1984 Merc 150 V-6 Overheat

[JimDD]

I recently spent a good bit on some work done by a local marine dealer. It was work to remedy an overheat situation that I had. The dealer ended up putting a new water pump, new overheat sensor, and had to bead blast some foulant in the heads on the water side. There was a bad gasket in the midsection that was also replaced and the poppet valve was rebuilt. I do not run therms in this engine and have not for over 15 years. This overheat problem just happened recently before I shopped the engine.

Since the repairs, I am not seeing much water pressure at idle (on a new gauge), it barely comes off the stop pin. The engine does not overheat at 5500 rpms and has plenty of water pressure (22 psi) and on throttling back I can see the pressure swing indicating the poppet valve is functioning best I can tell anyways. The overheat horn comes on when I try to run the engine at planing speed around 2600-3000 rpms. When it blows, I am indicating around 10 psi of water pressure.

The dealer said the water tube was OK and the top and bottom seals on it were good.

Does anyone have any idea what was missed on the repairs? Defective water pump repair? The dealer is working the problem but I just wanted to know from some of the forum participants.

Thanks for any help provided.

JimDD

 

[j_martin]

Re: 1984 Merc 150 V-6 Overheat

I recently spent a good bit on some work done by a local marine dealer. It was work to remedy an overheat situation that I had. The dealer ended up putting a new water pump, new overheat sensor, and had to bead blast some foulant in the heads on the water side. There was a bad gasket in the midsection that was also replaced and the poppet valve was rebuilt. I do not run therms in this engine and have not for over 15 years. This overheat problem just happened recently before I shopped the engine.

Since the repairs, I am not seeing much water pressure at idle (on a new gauge), it barely comes off the stop pin. The engine does not overheat at 5500 rpms and has plenty of water pressure (22 psi) and on throttling back I can see the pressure swing indicating the poppet valve is functioning best I can tell anyways. The overheat horn comes on when I try to run the engine at planing speed around 2600-3000 rpms. When it blows, I am indicating around 10 psi of water pressure.

The dealer said the water tube was OK and the top and bottom seals on it were good.

Does anyone have any idea what was missed on the repairs? Defective water pump repair? The dealer is working the problem but I just wanted to know from some of the forum participants.

Thanks for any help provided.

JimDD

You may think this is a stupid, irrelevant question, but what is your RPM at WOT.

 

[JimDD]

Re: 1984 Merc 150 V-6 Overheat

Capt. J,
Mercury marine engines are well reknown for being able to operate slightly above their recommended rpm range. Mine is no exception and has run at the 5800-6000 rpm range for more than 20 years. If you are implying that the engine is worn out, you lose. So tell me why you asked the question...

 

[Dukedog]

Re: 1984 Merc 150 V-6 Overheat

Sounds like a bad water pump. Not JUST tha impeller. Why no stats? Any kind of restrictor washers instead or just an empty hole?.............

 

[achris]

Re: 1984 Merc 150 V-6 Overheat

.... I do not run therms in this engine and have not for over 15 years. ....

Any particular reason?

Since the repairs, I am not seeing much water pressure at idle (on a new gauge), it barely comes off the stop pin.

Because you have no thermostats!

The engine does not overheat at 5500 rpms and has plenty of water pressure (22 psi) and on throttling back I can see the pressure swing indicating the poppet valve is functioning best I can tell anyways. The overheat horn comes on when I try to run the engine at planing speed around 2600-3000 rpms. When it blows, I am indicating around 10 psi of water pressure.

That's right at the changeover from thermostat control to poppet control. But you don't have thermostats, so the poppet isn't opening soon enough.

The dealer said the water tube was OK and the top and bottom seals on it were good.

Does anyone have any idea what was missed on the repairs?

Fitting a couple of thermostats???

Defective water pump repair? The dealer is working the problem but I just wanted to know from some of the forum participants.

Thanks for any help provided.

JimDD

Call me crazy, but in all the years I've worked on these engine, not one has left my shop without thermostats.

Chris.........

 

[JimDD]

Re: 1984 Merc 150 V-6 Overheat

I am inclined to suspect same as you are in that there is something clearly faulty within the new water pump. This was my first request to the dealer, to redo the water pump assembly and confirm water tube and seal integrity following that.

With regard to the no therms, I occasionally fish brackish and salt water here in the coastal regions of Louisiana. Along with some of my colleagues that did the same and had the same design older split cowling motors, we began running without therms just to have water flowing asap at the heads because we fish in shallow muddy shell bottom water and used the motors trimmed way out. Much easier to see tell tale water than a needle on a gauge. Because we are mostly warm in temperature I just left the therms out. It did not seem to harm the engine in doing so. But, that scenario may have led over time to the buildup of the foulant recently discovered that required bead blasting.

I dunno but this has been one heck of a fine fishing motor for me. I am the original owner.