175 efi overheating alarm.

[Dlanders91]

Hey guys, new to the site. I recently purchased a 96 mercury 175efi that had been sitting 5 years. After replacing injectors, water pump lower unit and thermostats, the shop still can't figure out my issue. Now the only problem is that once on plane the boat gives an overheating alarm. It will go off if you set it back down at idle. They replaced a temp sensor thinking it was bad but it is still alarming out. The alarm sounds but doesn't go into limp mode. Any ideas? Thanks!

[Dlanders91]

Just heard back from the shop. It is hitting 240° on a gauge. They said something about an exhaust plate or gasket under the power head? Any input?

[leonreno]

The gasket in adapter/ exhaust plate could leak causing exhaust gas to enter into the water flow causing overheat. It happens but I would try other things first like the poppet valve. Could be stuck closed, it opens up at higher rpm to provide for more water flow if stuck closed it would cause overheating at higher rpms but would be ok at idle. It’s easy and cheap enough to check or replace, should be done before exhaust plate, which requires removal of the powerhead. Also water pump was replaced but I would also have the water pump base and it’s gasket replaced too, if not done, which many times is not done.

[JOE54]

no limp mode on this engine,agree EVERYTHING else in cooling system should be checked before gaskets are replaced,and I would have to know what the wpsi is ,I have worked on hundreds of these engines and have not found that to be a common problem,now 3.0L is a problem,start at lowest wp base gasket and go up hope this helps ya JOE

[JOE54]

also check the spring on popett for exhaust residue [black carbon] good indicator gasket problem

[EuropeanAM]

Please see the Forum Announcements, and provide the REQUIRED info when posting. Thanks!

Two areas:

1. GASKET under the impeller plate or below the water pump base. Commonly found to be broken/torn, will cause current symptoms.

2. Poppet valve sticking (in any position).

[Dlanders91]

Thanks to all for the responses. They replaced the gaskets at no avail. Today they are replacing the poppet valve and I'm picking the boat up win or lose! If this doesn't help I will try to figure it out myself

[Dlanders91]

Well $875 later it was a $40 poppet valve!

[leonreno]

Glad you got it fixed, BUT they would have an issue with me if they pulled powerhead to replace gaskets before checking/replacing the poppet valve. The three things that should be checked for cooling problems are the water pump, thermostats, and the poppet valve. It’s in the Mercury service manual.

[JOE54]

agree EVERYTHING else in cooling system should be checked before gaskets are replaced,and I would have to know what the wpsi is ,I have worked on hundreds of these engines and have not found that to be a common problem,now 3.0L is a problem, hope this helps ya JOE