I have a 1994 LT1. It hasn't been driven for awhile, and when I started it up, I thought it was overheating. I shut it down when the analog temperature gauge hit 250 and was near the overheat line. It got that hot idling in the driveway on a cool (60 degree) day.

My first thought was to check the antifreeze with a tester, but it tested fine for both hot and cold temperatures and did not appear to be low. I ran it with the pressure cap off, hit the gas a bit, and the antifreeze got sucked up from the secondary reservoir, which I believe is what it is supposed to do.

Next I replaced the thermostat but nothing changed.

I also replaced the fan relay, but still nothing changed.

I had an aha moment and tried playing with the heater and AC, and when I turned either of them on, the engine cooling fan kicked in right away and the car cooled off. With both the heat and AC turned off, I noticed that the main fan didn't start until 250 degrees and the second fan never ran.

Then I noticed that the digital gauge (the one that says 'COOL °F') was far lower than the analog needle (the one that says TEMP °F). Which one should I be looking at?

Here's what I came up with through some experimentation: With the heat and AC off, the main fan comes on at 250 according to the needle and 228 according the digital gauge. It turns back off at 235 (needle) and 217 (digital). The temperature fluctuates between those values depending on whether the fan is on or off.

With the AC on, the main fan comes on right away and never turns off. The second fan constantly turns on and off at approximately one-minute intervals. The temperature stabilizes at 215 (needle) and 201 (digital).

With the heat on, the fans do the same thing, but the temperature stabilizes at 210 (needle) and 197 (digital).


So in summary, I think the car is running fine with the heat or AC on, but with them off, I'm not sure. The needle gauge (250) makes it look like the car is about to explode, but the fan does kick in and cool things off. But this is just idling. Maybe the needle is wrong and the digital gauge is right? Or maybe they are measuring two different things? Or maybe I'm just paranoid and all of the gauges are correct? Please help.

 

Everything appears to be fine except (1) you're looking at the wrong temp gage (analog), (2) it's reading high (maybe bad sender), and (3) you just replaced a perfectly good thermostat and fan relay.

 

I just took it for a drive and it runs great. I guess I was just being paranoid. Thanks!

 

The analog sensor is in the right (passenger) head between spark plugs #6 and #8 if you want to replace it. This sensor is only read by the analog gauge.
The digital sensor is located by the water pump on the front of the engine (close to the thermostat you just replaced) and is read by the PCM for fan(s) engagement at the numbers you stated in your first post. Go by the digital gauge, it appears to be correct.