1998 Cherokee Sport 4.0. My thermostat was in a stuck open position for about a month before I got to change it. Had to check engine light on and the code red stuck open thermostat. I changed the thermostat and it started right up and ran fine for about 10 miles and then quit running... When taking out the plugs they all seem saturated and soiled even though I just did a plug job a month ago. I've been reading that if the temperature of the engine is running too cool the engine tries to compensate by flooding gasoline into the motor. I've also read that if I groundout the temperature sensor so it thinks that it is running at a hot temperature and leans out the mixture so I can start the Jeep. I took the plugs out tonight and opened the throttle body with a screwdriver to let it sit overnight and hopefully dry the gas out of the motor. Am I on the right track? Which color wire from the temperature sensor is the one that I groundout yellow or tan?

 

On my 1996 XJ, the temp sensor is screwed into the thermostat housing. I don't know if grounding it out is a good idea, but removing the electrical connector and jumpering the contacts in the wiring harness side is probably what you are describing. Paper clip or the small gauge wire might do the trick. On a GM Saturn Ion, that is how you get the low coolant alarm to stop when the coolant float level system has failed.

 

It sounds more like you've got a bad O2 sensor to me. Once the engine is warmed up it uses the O2 sensor to adjust the mixture. Check that it's getting power, and the fuse is good (I don't recall if it has a separate fuse for the O2 sensor heater). Mucking with the temp sensor won't accomplish what you want. I would however get out your multimeter and test that the coolant temp sensor is working properly. At 50*F it will measure around 17-20,000 ohms. At 212*F it'll be 640-720 ohms. You're not looking for the ohms to be a perfect match, but they'll be in the ballpark and you should see a bit swing in the resistance as it warms up. It's the same part number as the intake air temp sensor, btw.

 

A couple of years ago the wires to my upstream O2 sensor were fried and shorted because the harness was too close to the exhaust manifold and was blowing a fuse. A shop diagnosed it and repaired, extended the wires.

Can't remember which fuse.

 

I checked all fuses and relays. All good. Is there any way I can bypass an O2 sensor? Unplug it just to get it started so I can verify that's it?

 

yes, just unplug it, examine the upper part, a short in the wires can confuse or even toast your PCM

I believe the computer will run the engine as though it has just started, see if it changes the running state, then plug back in

a failed O2 sensor (or wiring) should throw a specific OBD2 code

 

It's not. It grounds through the body.

 

The coolant temperature sensor (CTS) is not grounded through the body. It is a 2 wire sensor receiving both positive and ground. It contributes resistance to a circuit, which the PCM tracks to know the current engine coolant temperature. "Grounding it out" would be equivalent to a short circuit. Creating a short circuit on any circuit that is used directly by the PCM for measuring is a terrible idea, and can damage the PCM internally.

 

There was a thread awhile ago where the temp sensor was screwed in too far and was touching the housing itself causing erroneous readings. IDK, I'm just saying.

 

That would almost certainly make it read too hot by getting heated by the housing instead of just coolant.

 

I think he meant the sensor was shorting against the housing. I'm pretty sure the housing it going to be pretty close to the same temp as the coolant.