False CHT Over Heat W/ Check Engine Light

Keith Browning · October 20, 2012

It's a 2001 F350 with a 6.8 V10 triggering a false over heating condition intermittently.

I know we have discussed RFI type failures before and I might have a truck with such an issue however I am not coming up with anything. Here is my HotLine report and beyond that I have completed the Alternator Ripple test (good) and have stress tested about half of the 10 ignition coils so far without identifying any issues.

I am curious if any of you fellas have come across a similar false CHT overheat condition like this and what you found. The PCED points to a PCM which I am always hesitant to replace without some positive reason. I thought perhaps an issue with the alternator might be inducing voltage on the CHT wiring that sits in very close proximity... but no evidence of that as of yet. I have a sick feeling that this customer will have to start throwing expensive parts at this truck to resolve this - I want to consider and test everything possible before having that discussion.

Description of Vehicle Concern:

Customer States: While driving, the temperature gage will be reading in the normal range and at times will instantly peg to the hot mark and at that moment the Check Engine light also comes on. The truck is not over heating. If he pulls over, turns off the ignition and restarts the truck all is restored to normal.

DTCs: P1285 and P1299

Diagnostics Performed:

Verified cooling system operation. KOEO and KOER self tests pass with continuous memory codes in the PCM. Performed pinpoint tests DL-30, DL-3 and DL-4 with no concerns found. Performed harness wiggle test. I also removed the CHT sensor and inspected it along with the jumper harness.

Parts Replaced:

Prior repairs include a new water pump and thermostat. The customer has recently replaced the CHT sensor with an aftermarket part and the concern still exists. He then replaced that sensor with a new OEM sensor which also did not correct anything.

Question: Body Conversion:

Answer: STOCK

Technician's Question:

Pinpoint test DL-4 results lead to the replacement of the PCM. Prior to doing so I am looking for known concerns and further diagnostic recommendations if you have any. One idea thrown out was if it is possible to have an alternator problem that "might" induce voltage into the CHT sensor wiring that runs next to/touching the alternator. Is that even possible?

Additional Comments

Comment From: Ford Comment Date: 10/19/2012 1:36:36 PM

Keith,

Yes, the alternator theory is not only possible it is a relatively common cause of 'false overheats' as this truck is exhibiting. To rule this out, perform an alternator ripple test using IDS. If the alternator fails, replace it. If the alternator passes, continue with stress testing of all ignition coils, replace any that fail.

Keith Browning · October 22, 2012

Coil #5 stress test failed - sold the customer a new one and told him to re-evaluate.

mrbudge · October 23, 2012

I just had an F150 doing exactly the same thing. The parts cannon was being loaded up but I thought I should take a good look first. Found the CHT sensor wire worn through on the AC accumulator, and when the AC was on the accumulator would sweat and allow the wire to short to ground. I know that a Super Duty is layed out differently, but when weird shit starts to happen I am reluctant to call a module. It is so often a wiring problem.

Keith Browning · October 23, 2012

Yep, I am reluctant to throw a module as well... and this truck is not fixed. I think it is weird that Ford put both an ECT and CHT sensors on this engine and completely ignores the ECT. There is no PID to use to compare the values. Now I have my infrared pickup out and the CHT is reading 20-25 hotter than my readings. My meter shows around 200 at idle on the head... CHT is reading about 228.