[Crownie2204352]

I have a Crownline 220ccr with a 5.0L EFI that until last weekend ran fine. Now it will not go past 120 degrees. I think it's a stuck thermostat. Anyone have any other ideas?

[burp]

Thermostat....that is where I'd start.......the other possibility is a defective temp sending unit. Highly unlikely that it is the gauge itself. Here is something that was posted on another forum......

Your problem could be caused by several things.......If the thermostat is stuck open by debris, then the engine temp will most likely be lower than normal. If you have had the raw water impeller replaced and it was missing some of the rubber blades, the missing blades can be "found" at the thermostat. The thermostat will have to be pulled to check for debris keeping the thermostat open. If you find rubber blades at the thermostat and you have not had the raw water impeller replaced........it's time to do so. For the Merc Alpha outdrive, the raw water impeller is located inside the outdrive.

There are a couple of other things that could potentially cause the problem...a bad temp sending unit or gauge. These items are easy enough to check, one of which will require a DVM/ohm meter.

Locate the temperature sending unit on the engine and unhook the single wire attached to it. Turn the boat's ignition key to the "on" position (without starting the boat) and ground the wire that you removed from the temerature sending unit to the engine block. The needle on the temp gauge should fully deflect, indicating maximum temperature. If so, then you know the power, ground and signal wiring to the gauge is intact and the basic function of the gauge is working.

Next is the check of the temperature sending unit. For this you will need a DVM or ohm meter. Leave the wire that you previously unhooked from the temperature sending unit disconnected. Connect one lead of the DVM/ohm meter to a good ground on the engine block and the other lead to the post on the temperature sending unit (where the wire was disconnected from). Prep the boat to be started (if on land-muffs and water supply to the outdrive), set the DVM/ ohm meter to measure resistance and start the boat. As the engine temp increases, then the resistance value measured by the meter should change as well. As the temp increases, the resistance should decrease. The change should be smooth. If it doesn't change or if the measured resistance is irratic, the temperature sending unit is defective.

Some basic resistance values (for reference) of the temperature sending unit:

Water Temp Temp sending unit resistance

100 degrees (F) approximately 450 ohms
175 degrees (F) approximately 99 ohms
250 degrees (F) approximately 29 ohms
Should the temperature sending unit be defective, do NOT use teflon tape on the threads of the new temperature sending unit.

I suspect you will find the thermostat is stuck open. If it's not, you at least have more info to help locate the problem.