Engine Temperature Gauge Reading Low & Erratic

[AcetyleneDreams]

Hello fellow forum members! I recently noticed some odd behavior from my engine temperature gauge and I would like to see what your thoughts are.

 

Vehicle Remarks: 2005 Sierra, 5.3L V8 (VIN code B), 100% factory everything with no modifications to anything (even the fluids are factory). Current mileage is ~93k. Coolant system was last serviced at ~48k miles and I have the 100k mile service coming up in September. No abnormal conditions exist (i.e. check engine lights, knocking, abnormal emissions, etc.).

 

Environmental Remarks: Temperatures in my area are roughly 75 degrees F to 95 degrees F, with relative humidity of 90% - 100% on most days.

 

Problem Statement: During normal driving (around town and highway, no trailer, and minimal cargo in the bed), the engine temperature gauge indicates normal warm-up between 160 degrees F up to about 185 degrees F (which is the “half way” mark on the gauge). Once the gauge indicates a temperature of 185 degrees F, the gauge response changes. The temperature never exceeds 210 degrees (normal operating), but it will fluctuate between 185 and 210 degrees F quite frequently. The two electric cooling fans run periodically as they always have, but they are certainly not running continuously.

 

I have owned the truck since 2005, so I’m very familiar with how the gauge has always responded. Normally, the gauge sits straight up at 210 degrees F all the time, load or no load, around town or on the highway, trailer or no trailer. The fact that the gauge is now sporadic leads me to believe the temperature sensing unit or the gauge itself (or both) is faulty, or perhaps there is a bad electrical connection somewhere.

 

I’ll also comment that I am aware the electric fans (or at least one fan) operate when the air conditioning is on. I run the climate controls in “Auto” mode all the time, which means the compressor is always on call. However, I have driven without the climate controls active (no heat, no A/C, no fan, etc.) and I have observed the same behavior in the gauge. Further, realizing a high A/C charge pressure can cause the fans to operate, I had the A/C serviced at roughly 70k miles. This problem began within the last 5k miles.

 

The only cause for the gauge behavior I can conclude is that the sensor or the gauge or both have gone bad, or perhaps there is a bad electrical connection. I’ve never had a problem with this truck, so I’d like to believe that the gauge isn’t lying to me. Since the temperatures haven’t climbed over 210 degrees F, and since there are no other indications of a cooling system problem, it hasn't given me too much cause for concern.

 

When this truck goes in for the 100k service, I will ask the technicians to chase the problem. But in the interim, what are your thoughts?

[AcetyleneDreams]

Update: I've read the service manual (I bought the entire book set) which has a section entitled "Engine Fails to Reach Normal Operating Temperature." The manual suggests the following:

 

1) Check for codes using a scan tool. I don't have a scan tool handy, so I'll have to check for codes later today at the parts store.

 

2) Check that the coolant level is above the "ADD" mark. I'll verify this.

 

3) Inspect for stuck open, missing, or incorrect thermostat. I know the thermostat is installed and it is the original thermostat. If it's stuck open, that could be the culprit I hadn't identified previously.

 

I have some homework to do. I will report back soon.

[AcetyleneDreams]

I have inspected the coolant level. Hot and cold are exactly where they should be, and the coolant itself is clean. It looks brand new despite having ~45k miles usage.

 

While in neutral, A/C off, I let it idle up to 210 degrees per the gauge. Again, it warms up as it always had until ~185 degrees, and then it is very slow coming up the rest of the way. I noticed that any change in engine speed (i.e. increasing engine speed to ~1200 RPM, again in neutral) causes the temperature to decrease which makes sense since the water pump speed and engine speed are related.

 

So, still in idle (no revving the motor), neutral, A/C off, I let it run awhile longer. The cooling fans never kicked on, but the temperature continued to fluctuate between ~185 and 200 degrees.

 

So, I took it for a test drive. Very low speed around the neighborhood, no A/C. Every time I stopped, the gauge was at 210 degrees. Touch the throttle, and it went down to 185 degrees. Cooling fans never started.

 

I drove it home and parked it in neutral with the A/C off. Once the gauge said 210 degrees again, I turned on the A/C. The cooling fans (both fans) started in slow speed and the temperature gauge stayed on 210 degrees for several minutes. I turned the A/C off, cooling fans stopped, and the gauge remained at 210 degrees.

 

All that data suggests to me it may be a stuck open thermostat. Any thoughts would be appreciated! And, has anyone else had experience with a thermostat stuck open (or even removed)? I'm unfamiliar with what the temperature response would be in that case. Any symptoms might help me chase my issue.

 

Thanks!

[kstruckcountry]

I would start with a full coolant change and new thermostat (hoses too if they're original). Most people replace the thermostat whenever a coolant change is done.

[Kent2500]

Might as well pressure test the radiator and radiator cap for additional data points. Should be able to rent/borrow from a local parts store.

[Zebrafive]

Since you are not overheating. I would wait for the 100K service next month. I assume that includes coolant change. Ask if it includes new thermostat, if not have one put in.

At the age and mileage, I would change radiator caps now and see if any change. Do the easy cheap things first.