Temperature Fluctuation

This weekend, I noticed my temp gauge fluctuating from the bottom of the operating range to nearly the top, then back down, back up, etc. This cycle happens irregularly, maybe every couple of miles. MPH or RPM don't seem to matter. I have 11K on the engine and the temp has always been very steady, regardless of ambient temperatures.

My first thought was maybe a sticky thermostat, which might not be reacting to temperature changes. I changed it out, but that didn't fix it. The store didn't have a 180 degree, so I got a 190. It still fluctuates, but in a slightly higher range now.

• - The system and coolant are very clean, no leaks, and a coolant recovery tank keeps the system full.
  - When the thermostat opens, there seems to be good flow at the radiator inlet.
  - All hoses are good -lower hose has a spring to prevent collapsing.
  - There's no water in the oil or oil in the water so I don't suspect the head gasket. I pressure test it to be sure.
  - Drive belt is good and properly adjusted.

I'm thinking of changing the water pump, simply because I can't think of anything else that could be the cause. Maybe the impeller is slipping on the shaft? Have I overlooked anything?

Thanks for any advice.

-Stu

Hi Stu,

You didn't mention whether your temperature gauge was a stock type electrical gauge with a sending unit, or a mechanical aftermarket gauge. If it's a stock type with a sending unit, the sending unit could be failing or you could be experiencing an electrical issue with your gauges - I believe there's a small resistor that's used with the gauges.

Personally, I believe that oil pressure and temperature are too important to trust to sending units and electrical gauges. I use Autometer mechanical gauges that don't depend on sending units and work even if my entire electrical system dies.

My only other thought, but it's unlikely: could the impeller be hanging up on the right front head bolt, that goes through the water jacket near the water pump?

On an unrelated note, I really like your paint scheme. I'll be painting my '62 Ranchero eventually, and may do something similar.

Thanks
Bob

Thanks for the reply. I hadn't really thought about the gauge and sender, which are the stock units. The sender would be a quick and easy thing to try. I sure hate the thought of pulling the water pump, since there's so much in the way.

There's a constant voltage regulator that supplies something like 5 volts to the fuel and temp gages. The fuel gage works fine, so the regulator is probably not the issue.

I'm glad you like my paint job. It gets a lot of attention - just different enough to stand out.

-Stu

Hi Stu.Something else to consider.Check the sending unit wire for bare spots.If there is a bare spot tape it up.When the bare spot gets against the block or other bare metal,it will be grounded and the dash gauge will go over to the hot side of the gauge.Or anywhere in between.Ask me how I know.Hehehe.
Good luck.Have fun.Be safe.
Leo

A new sender seems to have fixed it. I also put a 180 thermostat back in and all seems to be back to normal. I still won't trust it until I drive it to work for a few days, but fortunately, the weather here has been great for cruising. I'm almost anxious for Monday morning!

Also, some bad news/good news. Turns out, the sender was only 2-1/2 years old, but O'Reilly has a lifetime warranty on it (BWD brand - "hecho in Mexico"), so they gave me the new one. Crummy imported parts backed by a lifetime warranty! They won't make any money off me, but I sure get tired of turning wrenches.

Also, I played around with the IR thermometer. With the 180 stat, I observed the following readings:
- Head surface near the temp sender - 170
- Thermostat housing - 178 - 180 on the engine side
- Outside of lower radiator hose (water pump inlet) side - 135

I know the outside surfaces are not the same as the water inside, but it's interesting to see the variation between theory (180 degrees) and actual dynamic operation. I wonder how hot the water actually is at the sender, since by the time it reaches the back of the motor it has cooled all six cylinders. Typing this, I realized that I should have shot the upper and lower radiator tanks. That thin metal would have given pretty accurate temps, I think.

Anyway, thanks for the input and help.

-Stu