Weird Overheat/False Overheat?

[JayT]

I had the truck on the highway this weekend, and the temperature gauge needle spiked from the middle where it normally is to full HOT in around 2 seconds. I immediately got off the gas, and coasted to the shoulder. By that time the temp returned to normal. I checked under the hood and nothing seemed abnormally hot.
Temp gauge stayed at normal, but I was watching it more closely of course. After 10-20 minutes it would do a quick spike to full H, and when I coasted I counted 7 seconds and it would drop straight back to normal.

I did a search on this and found a thread describing the same problem, but they never said what the solution was. I don't know if I have a bad sensor, bad gauge, or what. I've never seen an engine overheat instantly without a cloud of steam from a blown hose, and I've never seen an overheat cool down to normal in 7 seconds.

The only think I can think is there is something going on with air in the system, because I do get some gurgling sound when I turn the heat on in the morning. Reservoir is at the full mark. I have "burped" it a few times to relieve whatever air may be in it, but the problem continues every 10-20 minutes of highway driving.

 

[JayT]

Update

Pulled OBDII code P0597 from it today. It said "thermostat/heater control circuit open". Whether this means I need a new thermostat or not, I don't know yet. I will appreciate any input on this.

 

[swetrid]

Here is pinpoint test for you.
2003 PCED Gasoline Engines SECTION 5: Pinpoint Tests
Procedure revision date: 08/19/2002

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KQ: Thermostat Heater Control (THTRC) KQ: Introduction

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KQ10 DTC P0597/P1432: CHECK VPWR AT THTRC HARNESS CONNECTOR
Note: Refer to the PCM connector pin numbers in the beginning of this pinpoint test.

Disconnect the thermostat harness connector.
Key on.
Measure VPWR circuit voltage at THTRC harness connector.
Is the voltage reading greater than 10.5 volts?
Yes No
KEY OFF. GO to KQ11 . KEY OFF. REPAIR VPWR circuit.

KQ11 CHECK THERMOSTAT RESISTANCE
Measure resistance of the thermostat at the two pin terminal connector.
Is resistance between 14.0 and 16.0 ohms?
Yes No
GO to KQ12 . REPLACE thermostat.

KQ12 CHECK THTRC CIRCUIT FOR OPEN IN HARNESS
Disconnect PCM.
Measure resistance between THTRC circuit between PCM harness connector pin and the THTRC harness connector.
Is the resistance less than 5.0 ohms?
Yes No
GO to KQ13 . REPAIR open circuit.

KQ13 CHECK THTRC CIRCUIT FOR SHORT TO GROUND OR VPWR IN HARNESS
Measure resistance between THTRC PCM pin and PCM pins VPWR, SIG RTN and PWR GND.
Is each resistance greater than 10,000 ohms?
Yes No
For Continous DTC P0597/P1432, GO to Z1 .

For KOEO or KOER DTC P0597/P1432, REPLACE PCM. REPAIR short to GND or VPWR.




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[JayT]

Thanks

Swetrid: Thank you. Fortunately the temp gauge is not spiking in city driving, so I will perform those tests when I get time later this week.

 

[JayT]

Update:

I finally decided to get a new thermostat at the beginning of this month. I could not get anyone to do the tests noted above for a reasonable price, and I am lost on automotive electronics. The old thermostat was clogged with rust and gunk, and the whole system needed flushed. Now it runs like new, and the heater works much better. I am blessed to not have blown my head gaskets. Now I have a lot of good things to say about this 2.3L engine.