I have been reading online all night, so many threads just stop and they never have a final answer when they fix the problem. I have a '99 250 with the v10. Today out of the blue it idled rough for a second the check enine light came on and the it went right back to normal. I have a scan guage on it that gives me codes and digital readouts. P1229 cylinder head temperature sensor. From what I understand when this happens it shuts off 4 injectors allowing nothing but cold air to pass through the motor and cool it down. My first thought was a bad sensor. After it happened a few more times, some diagnostic work and research this is what I found and anyone with some information I would greatly appreciate it before I start changing everything.

​​​1. When the temperature guage spikes the scan guage shows both the water temp and cylinder head temperature (CHT) show about 260F, they are always both within 2 degrees of each other.

2. When it spikes it is instant. With in a second it goes from about 202F to 260F then sometimes it is a second or 2 before it drops to 202F. Other times I have to turn off the truck and restart it. One time I had to do 3 or 4 times.

3. I have a Cat S60 phone (it has Thermo-Imaging) the thermostat housing reads 200ish F. (Amazing phone for anyone that has never seen one you should check them out). Which leads me to think it is not the thermostat sticking. (Expecially with such a extreme spike in temperature if it was a more gradual I would assume it was the thermostat.)


4. From what i have read thee PCM gets reading from the CHT and a water temperature sender. The dash dial guage has its own sensor.

5. If I unplug the CHT sensor the scan guage reads -40F. If I unplug the water temp sensor on top of the intake the dial guage drops to cold but the scan guage still gives a digital readout.


Questions

1. Has anybody ever had this problem and what was the fix.

2. Why would all 3 sensors be giving me close to the same readings?

3. If the dial guage on the dash has its own sender why would it spike hot as well? (Pretty much the same question as 2)

4. Where is the sender for the dial guage? (I have found the CHT and PCM water temp sender)
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Any help would be awsome.
Thanks in advance
When I find a fix I will make sure I post it for everyone so in the future hopefully someone will read this post and have an instant answer.
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This sounds like a wiring/ground issue. If all the temp sensors are spiking to max at the same time it could be a bad ground causing a false reading, or the wire harness coming into the firewall/PCM could have a bad connection.

 

Yep, either a fualty sensor or a bad connection to the CHT sensor. The wiring harness has been known to fail internally (broken wire inside the insulation) and cause these symptoms.

 

One other thing to keep in mind, around the 99-2001 range, we had some members report that the PCM was going into 5-cylinder mode, and no amount of checking wiring or changing sensors made it go away. While the "failure-mode" is different (in their case, it would just happen and stay that way), the PCM was ultimately the issue.

I forget which years, but I think the PCM only uses the CHT - the sensor in the intake is for the gauge. You can read the CHT temp as both "coolant temp" and "cylinder head temp" from the PCM but I think they are in fact the same thing.

Another couple of notes: The fact that you unplug the CHT and it drops to -40 means that a wire break would do the same, not peg it at 260F. Granted it could be a SHORT - to ground, most likely, so a worn-through wire loom grounding to the block is a good possibility. The fact that you restart and it goes away also leads one to think that the wire is grounding out, OR the PCM is faulty.

 

I had to drive home or pay a 500 dollar tow bill this morning. I chose to drive home. I got about 5 miles and the truck started acting up again. Pulling a trailer with less than all 10 cylinders isn't fun.... neither is trying to get back into traffic middle if downtown denver after I pull over and restart the truck. After the 3rd time I unplugged the cylinder head temperature sensor. The scan guage showed me a water temp of -40 and a CTH of 200 (correction to my first post) the rest of the trip. The dial guage on the dash read normal temp. So thinking the CTH and water temp through the scan guage are from the same sender. Anyways it drove home just fine(200 miles) and I have a new CTH sender on order with a round of intake and valve cover gaskets. (Valve cover has been leaking for a while.) Any more input from anyone would be great. Will update when everything is replaced.