Hey all, I'm relatively new to this site and looking for some wisdom. I have a 08 Expedition (5.4) that is giving me fits. It started with intermittent engine performance/limp mode and Coolant temp gauge doing strange things. I was getting P0119 and 128a which led me to replace the CHT and cable harness. That did NOT solve the issue and the engine would slowly overheat while idling. I took the thermostat out and ran it without it yesterday to see if that might be causing the issue. Removing it eliminated the P0119 and 128a errors as far as I can tell since they haven't returned. However, its still overheating some.

When engine is cool, it starts and runs well. It heats up normally to 90 degC and idles fine for a while. While monitoring the CHT sensor data (voltage and temperature) with my scanner, I can see that the dual operation of the CHT sensor switching resistors - showing 0.8v one minute and then switching to 3.8v the next - all while the calculated temp from the PCM doesn't change (as designed). The dash gauge will appear in the middle indicating everything is ok. Then, all of a sudden, I hear the engine RPM's change and observe that the calculated temp has gone from 90 degC down to 30 (or something low like that). Over the next few minutes the engine starts to heat up even more (monitored by external temp gun) and coolant starts to get hot enough that it starts boiling out of the de-gas bottle. When it does, my temp gun reports somewhere around 240/250F (115/121C). The Fan appears to be running at minimum. I can easily stop it. I had thought that perhaps the fan clutch might be the problem but since the PCM was reporting 30 decC it was likely telling the fan not to run - which it wasn't. Its not that hot outside these days so it takes it about 45 minutes to heat up enough to boil out.

Several possibilities went through my head for overheating issues like Water Pump, Radiator plug, Engine plug, Head gasket, etc. but none of these explain the PCM reporting a temperature that is much lower than actual all of a sudden - especially when the voltage reading did not change. And since there is still a Voltage reading from the CHT, I don't have any reason to believe a wiring issue or that the CHT is bad (I just replaced both). A bad PCM has entered my mind also but I don't have another to test that theory.

So I need someone else to help me rationalize this. I'm about to head out there and see if I can modulate the fan with my scanner and see what that tells me.

Thanks guys!

 

Update: The intermittent CHT has returned. I have P0119 and P1288.
I was unable to modulate the Fan from my Scanner and it is still running at a minimum as far as I can tell. Its not making and noise and its easy to stop.
Engine runs ok when Temp shows normal (96 degC right now) but all of a sudden the Temp will display something like 30 and the engine starts running poorly. Voltage from the CHT sensor can move from 0.7 to 4.x and the calculated Temp reading will not change on the dash gauge or my scanner.
There is SOMETHING wrong with how the PCM is coming up with the Temp reading. I suppose its possible the new CHT is bad but its hard to tell since the CHT has the dual-more functionality - switching from low voltage to high by design.

 

I might have missed it but did you purchase a Motorcraft CHT?

 

No, my son said he purchased it from AutoZone. Why?

 

IMHO, there is the potential issue- the aftermarket sensors (especially) are very prone to cause "goofy" issues ranging from ECM, brakes, traction control, etc. It would be my best swag to replace the CHT sensor with a MC unit and I'm betting the issue goes away!

 

Thanks for the insight. I may go through the process of changing it out to be sure. It's a pain in the rear to change. I don't know why engineers would put the CHT under the stupid Intake!

 

Understood..... I made myself a long "pick" to remove the release clip then purchased a flex head box end ratcheting wrench to remove it.... made life almost easy...............

 

Update: I changed the CHT with a OEM sensor from the deler... no difference. See the photo below. The Voltage and Temp are unstable. The actual temp during this test was 220F. I put my volt meter on the connector entering the PCM and the voltage matches what you see in photo.
What's the possibility of the PCM being the problem?

 

Here's another view

 

And the codes