Temp Gauge indicates Hot, But engine is Not Hot

[ShoreFlyer]

I have a 7.4L EFI Horizon Mercruiser Raw Water Cooled. The temperature gauge has been warming up to about 200-220 deg.

1. The computer indicates is running at 158-163 deg. (mercruiser scan tool)
2. I have changed the sending unite twice.
3. I changed the Thermostat.
4. Changed the Thermostat gasket twice, it does have the brass rivets in it.
5. Changed the gauge twice.
6. Hooked the gauge up directly to the battery using new wires. New wires to sending unit. by passing he harness.
7. Engine is normal to the touch.
8. Used an Infrared gun to take temps. Seems normal
9. Manifolds and Risers changed last year
10. New Water pump last month. It had broke.
11. The Alarm doesn't go off.

The engine doesn't appear to be overheating, as confirmed by the mercruiser scan tool. I believe its an issue with the wrong resistance going to the sending unit from the block. But I can't pinpoint what would be causing this.

Andy ideas ?

 

[SCORPIO]

Verify that the sending unit is matched to the gauge. I had a problem with my new engines, the senders they came with showed 160 degrees on my gauges even when the engines were stone cold. Turned out the sender output different resistance values than what my gauges were expecting to see. I got a set of older style senders and installed and all is well. I suspect your issue is something similar.

 

[kap2]

This post is over a month old but here's some info and ideas.

You can test the sending unit using an ohm meter. Connecting the meter between the terminal connector and the metal housing that threads into the thermostat housing. Obviously, you would need a container with water, heat source and thermometer.

Temp: 140F = 121-147 ohms
Temp: 194F = 47-55 ohms
Temp: 212F = 36-41 ohms

You could also test the gauge using the above resistance to ground to see if the temperature reading matches.

With the gauge reading higher then the actual engine temperature and if the sending unit and gauge are fine there could be some corrosion at one of the connectors which is creating a low resistance short to ground. You could measure the resistance at the sending unit (between its body and terminal) with the wire disconnected then remeasure with the wire installed. The resistance value should be similar. If there's a large difference, start tracing the wires and testing at the connectors.

Good luck. Let us know how you make out.