Re: Engine temp gauge will not read past 120 degrees.
General Instructions:
1 Access the back of the water temperature gauge. Remove any panels that allow you free access to the instrument panel. Turn the boat's battery switch off.
2 Inspect the connections on the back of the gauge. Ensure the crimp terminals are connected, tight and corrosion-free. If corrosion is present, remove the connections and clean with a piece of sandpaper until the crimp terminals are bright metal.
3 Measure the voltage at the gauge terminals. Set your volt-ohm meter to the 12 volts DC setting. Attach the red lead to the gauge terminal marked "I" for ignition and the black lead to the terminal marked "G" for ground. Turn the battery switch on. Turn the engine start key switch to "I." The meter should read 12 volts. If the meter does not read 12 volts, ensure the circuit breaker for the engine instrument is on.
4 Remove the wire from the "S" terminal on the gauge. The temperature gauge should read below 120 degrees (max cold) with power still applied.
5 Connect the "S" terminal to the "G" terminal with a jumper wire with alligator clips on each end. The gauge should read above 240 degrees/max out/hot. Also with the key on, engine warm, try tapping on the gauge to see if the needle will move up. The gauge seems to be working fine if it passes these tests. Therefore, the problem may be within the temperature sender.
6 Turn off the battery switch. Remove the jumper from between the "S" and "G" terminals. Reconnect the "S" wire to the "S" terminal. Reinstall the access panels.
7 Locate the engine's temperature sender. The sender is a brass fitting with a single tan-colored wire on a screw terminal. Consult your engine's service manual if you need help finding the sender.
8 Disconnect the wire from the sender terminal. Set your volt-ohm meter to the 1K ohm scale. Measure the resistance between the sender terminal and the engine ground. The temperature sender is faulty if the resistance measures 0 ohms when the engine is cold, or if the resistance is infinite or open. A functioning sender has a resistance of between 600 to 800 ohms at 70 degrees Fahrenheit and 39 ohms at 212 degrees Fahrenheit.
9 Remove the sender from the engine with a box wrench. Ensure there is no Teflon tape or a heavy amount of sealant on the threads. A proper temperature reading is dependent on a good electrical contact between the sender's threads and the engine block.
10 Clean the sender threads. Reinstall the or new sender only using loctite pipe sealant with teflon to threads and install. Note: The Mercruiser OEM sender or the Mallory Marine water temp sender comes with the proper sealant already installed on the sender that is ok to use. Just install it. Available here on iboats.
Mercruiser Water Temperature Senders - iboats
11 Reconnect the tan wire to the sender.
12 Start the engine. Check for leaks at the temperature sender.
Water Temperature - Ohms Reading
75?F (24?C) : 600-800
100?F (40?C) : 450
140?F (60?C) : 121-147
149?F (65?C) : 99
194?F (90?C) : 47-55
212?F (100?C) : 36-41
250?F (121?C) : 29.6
Engine Coolant Temperature Sensor Resistance Checking - YouTube
Also be sure the engine temp is really getting up to 163 degrees using an infrared temperature gun.
Also be sure the gasket between the thermostat housing and intake manifold has two little rivets on each side of the gasket and the gasket goes on dry. If the rivets are covered with sealant, the sender will lose it's ground and not function properly.
Also this should be helpful as well.
http://www.boatfix.com/merc/Servmanl/13/13D4R2.PDF
Hope this helps, Good luck.
