Mercury 175 Black Max Alarm Help!!

[fastmike22]

I recently purchased a used bass boat with a 1986 merc 175 Black max on it. The boat is in nice shape and the engine looks like it is brand new under the hood. Well my first trip out it ran good all day at quarter throttle while fishing but when i started getting into the throttle the alarm went off. Soon as i get the rpms down no alarm and i could run it like that all day. Also if the alarm was going off if i shut the engine down and start it back up there was no alarm on. So after reading up i think its the overheat alarm because it was a steady tone. I took the temp sensor out of the head and put it in a cup of heated water and the alarm went off at 145 degrees. I also have a steady stream on the garden hose but when i rev it up the stream starts to get smaller. Im not sure if that means the impeller is going bad or if the garden hose just didnt supply enough water. Also i could not get that alarm to go off on the hose. Both oil tanks are full and seem to be working so i dont think its a oil problem. Any help or pointers would be appreciated. Thanks!

 

[Barnacle_Bill]

Re: Mercury 175 Black Max Alarm Help!!

A steady alarm is an overheating alarm. When is the last time the impeller was replaced? If you don't know then replace it. When using the hose the impeller doesn't have to do anything as pressure from the hose is forcing the water thru.

 

[fastmike22]

Re: Mercury 175 Black Max Alarm Help!!

Im not sure when the impeller was replaced last. I have the boat in the shop right now and they are replacing it just to be safe. I hope thats the problem. I just was trying to see if anyone else had a similar problem before and what they did to fix it.

 

[Dukedog]

Re: Mercury 175 Black Max Alarm Help!!

I took the temp sensor out of the head and put it in a cup of heated water and the alarm went off at 145 degrees. Thanks!

Its not supposed ta go into alarm at that temp. Thats just barely warm enough ta open tha T-stats. Should be over 190/200+ degrees. Some vary a little. A "complete" water pump kit is usually tha best thing for a "new" to you motor no mater what you see or are told...................jmo

 

[fastmike22]

Re: Mercury 175 Black Max Alarm Help!!

I thought that 145 degrees seemed cool for an alarm but im pretty new to the boat world so i wasnt sure. I guess i will see once the impeller is changed if that was the problem

 

[Dukedog]

Re: Mercury 175 Black Max Alarm Help!!

guess i will see once the impeller is changed if that was the problem

Put a "complete" oem water pump kit, not just tha impellar.........jmo

 

[Texasmark]

Re: Mercury 175 Black Max Alarm Help!!

As mentioned, the pellet on the stat should be stamped 143 and that's F and the temp where the stat starts to open. The OT sensor is, as mentioned 195 +/-. You are correct the alarm went off as a result of your sensor malfunction, not a cooling malfunction.

Mark

 

[fastmike22]

Re: Mercury 175 Black Max Alarm Help!!

So it looks like i will be replacing the sensor then. I also am going to order a temp gauge so i can keep an eye on things.