Originally Posted by Rossterman
I think you are saying that that there is a temp switch, a temp sender, and a MEFI temp sensor as well? The switch has a single wire and is up by the thermostat at the front of the engine and screws into a horizontal opening. it causes the CEL/ alarm when above 181 degrees and the bimetallic switch closes the single wire link to ground (through the MEFI). This may affect other aspects of the engine operation- I'm not sure. This doesn't feed the temp guage ( through the mefi or directly) as is only open or closed signal. The second single wire sensor in the manifold that sits nearby and screws in vertically and feeds the temp guage. My temp temp switch was bad but ran boat for a week with the CEL and couldn't tell the difference in performance so believe it acts solely as a warning w/o affecting anything else (realize later models have limp mode to better protect engine but if a 2001 tbi does, mine isn't working).
The temp sender (2wire plug you refer to) provides variable resistance signal to the MEFI to tell iit how hot it is so as to adjust one aspect of the fuel curve map. If bad, it will cause engine to run rich. As said, There is also another single wire sensor in the manifold so that one must feed the gauge correct? So gauge could read fine showing the engine is up to operating temp but MEFI thinks engine is cold and dumps in more fuel than needed? Changing any of the single wire sensors won't fix the rich condition. He needs to check the sensor that has 2 wires and determine the resistance hot and cold then replace if out of spec
Jimn- is this logic correct?
The one-wire temperature sensor is for the gauge. The MIL (Malfunction Indicator Lamp/Light) which is sometimes called the CEL is triggered by the ECM, nothing else. RPM Reduction, which far too many take to be "limp home mode" is triggered by the ECT and controlled by the ECM, nothing else. RPM Reduction was introduced as soon as injection was added, but they may have used a bi-metal switch as a backup on the '93-'94 models- I would have to refer to my first notes and manual for later TBI to be absolutely sure. I don't remember being told about anything happening at 181°F- overheat is triggered at 200°F, although boats used in hotter locations can be recalibrated to avoid hot start issues when heat soak occurs. There's really no reason to add a switch and the associated problems that come from another sensor when the ECM is already programmed to perform these functions, other than a level of redundancy. Relying on one sender for voltage signal when the resistance could vary isn't the most fail-safe situation, but a bi-metal switch isn't infallible, either.
The logic is correct WRT actual temperature vs two-wire sensor's resistance, but it could fail with lower OR higher resistance, so low resistance would cause hard cold start problems, yet it could run fine at normal operating temperatures if the resistance is within tolerance in that range. If the resistance is too low, it would immediately go into RPM reduction. If the resistance is extremely high, it will run rich at all temperatures and be very hard to start when hot.
Again, I would have to refer to my notes and manuals to be absolutely sure about the temperature switch, but I seriously doubt that's causing any of the problems on this boat. Rich fuel ratio could be a combination of faults, too. Bad ECT and TPS or MAP sensor (or both), leaky valves/rings with extremely bad compression loss could all contribute but, unless the engine was abused, I don't think the cylinders/rings/valves are so bad although no actual proof has been presented.