Sudden 147C ECT Temp.

OldSchoolNoe

Member
Nov 13, 2019
28
5
13
33
California
2002 Mustang, this entire month, every now and then I get a check engine code: P1285: Cylinder Head Over Temperature Condition.
Freeze data shows insane ECT temperatures ranging from 130C to 147C at VSS = 0mph. "Overheats" during idle and on startups after the car has been driven for about 4 miles. Might as well try the first fix, which is to purge any air that's in the system.

Hooked up one of those "easy fill" funnels from EPAUTO, and idled the car for about 25 minutes. It all looked good, bubbles were appearing, had heat on full blast. Monitored the temp, it slowly rose to 93C, then all of a sudden it jumped to 155C, that's 311F, what in the world!? Of course check engine light came on. The coolant in the funnel did not boil and no steam came out. Everything looked normal. Turned everything off and then put car in "aux mode." Here I monitored the ECT temp, 20 minutes later it has dropped to 119C.

About 8 months ago I changed the CHT Sensor (which is also the coolant temp sensor)(Located behind driver-side valve near the firewall).
Here's what I'm considering as a fix.
1. Maybe the CHT Sensor Connector is faulty, will splice a new one in. (Coupled with another new CHT Sensor)
2. Water Pump , Could it be that it is weak and not moving coolant out of the engine fast enough?
3. Radiator, I doubt this is the problem, it can be internally clogged, is still original, might be under performing due to wear.
 
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HemiRick

I'd be looking at jacking under the house
Jun 28, 2020
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Memphis TN
it obviously did NOT get to 311 or it would have boiled over, I bet its a bad sensor.
 

OldSchoolNoe

Member
Nov 13, 2019
28
5
13
33
California
Today replaced CHT sensor. Idled motor for 20 minutes then drove for 30 minutes. Near the end the coolant temp went from a constant 88C-90C to 97C, then 159C (318F). This time it threw a new code along with the usual codes.

P1290; Cylinder Head Temp Sensor Circ. low input

Looks like I'll have to cut that connector and splice in a new one.
 

OldSchoolNoe

Member
Nov 13, 2019
28
5
13
33
California
Spliced in a new CHT sensor connector and drove for an hour. ECT temp read between 85C to 92C . Got home and left the transmission on Park, engine idling. Went to the mailbox (3 minute walk) came back, and temp read 159C.

I've read something about an "incorrect" thermostat doing this. At this point I don't know what could be causing this issue.

Connected a better scanner (MK808), it found C1284 "Oil Pressure Switch Failure" and
U1073 "Standard Engine Protocol; Invalid or missing data for engine coolant."

Something interesting, parked, key on, engine off, scanner connected. The ECT temp was 119C, in 2 minutes it rose to 128C. Haha the engine's not even on!

I have one of those Duaralast Thermostats. Going to replace with a better one and change that oil pressure switch.

Could it be that the oil pressure switch shares the same circuit as the CHT sensor, and that's what's making it give false readings?
 

OldSchoolNoe

Member
Nov 13, 2019
28
5
13
33
California
Recorded this gremlin in action, I noticed that the fan kicks in at a much later temp (later verified that it "works."). Noticed temp was 105C and the fan wasn't on. It turned on around 110C (230F), weird. I'm starting to suspect the "Radiator Fan Relay Controller" some call it the "CCRM Module." It might have gone bad, and is shorting things out. The temp seems to jump wildly when the "fan is supposed to come on" at around [[96C ? ]]. Cleaned body grounds and checked wires, all looks good, should I proceed with buying this near $100 part? View: https://www.youtube.com/watch?v=YUrM1pTDny8
 

KRUISR

5 Year Member
Apr 16, 2015
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For the cost of the cylinder head temp sensor, just change it and see what your scanner says then.
 

OldSchoolNoe

Member
Nov 13, 2019
28
5
13
33
California
I've already gone through 2 new CHT sensors, I even spliced in a new CHT connector, today I learned something critical. It's a radiator fan problem.

The thermostat should begin to open at (86.7C to 90.6C). That's (189F to 196F). My thermo is rated at 195F
The radiator fan DOES NOT turn on at any of those temps, not even up to 97C (after that temp "jumps" to 159C).
The radiator fan DOES turn on when AC is turned on

Here's the test I did: Idled vehicle until 95C, then turned on AC
= no Overheats!
Took a look at the radiator fan wiring diagram......guess what?? The CHT Sensor / CCRM / PCM share the same circuit.
The CCRM contains the fan relay
We have, without a doubt, ruled out the CHT sensor
Will order a new CCRM and update y'all
(oh yeah, doing this test triggered another code, P1117 . It means the CHT sensor circuit is faulty)
 
Oct 9, 2021
12
3
13
Detroit
So I'm having an issue with my sensor as well. How did you get yours out? Mine is seized in pretty good. I have 19 mm wrench and socket wrench but that clearance is a B. Any advice?
 

OldSchoolNoe

Member
Nov 13, 2019
28
5
13
33
California
So I'm having an issue with my sensor as well. How did you get yours out? Mine is seized in pretty good. I have 19 mm wrench and socket wrench but that clearance is a B. Any advice?

Did you disconnect the black connector and move it out of the way? That black connector is held in place by "plastic screws" that come out with a molding tool. Once you move that out of the way you'll have a bit more space.

UPDATE: When the CCRM was shorted out, the PCM also must have been affected. The car ONLY throws an overheat code if the following conditions are met:
- Car is driven to operating temp.
- Shut off the vehicle
- If the car is IMMEDIATELY started within 10 minutes it throws an overheat code (but monitoring the temp each time shows normal operation)
The work around this is to put the key in aux mode for at least 3 seconds, THEN turn the car on.

I'm sure this is a very rare situation to get with these cars. A new PCM should take care of the issue, but waiting a few seconds to turn the car on as a work around is fine with me for now.
 
Oct 9, 2021
12
3
13
Detroit
Did you disconnect the black connector and move it out of the way? That black connector is held in place by "plastic screws" that come out with a molding tool. Once you move that out of the way you'll have a bit more space.

UPDATE: When the CCRM was shorted out, the PCM also must have been affected. The car ONLY throws an overheat code if the following conditions are met:
- Car is driven to operating temp.
- Shut off the vehicle
- If the car is IMMEDIATELY started within 10 minutes it throws an overheat code (but monitoring the temp each time shows normal operation)
The work around this is to put the key in aux mode for at least 3 seconds, THEN turn the car on.

I'm sure this is a very rare situation to get with these cars. A new PCM should take care of the issue, but waiting a few seconds to turn the car on as a work around is fine with me for now.
Too late the sensor broke already. I'm just getting rid of it