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Waving water temp gauge

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658 views 5 replies 4 participants last post by  Bob Cowan  
#1 ·
I have a fairly basic 302 and I have always had a bit of a strange issue with the water temp gauge. In the summer, after an initial start the temp will rise steadily to 210 or so and them drop quickly down the around 160 or 170. Then it creeps slowly back to around 190 and sits there. My water temp sensor is the in the aluminum tube that goes through the F panel to the upper radiator connection. It is so consistent that I decided it is just the placement of the sensor responding to the thermostat opening.

I have been driving in the cold and the temp gauge is now doing an ever diminishing cycle like the one described above. Starts out with the big swing from 210 or so down do 160 and then it will go on moving up and down for 15 plus miles. Eventually it settles in to the fairly typical 190ish range.

Do I have a problem? The car has 2,300 mile on it now and has never overheated in cold weather or 105 degrees Texas summer weather (even in heavy start and stop traffic).

Thanks,
Jerry
 
#2 ·
Sounds like the thermostat is acting up - opening and closing. Do you have a hole drilled in it?
 
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#4 ·
The sending unit should be on the upstream side of the thermostat. You need to monitor the engine temp, not the water outlet temp.
Install the sending unit in the manifold, in back of the thermostat. This should make the gauge read steady and keep it from swinging.
 
#5 ·
I think it's acting normal. Once the thermostat opens, the cold water from the rad enters the system, it then soon mixes and warms up with the hot water from the block, and then levels out. I think that larger capacity rads may tend more to do that.

My boat does the exact same thing. I know when the thermostat has opened just by watching the gauge, for that drop, before it climbs to normal. Seems to always be the hottest just before it opens, but doesn't ever climb back up that high after.

You don't mention any problems in particular so I would not be concerned.
 
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#6 ·
A closed thermostat breaks the circuit, so to speak. There's no movement at all when it's closed. There's nothing wrong with that, when it's working properly. There are millions of cars on the road that work just fine that way.

Drilling a small hole in the thermostat makes it an open system. Water will flow through the circuit at all times. Not enough to delay the normal warming cycle an appreciable amount. But enough so that you get slightly more even warming. It also helps to get rid of air in the system when first filling it.

But, when the thermostat is operating correctly, you should not see the temp spike to 210*, and then drop like that. The thermostat or the gauge is not operating correctly. After 2,300 miles, it's hard to imagine that you still have air in the system. Possible, but unlikely.

My gauge sensor is sort of where yours is. It's behind the thermostat, in the housing. I have a 180* stat, with the extra hole. I never get the swings you do. The gauge slowly warms up, until the thermostat starts to open. Then it rises a little quicker until it hits operating temp. Then it stays fairly stable.
 
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