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Old 11-12-2004, 02:55 PM
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Temperature Sending Unit problem

I'm running a set of Teleflex gauges on the hemi in the truck and I believe I have a temp sending unit thats lying to me. The gauge reads 220 but I know the engine is nowhere near that warm. I'm running a rather large custom made aluminum radiator with a shroud and mechanical fan. When the gauge says 220 I can hold my hand on the top of the radiator for several seconds and if it was indeed that hot I shouldn't be able to do that. Also the hoses are only warm to the touch and with a 7 lb. cap I have no coolant boiling over into the overflow tank. So everything says the engine is at a normal operating temp but the gauge is just not telling me the truth.

The sending unit is located in the side of the intake manifold and extends into the head. I used a brass adapter to fit the sender in the manifold since the sender was an 18 NPT and the intake needed a 14 NPT fitting. This resulted in the sender being mounted about 1/2" higher than it should be which means the probe isn't sticking down into the water flow as much as it should. I suspect this is the problem since I've had this happen once before on a different engine. The solution that time was to purchase a sender with a longer probe but that was a GM installation with a GM gauge. This setup is strictly aftermarket so I have to rely on the gauge manufacturer to send me a new sending unit that is the proper size to fit the manifold so I can eliminate the adapter. Hopefully that will solve the problem.

I just wanted to run this buy you guys to see if what I suspect the problem is makes sense and if anyone else has ever run into this same problem.



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Old 11-12-2004, 03:55 PM
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Doc here:

First off, If the Sender isn't in the water flow it's probably inaccurate, and may have an air pocket trapped in it..a sender can't read air, Get the sender down in the stream.

The Radiator temp will be MUCH cooler at the top than the manifold when the thermostat is closed...Can you hold you hand on the manifold..? I'll bet not...at least until the thermostat opens.

Also, If you can't get a response ("Burp") into the recovery Tank, You may have trapped air in the system or a pinhole leak (air in, no water out) Causing an air bubble..which will restrict proper flow and raise the operating temp.Look for tiny leaks.

Check the cap, does it actually open at 7 lbs...and is that the right pressure? and does the cap perhaps leak a little air?

90% of your problem is going to be that sender out of the water stream, repair that and see if it comes back to normal, but also look at the other things I mentioned.

Hope it helps!
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Old 11-12-2004, 04:14 PM
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I'd bet it is an air pocket. My AFR heads do not have a tap to install a sending unit. And my Edelbrock Pro-Flo fuel injection uses the only intake port for its temp sending unit. I tried installing a brass tee to allow both sending units (fuel inj & gauge) to tap into the same intake port. Bad idea. Both would run hot 215-220. But I too could touch the upper rad hose, etc and is was not as hot as the gauge was reading. I removed the tee and reinstalled the fuel injection sensor and temps were normal. I ended up drilling and tapping my temp. sending unit for the gauge under the water neck. If your gauge sending unit is a small 1/8 NPT, you could do the same. Drilling and tapping into the intake in another location is an easy fix, especially with the 1/8 NPT size tap. Ah, a picture is worth a thousand words ...


Good luck, Ed www.edgesz28.com
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Old 11-12-2004, 05:13 PM

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Edge, which Pro Flow are you running? Mine, the 3507 had two like ports for the coolant sending units and the FI and guage read within two degrees of each other. Where in SC are you located? PM me and maybe we can get together and do some tuning comparisons. I feel I may be leaving some performance on the table and would like to consume it.

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