1987 mercury 45hp classic 50 shut off in the middle of running

[pgebhard0212]

I have a old mercury that runs good until it doesn't. Only when I take it out and drive it the overheat alarm goes off at full throttle and will put it in limp mode. I've change the whole impeller system and opened the water jackets and no clogs. It's pees with a strong stream and ive checked it with an infared thermometer after running wot and it was at normal temps 130 140. I unhooked the temp sensor and it ran fine for awhile took it out today and it completely shut off after running it fast for 5 min let it rest for a couple hours and fired it back up and it ran for a few then shut off twice at idle then wouldn't crank over. Anybody have any ideas ive put to much time into this engine already and just need it to work good or I gotta get a new one

 

[Mc Tool]

Have you tested the temp sensor ?. It could be telling lies ......not unheard of

 

[pgebhard0212]

Have you tested the temp sensor ?. It could be telling lies ......not unheard of

I haven't tested it but I unhooked it completely but the engine still shuts off

 

[jimmbo]

How Old is the Water Pump Impeller?

 

[airshot]

Does this motor have a CDI box ? The reason being is years back I had the same issue with an OMC outboard. As a last result, we put on a new cdi and problem was fixed.

 

[racerone]

Yes , these motors have a switch box.

 

[Nordin]

You have to check for spark when it shuts down.
The switchbox could fail when getting warm and it loose spark.
When getting cold it will work for avail again until it get warm.
Have to figure out if it is a spark issue or overheating issue.
Seams to be a spark issue as if it is an overheating issue it would loose in compression.
A compression test is still a number one to check.

 

[Chris1956]

There is no limp mode on those motors, from the factory. Overheat also does not cause them to turn off.

Nordin could very well be correct about warm fail of the switchbox.

However, I would also trace the black/yellow wire, which kills spark, from the switchbox to the wiring harness plug and look for places it could ground to the frame.

 

[Nordin]

I have had that kind of issue with two smaller Mercury.
One 9,8Hp 1977, started fine and ran for some minutes then shut down.
Put it in a large bucket and force heat to the switch box with a heat gun.
Shut down after some minutes.
Similar issue with a 20Hp 1976, shut down after avail.
Did the same test with a heat gun, same result.
Changed switchbox to both of them, ran fine after that.