Engine instantly dies when Coolant Temp Sensor is plugged IN

[CaptainMak]

I have a '99 Merc 5.0L with the TBI EFI. There are 2 sensors in the thermostat housing. One on strbrd side is one stud, tan wire. The other one is on the port side of the housing and has a black and a yellow wire (2 wire plug). If I keep this 2-wire sensor unplugged, boat runs good, but seems to run rich and no power after about 3/4 throttle (almost seems like past 3/4 throttle the TBI starts getting flooded). Back off the throttle, and it runs good again.

From what I can tell, this sensor sends engine temp to the ECU (MEFI-3 setup) which in turn controls the air/fuel mixture to the engine. Wondering if it's related to the flooding past 3/4 throttle. Is there a way to test these or should I just cough up $22 at the local parts store for a Chevy sensor and replace the old one ?

 

[puddle jumper]

I would bet that your sensor has gone to ground and needs replacing. The engine computer is seeing an over heat and shutting it down. With it un plugged the computer is seeing -40 and is richen it up if it an fuel injected engine.

 

[CaptainMak]

Thanks for the info Puddle Jumper. Is there a way to test resistance or anything else on these sensors just to make sure it is the problem ?

One other thing that I forgot to mention. When I pulled out the sensor, the tip had a slight coating of what looked like rust. Is this normal and would it screw up the readings?

 

[CaptainMak]

Tested new vs old sensor today. Both sensors test same resistance levels +/-.

Next on the to-do list : test voltage drop, continuity, and grounds to the ECM


One question: With the ignition ON, engine OFF, should I be getting 5 volts at the yellow or black wire while the pigtail is disconnected ? Currently I'm not getting the 5 volts with ign ON , engine OFF

 

[CaptainMak]

Okay, so there is 5 VOLTS coming into the pigtail of the coolant temp sensor.
At about 140*F, the voltage drop is to 2.5 VOLTS. This seems okay.
There is continuity between the MEFI3 connector and both the signal and return wires.
I'm stumped why the engine dies with connector plugged in. Leaning towards a bad ECM.

Over 60 views, . anyone got any ideas ?