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Water Temp rising on a Sonoma


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Let me describe what this truck is doing. Its a 96 Sonoma with a 4.3. When you start it up cold and drive it until it warms up the temp guage will shoot over to about 230 for a few seconds and then settle back down where it has always run at about 180 or so. It scares the bejesus out of me each time thinking its going to over heat.

 

Its been doing this for a little while now. What would cause this? I shut the truck off in the driveway tonight and went in to grab a bite to eat. (30 min maybe). Came out and pulled the truck around back to unload it and it did the same thing again. Any help is appreciated. Water pump is pretty new on it and it has no leaks that I know of.

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  • 10 months later...

I think you may have a short or direct ground on the thermostat guage. The sender is a vaiable resistance ohm ground, so a momentary short may cause the guage to fluctuate like it does.

If you do not have access to a gauge tester, you may always start with the sender. To check the sender, warm the engine up to a stable temperature. Have an ohm meter ready, as well as the tool(s) needed to remove the wire off of the sender.
With the engine running/idling, notice what the gauge is reading, and write this down the reading. With the engine still idling, remove the wire off of the sender, then hook the positive wire of your ohm meter to the sender
where the sender wire was. Now hook the negative wire of your ohm meter to ground. What resistance do you measure?
I will give you some examples to help you determine what to do next. Let’s say that the gauge read 210 degrees, and then you took a measurement of 123 ohms. This would mean that the gauge was reading exactly as it should. And if you thought the engine was running hotter than that, or colder than that, either the sender is the problem (remember the gauge was reading perfectly based on the information from the sender), or the engine really is running that temperature.
Another example for you: You suspect the gauge is reading hotter than actual, and you measure the resistance of the sender again. You measure 253 ohms, and the gauge reads 210 degrees. In this case, the sender is at 170 degrees (based on the resistance measurement), and the gauge is not responding the way it should. In this case, either the ground to the gauge itself is insufficient (which will cause a too high reading), or the gauge is out of calibration.
Never trust how a ground looks. If your gauge shows higher than normal and the sender tests where it should, then run a new dedicated ground from the threaded portion of the gauge ground terminal to an engine ground and see if the problem persists.

Good Luck, Hope this helps a little.

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I think you may have a short or direct ground on the thermostat guage. The sender is a vaiable resistance ohm ground, so a momentary short may cause the guage to fluctuate like it does.

If you do not have access to a gauge tester, you may always start with the sender. To check the sender, warm the engine up to a stable temperature. Have an ohm meter ready, as well as the tool(s) needed to remove the wire off of the sender.

With the engine running/idling, notice what the gauge is reading, and write this down the reading. With the engine still idling, remove the wire off of the sender, then hook the positive wire of your ohm meter to the sender

where the sender wire was. Now hook the negative wire of your ohm meter to ground. What resistance do you measure?

I will give you some examples to help you determine what to do next. Let’s say that the gauge read 210 degrees, and then you took a measurement of 123 ohms. This would mean that the gauge was reading exactly as it should. And if you thought the engine was running hotter than that, or colder than that, either the sender is the problem (remember the gauge was reading perfectly based on the information from the sender), or the engine really is running that temperature.

Another example for you: You suspect the gauge is reading hotter than actual, and you measure the resistance of the sender again. You measure 253 ohms, and the gauge reads 210 degrees. In this case, the sender is at 170 degrees (based on the resistance measurement), and the gauge is not responding the way it should. In this case, either the ground to the gauge itself is insufficient (which will cause a too high reading), or the gauge is out of calibration.

Never trust how a ground looks. If your gauge shows higher than normal and the sender tests where it should, then run a new dedicated ground from the threaded portion of the gauge ground terminal to an engine ground and see if the problem persists.

Good Luck, Hope this helps a little.

 

Are you serious? It's probably his thermostat, and his problem was over 9 months ago. I'm sure he figured it out.

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