Hi all, I tried doing a search, but I didn't find anything useful. anyway, I'm having an issue with my temp gauge. it will randomly peg hot, setting off a dash alarm. the truck isn't overheating, (verified with laser thermometer) and when the key is cycled (truck off and restarted) it goes back to the normal halfway mark. this is an 05 fx4 150. anyone else have this happen? I'm tempted to throw a temp sensor at it, but thought I'd ask around first. Thanks in advance!

 

Check the CHT sensor wiring and connector. If that proves out, replace the sensor itself.

 

It is true it could be wiring or connector or the CHT itself. And therefore you can't just 'throw a temp sensor at it' because @projectSHO89 is correct - that function is totally fulfilled by the Cylinder Head Temperature sensor.


Couple of thoughts that might help, since it is a such a PITA to remove the intake manifold to get to the CHT. The PCM calculates engine coolant temperature based on its reading from the CHT, (therefore it could be wiring or connector related). But the PCM sends information (IDK exactly what) via the OBDII bus to the microprocessor in the instrument panel. IT drives the actual temperature gauge and generates the alarm. However, the PCM -ALSO- presents that calculated ECT reading on the OBDII port (PID # 0105). If you have a scanner, or can have someone read the ECT temperature from the OBDII port while the symptom is present, it could certainly provide direction for further investigation and save you lots of trouble.


Also. The PCM logic places such high reliance on the CHT to determine proper engine cooling, there are a number of DTC's that __"SHOULD"__ show up if an open or short appears in the wiring. It does a 'recalibration' routine on the CHT sensor every engine run cycle. And the PCM will throw the engine into LIMP mode, (alternately shutting off cylinders) in an effort to cool things with air if it thinks the engine is overheating.

For these reasons, I suspect your problem may well not be more within, or in the direction of the instrument panel?

A very good article on the Ford CHT is here:
Diagnosing Ford?s Cylinder Head Temperature (CHT) Sensor | Search Autoparts

 

I have had problems caused by my tach gauge - it would drop to zero causing other strange cluster responses. The problem was solved for a few years by opening the cluster and sliding the gauge on its contacts. It has started to happen again so I will repeat the "fix".

Here are some instructions on removal https://www.ford-trucks.com/forums/1...r-windows.html

 

Thank you, I will check the wiring, read up on that link, see what I can figure out.

 

Finally had a chance to scan while the issue was present. while the scanner I used couldn't tell me the temp, it did show a KOER code of P1285 (cylinder head overtemp). after shutting the truck off, restarting and running the test again, the code was gone, as was the gauge issue. I checked the DTC's after that, and found 2, P0171 (system lean, bank 1) and P0303 (cylinder 3 misfire) I don't think the DTC's and the KOER codes are related. the CHT sensor isn't particularly expensive, and I don't have the ability right now to test the voltage so I think I will just replace it and see if the problem still presents.

 

Actually, there is one specific failure that could tie all three issues together - a blown head gasket at cylinder #3.

Good luck and hope it's only the CHT and that the other stuff is more mundane!

 

Didn't think about that, but you're right. :/