I was doing some work on my 2000 XJ when i noticed there was a wiring harness unplugged in the front of the engine near the radiator cap. I looked it up and its the ECU throttle position sensor, so should i plug it back in or keep it unplugged? There was also no check engine light and the engine was running perfectly with it unplugged

 

Does you check engine light work ? and how's it run with it plugged in ?

 

How many wires on this pigtail? Post a picture.

 

The TPS connector shouldn't be in that area. The TPS is in the back of the throttle body facing the firewall. Could it be for the electric fan?

 

Its this connection here right by the radiator cap, no idea how long it was unplugged for but when i plugged it back in i noticed no difference.

 

I'd have to look at mine to be 100% sure, but my guess is also the electric fan which is right in that area. That's not a TPS wire, like previously said the TPS is attached to throttle body.

 

That connection is for the electric fan.

 

If that were the TPS, you wouldn't be running at all. I'm with the "electric fan" vote.

 

X2.... just had mine disconnected recently to get the electric fan out of the way for replacing my water pump. You definitely want that plugged back in.

 

Unplug it and switch on a/c: electric fan doesn't run.
Plug it, switch on a/c: electric fan must run!
That's the easiest check.

 

Well since ive replugged it in, my mpg has gone up about 5 to 17mpg. ill try the electric fan test too

 

The TPS would have 3 wires, not 2.