Engine just shuts off after it warms up

[juicejug]

I have a 1984 140 HP Mercruiser. Since the end of last season I've noticed a problem where the engine will just shut off. What first started happening is that it wouldn't run when just placed into gear but would be fine at higher RPM's. Then, when I went to winterize the boat, I couldn't keep it running. I figured I'll deal with it when it comes to getting it ready for the summer. Well, that was this weekend and I'm having the same problem.

I started doing some more diagnosis and found that it starts right up and runs fine when it is cold and then when it warms up, it just shuts off. I believe it is electrical (possibly the choke) because it doesn't sputter and then shut off, it just stops dead.

Any suggestions on what to check?

 

[gm280]

Re: Engine just shuts off after it warms up

I have a 1984 140 HP Mercruiser. Since the end of last season I've noticed a problem where the engine will just shut off. What first started happening is that it wouldn't run when just placed into gear but would be fine at higher RPM's. Then, when I went to winterize the boat, I couldn't keep it running. I figured I'll deal with it when it comes to getting it ready for the summer. Well, that was this weekend and I'm having the same problem.

I started doing some more diagnosis and found that it starts right up and runs fine when it is cold and then when it warms up, it just shuts off. I believe it is electrical (possibly the choke) because it doesn't sputter and then shut off, it just stops dead.

Any suggestions on what to check?

First of all welcome to iboats :welcome: great to have you aboard...

I don't think it is a choke issue myself. It sounds more like a heat related ignition issue. Try starting it again (when it is cooled off) and when it stops, check for spark to verify that the ignition is working or not... Post your results either way...

 

[juicejug]

Re: Engine just shuts off after it warms up

Since I posted, I started thinking it might be the shift cut out switch so I pulled that and checked the resistance when closed and opened. It gave 0 and full resistance as expected so I put it back on. Then I checked the resistance again with the switch installed and noticed that the ohmmeter was reading half resistance - exactly between full and 0 resistance (forgive my ignorance but I can't read the analog ohmmeter properly).

This was right after it stopped. I left the ohmmeter hooked up and tended to some other things while I let it cool down.

A few hours later I checked it again and there was 0 resistance and the engine started right up.

My schematic shows there is a resistance wire between the coil and either that switch or the choke but I can't read it as it's a photocopy. Is there any possibility that this has something to do with it? Should I just hook a voltmeter up to the coil and see if the voltage changes when it shuts off?

 

[gm280]

Re: Engine just shuts off after it warms up

Since I posted, I started thinking it might be the shift cut out switch so I pulled that and checked the resistance when closed and opened. It gave 0 and full resistance as expected so I put it back on. Then I checked the resistance again with the switch installed and noticed that the ohmmeter was reading half resistance - exactly between full and 0 resistance (forgive my ignorance but I can't read the analog ohmmeter properly).

This was right after it stopped. I left the ohmmeter hooked up and tended to some other things while I let it cool down.

A few hours later I checked it again and there was 0 resistance and the engine started right up.

My schematic shows there is a resistance wire between the coil and either that switch or the choke but I can't read it as it's a photocopy. Is there any possibility that this has something to do with it? Should I just hook a voltmeter up to the coil and see if the voltage changes when it shuts off?

Not sure what switch you are talking about, but it sounds like it may be the neutral safety switch. That switch keeps the starter from starting if the engine is not in neutral. When you say the engine will not start when it shuts off, do you mean it won't turn over with the starter? Again, when it does stop again, check the spark and let us know. If the starter itself is not turning then that is a different issue as well...

 

[juicejug]

Re: Engine just shuts off after it warms up

Yes, that is the switch I am talking about.

Yes, it turns over but there is no spark. Also it only reads about 8 volts when it stalls.

When it's cold, it is the full 12.5V.

 

[juicejug]

Re: Engine just shuts off after it warms up

Okay, here's what else I've recently found. I had switched out the electronic conversion for some old points I had laying around. After I had done that, it seemed to run fine and I was able to run it for periods of 10-15 minutes in the driveway. I figured at that point that it must have been the conversion for whatever reason. I went to take it out yesterday and when I put it in the water it started cutting out again.

I checked the coil and it was so hot it about burned me. I figured that must be it. I then remembered that the ignition coil was replaced last summer. I started doing some research and couldn't find a coil with an internal resistor, all coils said they required an external resistor. Since I don't see any resistor hooked up to any wires that are hooked up to the coil, I'm assuming that the lack of a resistor with the new coil is the problem.

I have a new question though. I've always known that this boat had an additional wire hooked up to the coil that provides 12v to the coil. The previous owner said that they had wired it this way for using the radio in some certain conditions or some such thing. Since it always worked, I didn't screw with it. Upon having these problems, I ran through all the wiring on this boat. I disconnected this power wire and found that the starter is not delivering any voltage to the coil at any point. My question, shouldn't the wire coming from the solenoid on the starter be providing 12v to the coil? The starter works fine to start the motor but I'm guessing there's something wrong in the solenoid. Is this possible?

 

[juicejug]

I'm aware that this is an old post, but I thought I had provided my resolution. It was in fact that the new coil didn't have an external resistor. I had them check at the marine and they found the resistor for me. They were also surprised to find out these new coils required the additional external resistor.

 

[GA_Boater]

You can post any time you want when you're the thread starter, juice.

And any follow up with what was causing the problem is always welcome. Too often it gets fixed and we never never knew if we were any help.

Thanks for closing the tread out and what was the cure. Thanks. :thumb: