2005 Expedition - Rough Idle with a P1289 Code (CHT)

[RandomHunter208]

I recently picked up a 2005 Expedition with a 5.4L 3v, and have been working my way through all of the little issues. I think I have it about wrapped up but I cannot seem to figure out what going on with the P1289 code for the Cylinder Head Sensor. I replaced the sensor and inspected the wiring all the way to the PCM and have no reason to think the new sensor is faulty. When I view the live data the scanner shows an engine temp of 55 c (doesnt change), and the gauge on the dash doesn't move. No lights are on the dash to indicate any other problems. When I clear the code and reset the PCM the truck will run great until the thermostat opens, then the code comes back and the engine falls into a rough idle and doesn't want to run. My suspicion is that the PCM is probably bad, unforcedly because of the year I can't seem to easily locate a replacement at the locate yards to check, and outside of a gut feeling I don't have a reason to spend another 200 bucks to throw parts at this thing. Has anyone had similar issues or is more familiar with this vehicle? Everything seems to be tied to the temperature issue.

No vacuum leaks
New TPS sensor (3rd gen)
New CHT sensor
No Misfires
The temp Gauge is not working

 

[RandomHunter208]

I just wanted to follow up after more diagnostics with the final result in case anyone runs into a similar issue. The initial symptoms were a rough idle, no temperature gauge, and the vehicle being in a limp mode state (little to no power getting up to 50+ mph). At a stop, it ran so poorly it would almost die. Last but not least it was throwing a P1289 CHT code.

The results are exactly what you might expect. The rough idle is a result of a vacuum leak from a cracked fitting on the back passenger side of the intake (looks like the hose routes over to the brake booster), once that was fixed the engine idled perfectly. However, it still has hesitation while accelerating, and has little to no power. as well as no temp gauge. After running around this thing for far too long I started chasing the wiring. The engine pigtail running to the CHT sensor was the culprit. It looked like the engine may have been swapped and that two-wire lead was soldered together with heat shrink. This splice looked okay but the corrosion was the full length of the pigtail. This connection is located behind the engine just over the transmission bell housing. The pug is a two-pin round style fitting (you should be able to feel the difference, it's pretty close to an o2 sensor plug but slightly more to the center of the engine). I unplugged the pigtail from the main harness, removed the alternator, and was able to pop the clip off the sensor with a long screwdriver and a pry bar and retrieve the 8" to 10" sensor wire. I cut the old pigtail up and made one roughly 28" long and reinstalled it coming around the front of the intake. Collectively under an hour once I was able to test the resistance and saw the OBD II scanner pick up the temp change. A key piece of information here is that the live data stream showed the engine temp stuck at 55 degrees Freiheit (I mistakenly wrote Celcius in my initial post). If your vehicle is having these symptoms, I would check the engine temp on the scanner first and go from there.

Best of luck.