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91 Ls400.. Temp Going Up Up Up.. Then Down, Down Down...

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Today my 91 LS 400 did something it has never done. It started to show it running hot on the temp gauge.

This car no matter how hot (like 130 in the shade) or how cold outside would always start, "warm up" get the temp gauge needle to dead on center of the gauge and stay there. I have never , ever seen it go higher then the center of the range. In ten years with this car.

Today after driving about four blocks, I could watch the gauge start climbing. It got to the first mark on the gauge just under the highest point on the gauge. Then drooped back to the center. Then it started climbing again. Would get to the same point and drop again... But only to go back up.

I did have the A/C on, but that should force the electric fans on, and help the cooling...

After about 5 or 6 times it stopped doing this. And stayed dead center, as always... I had driven a total of maybe 5 miles. After it stopped doing the up and down, it stayed at the traditional dead center point.

I drove about another 5 miles to my destination. Stayed there for at least 4 hours and drove home. The car did not do it once on the ride home. And it was even hotter out.

I am thinking that the only thing that could make it go up and down that fast is the thermostat malfunctioning.

Any thoughts on this?

About a month ago I had the A/C converted over to 134A and on doing the work it was discovered that one electric fan was dead and the fan switch on the bottom of the radiator was no good. A working fan and new switch was installed and the system checked OK.

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WOW 100 views and no one has any info on this... Just my luck.... :chairshot:

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Well I'm probably 4000 miles away but will agree with your thermostat theory, it could be jamming closed then suddenly opening, for what it costs I would replace it to eliminate it being the cause.

I"m assuming the coolant level is correct and there are no signs of leakage.

The expansion tank cap is another possible as is a water pump on it'd way out and and air lock in the cooling system.

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It could be your thermostat or your temperature sensor or an intermittent connection to the sensor. Could also be an air pocket in the system.

I would start by replacing the thermostat especially if you have not done so in the past 10 years. Coolant should be changed every few years. keep an eye on the temp gauge. If that does not fix it, go after the sensor.

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Maybe air pockets in coolant? Is coolant low? Maybe a leaky or damaged head gasket is letting coolant into the cylinder and being vaporized out the exhaust. Otherwise I'd check thermostat.Maybe just change thermostat. They are cheap enough and simple to swap out.

I doubt the fan would be a big factor as when you are driving airflow is fine. Fans solve the issue when your standing still in traffic. If the needle is bumping around while driving at a contant speed ignore the fan issue for now.

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I had a similar problem, I changed the Radiator cap, it improved but did not cure it, but when I changed the Fan Clutch all went to normal. Try to run the car without AC and see if it does not happen! Also if you have not changed your thermostat It may be a good time to do so! CCF

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