'choose' the temperature point, that you are comfortable with? And...in most cases, the auto temp switches will simply replace the OEM?

 

[ezeke]

Re: 1978 Johnson 140hp Overheating?

At 99 degrees C. You are approximately one Celsius or Centigrade degree below boiling water at sea level.

The point of the system is that when you hear the constant horn, you turn the engine off immediately and find the problem.


I think that many of us have added temperature gauges so that we can monitor the temperature and get a heads up before the overheat horn.

You cannot depend on the tell-tale to do more than let you know that the water pump is functional.

 

[emdsapmgr]

Re: 1978 Johnson 140hp Overheating?

There is a valve body in the thermostat housing. On yours it has two small pinholes that feed hot water to the back of the thermostats to signal them to open. One or both can occasionally plug up with debris like sand or weeds. Once one hole plugs up, it can overheat that side of the engine at idle/slow speeds, even with a good telltale.