Temp goes to hot then drops- replaced Thermostat??

[Beachrights]

1992 Sunbird Corsair with a OMC Cobra 5.0.

When 1st starting and warming up temp gauge goes to "hot" then it appears Thermostat opens and temp drops like a rock to normal- boat will run all day and not go back to hot.

Replaced Thermostat thinking stuck or defective....nope....same issue. Risers get very hot before temp change-will cool once temp is within normal range

Replaced Impeller- same issue.

Have good water flow since I took off hose

Any ideas? I'm stumped. Boat is clearly getting too hot till temp drops- Thanks

 

[alldodge]

Might need to replace the pump housing. Buddy had same issue with new rebuilt 5.7, did all kinds of things to help, and nothing worked until pump housing was replaced

 

[Beachrights]

Meaning sucking air vs. water?

 

[alldodge]

Meaning sucking air vs. water?

Was not involved in the repair, but guess the housing was worn just enough to keep the impeller from working at 100 percent. If blades were not making full contact it would cause some water bypass leakage

 

[MRS]

AllDodge is right I had same troubles till I changed out everything plus check the plate under the impeller. Make sure it is clean and no ruff spots on it. you can flip them over if needed.

 

[Beachrights]

Turned out to be a bad Impeller...new! I noticed it was a little warped from storage - bought another and problem solved! Don't judge a book by it's cover right?