I have a '93 that's basically stock except for a recent radiator swap - I now have a Koyo. It used to be that once the car warmed up, the temp gauge (which I know is a bit of a 'dummy' gauge) would never go above or below the halfway point.

Now with the new radiator and nighttime driving on the freeways (when the outside temperature is cool) at near 80 mph, the temp gauge actually *dips down* to about the 1/4 point. As soon as I get off the freeway though, and drop back down to idle...the add coolant buzzer and light come on for about 20 seconds, and then the temp gauge slowly rises back up to halfway - and the buzzer and light turn off. Then the next day I have to add more coolant.

Any ideas?

 

how long have you had the rad in the car for ? It can take a while to fully burp the coolant system on these cars

 

Questions to aid diagnosis:

1) Where is the coolant that you are adding going? It has to be going somewhere unless there was a lot of air left in the system due to the radiator swap. Any visible leakage?
2) Do you see a lot of white smoke on start-up?
3) How old is your thermostat? Could it be sticking open?
4) Do you still have an AST?

 

The radiator swap was in February. I do see a small white cloud from the rear on startup. I had a visible leak, but it was the hose to the overflow tank that had come off after driving over our horrible southern california roads (lots of potholes after the winter rains). What I'm really curious about is why the temp gauge dips so far down during nighttime driving on the freeways. Last night, just before this happened, it wasn't even 1/4 up. Maybe 1/6.

During the daytime this is never an issue, and the temp gauge stays at halfway all the time.

I wonder if it's possible that it's getting so warm, it's pushing excess into the overflow tank, and then not pulling it back in and so the coolant I'm adding just ends up going into the overflow tank. Not sure how to check on the thermostat, and yes, I still have the stock AST.

 

A "small" white cloud is normal. Losing coolant isn't. Check your tubing to the O-F tank again for leakage. If the T-stat is old, it could be malfunctioning - if it sticks open just a tiny amount, your low temperature readings will result. The T-stat gasket can leak a little due to the slightest corrosion - the coolant will run down the front of the engine from the housing.

You can check the T-stat by removing it and checking it in hot water to see if it works. However, I'd just get a new one with gasket (Malloy mazda - Ray Crowe, 888-533-3400).

If the coolant were just going into the O-F tank and not being pulled back, it would eventually overflow. Bad fill and AST caps can also cause this.

 

Thanks Dave, so much - you're very helpful. I got a pressurized AST cap from Pep Boys and I'm sure it's not correct - though the book at the store said it was. It's 16 lbs. Shouldn't it be more like 12 or 13? Is it possible that there's too *much* pressure in the system? The main filler cap (non pressurized) was just replaced a month ago - stock part with the yellow sticker on top.

I'm sure the thermostat has never been replaced. I'll have the shop look at that for me. But the readings have only been low since the Koyo radiator was put in - it's much larger and all aluminum, so I thought maybe that's why the gauge wasn't going up to halfway. I just didn't think it would be as low as it is sometimes.

 

The ast cap should be 13lb cap. I had a friend who was having issues like this and his coolant was actually being pushed out the overflow tank...His cap was worn out and pull air into the system.....replaced the cap and everything is fine.