Sender sends high temp

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_rgk

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The temp sender at the block works fine until the temp reaches 180F, then it sends a very high temp (240 or higher) and fluctuates.

I've checked the sender resistance and it checks out, so I don't think it's the gauge. I've cleaned the threads as best I could at the sender and block. The sensor at the thermostat is reading steadily around 190 once the thermostat opens, which I've confirmed with an IR thermometer. The top radiator hose is warm, the bottom hose is cold.

Air has been bled and no boiling or coolant overflow or any other signs of overheating. I checked cooling system pressure and it holds. Car runs and idles great.

I just want to make sure this isn't a coolant circulation issue before I replace the sender. When pointing the IR thermometer at the head near the sender, it reads more or less the same temp that the sender is sending. It seems that it's sending the head temp instead of the coolant temp, which I believe should be lower due to pressure and circulation.

Is my reasoning sound?
 
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Schurkey

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WHAT ENGINE? WHAT VEHICLE?

The temp sender at the block works fine until the temp reaches 180F, then it sends a very high temp (240 or higher) and fluctuates.

I've checked the sender resistance and it checks out, so I don't think it's the gauge. I've cleaned the threads as best I could at the sender and block. The sensor at the thermostat is reading steadily around 190 once the thermostat opens, which I've confirmed with an IR thermometer.
Do you mean the sending unit in the head?

What do you mean by "checked the sender resistance and it checks out"? The sender resistance is appropriate for the high and fluctuating reading at the gauge? Or the sender resistance is normal for a 190-degree temperature?

The top radiator hose is warm, the bottom hose is cold.
You don't have proper circulation. Upper hose should be HOT, lower hose should be somewhat less hot. Ambient temp plays some role in how much heat gets transferred out of the coolant by the radiator and fan...but that upper hose should be 190 degrees just like the sensor says.

If the temp sensor and infra-red thermometer are both showing ~190 near the thermostat housing, I have to think that either the radiator is plugged, the thermostat isn't opening, or the water pump isn't pumping. Of the three, the thermostat is most-likely and least-expensive.

When pointing the IR thermometer at the head near the sender, it reads more or less the same temp that the sender is sending. It seems that it's sending the head temp instead of the coolant temp, which I believe should be lower due to pressure and circulation.

Is my reasoning sound?
I'd say you have overheating problems that somehow aren't bad enough to blow steam out of the rad cap.

Open the rad cap when cold, start engine and allow to fully warm-up so the gauge is beyond 190. Do you see circulation in the tank? Is that water hot?
 

_rgk

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98 K1500 5.7. Sender at the head.

Resistance appropriate for high and fluctuating temp.
 

_rgk

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Ok, next step is to pull the thermostat and test it.
 

_rgk

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I tested the thermostat in hot water, it works. Also, I can see circulation once the thermostat opens.

It does seem like a circulation issue because when I hit the throttle the temp goes down right away. As soon as I let off, the temp starts rising.

I'm thinking belt/tensioner or pump. Fan clutch has resistance when the car warms up.

I think if the rad were blocked or the pressure wasn't high enough the entire system would suffer this issue.

I tried warming up the car without a thermostat, but it wouldn't go above 90 degrees. Circulation looked good at the rad.

I'm going to wait for the engine to cool back down and then hook up my pressure gauge to the rad to rule out a blown head gasket. If gasket is good, I'll run a couple more heat cycles to make sure all the air is purged and then start with a new tensioner and belt.
 
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_rgk

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Pressure builds slowly and steadily, ruling out a head gasket leak. Gauge temp got to just over 3/4, thermostat had not opened yet but temp there was about 190F. Pressure in system was 9-10 psi.
 

_rgk

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Holy moly, look at these pictures.

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Normal water pump outlet to passenger side head.

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Driver's side outlet mostly restricted by what appears to be a casting flaw!
 

_rgk

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I didn't want to open the back cover on the new pump, and you wouldn't be able to see much in a photo without that open. But I did see through the outlet that both sides were cast correctly.

I installed the pump and heated up the truck. With the thermostat sensor reading 190, the gauge sits steadily at the 3/8 mark. This case is closed.
 
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