Replace Engine Coolant Temp Sensor or ECM?

['TilWheelsFallOff]

On our 94 SLE after some engine warm-up, the check gauges light illuminates for approx. 5 to 10 seconds and I hear a chime, but no issue with the temp, oil, or voltage gauges. Also, the electric engine cooling fan(s) are "on" when the check gauges light is illuminated.

Using an OBD Diagnostics cable connected to the vehicle's 16-pin ALDL connector and my laptop, I monitored the engine coolant temp sensor. It shows the sensor temp increasing from a steady 154F to 296F in just 0.2 seconds. The temp was then erratic mostly above 250F for 5 to 10 seconds before dropping back down to the steady 154F. The engine cooling fan(s) were also "ON" in the data log when the temp was high.

I'm trying to determine if the sensor or ECM needs to be replaced. I'd rather not remove the difficult to get to sensor and test it if I can avoid it. I also know that this vehicle is supposedly notorious for ECM failures. I replaced the ECM 12-years ago (32K miles). Current mileage is only 81K.

Any help would be greatly appreciated.

[CMNTMXR57]

Dexcool can gunk up the sensor. It's a fairly known issue.

As always, start small/easy first. ECM replacement is the LAST thing you do.

['TilWheelsFallOff]

Dexcool can gunk up the sensor. It's a fairly known issue.

As always, start small/easy first. ECM replacement is the LAST thing you do.

CMNTMXR57, thank you for your response.

We are the original owners of the vehicle. I am the only one to have replaced/maintained the cooling system. I have always used traditional coolant, never Dexcool.

Does this change the thought process of the sensor being the likely cause of the erratic temperature behavior?

[CMNTMXR57]

Sensors still gunk up. It is a known issue.

Replace the simple/cheap stuff first. Even in GM's troubleshooting tree, ECM/PCM replacement is the absolute last thing in the process. Everything between that is troubleshooting sensors and the wiring itself

['TilWheelsFallOff]

This weekend I replaced the engine coolant temperature sensor with a new AC Delco part. Unfortunately, I noticed basically the same jumps in coolant temp when data logging.

Is it possible for this erratic behavior to be caused by an intermittent wiring issue? It seems to me it's now either wiring or the ECM. (Unless the new sensor is defective. I hope that's very unlikely...)

Could something like the 5 Volt reference voltage to the sensor be fluctuating intermittently? Is there something I can look for in the data logs to pin down that the ECM needs replaced?

[CMNTMXR57]

Yes. Testing the wiring is the next step after you verify the sensor itself.

['TilWheelsFallOff]

Yes. Testing the wiring is the next step after you verify the sensor itself.

The vehicle has the original spark plug wires (21-yrs old, 80K miles). Is it possible the spark plug wires now have deteriorated and leaking insulation allowing sparks to intermittently leak through the insulation to the engine coolant temperature sensor wiring creating an induced voltage?

This intermittent increased voltage would then be "seen" as an intermittent increased engine coolant temperature by the ECM.

['TilWheelsFallOff]

I replaced the spark plugs and spark plug wires. Unfortunately, I'm still seeing the same jumps in coolant temperature when data logging.

How do I go about testing the wiring for an intermittent short? (The glitch is prevalent at temperatures above ambient.)

['TilWheelsFallOff]

Shaking the wiring harness, rather harshly, eliminated the problem!

No more chiming, no more illumination of the check gauges light, and no more jumps in the coolant temperature when data logging.

Thanks everyone! (I apologize for not providing this update months ago. I forgot to close the loop.)