Disappearing antifreeze

'03 Chevy Venture with 126k. Several months ago, I had to get the intake manifold replaced due to DexCool corrosion. The clue was antifreeze dripping on my driveway. I replaced the DexCool with Green at that time too.

NOW, I still get a check engine light. I take it to the parts store for a diagnostic and they tell me low coolant temperature. The temp gauge was running low. I replaced the coolant temp sensor but no help. I top off the antifreeze and the light goes away for about a month but then comes back. There’s no dripping antifreeze on my driveway and none on the dipstick, as far as I can tell. If there was antifreeze in my oil, it would be creamy and milky on my dipstick, right? Someone suggested a stuck open thermostat could be making my antifreeze run cool but would that explain the disappearing antifreeze? Any ideas?

The low temperature, as shown by both the gage and the check engine light, is caused by a bad thermostat. The temp sensor was/is fine. To fix THAT, change the stat. I doubt that has anything to do with the disappearing coolant.

BUT, when I top off the coolant, the check engine light goes off and the temp gauge runs normal. How to explain that?

First thing to check would be if the coolant is circulating. With the car cold, remove the radiator cap and set it aside. Let the car idle for ten minutes or so. You should see the coolant circulating and the coolant in the radiator should heat up. If the coolant isn’t circulating, I think you are likely in trouble.

The cheapest cause of your problems would probably be a stuck open thermostat augmented by a cracked overflow container that allows the heated coolant to drip out on the road while you are driving. The most expensive would be a cracked block (although a leaking head gasket would be a lot more likely) and possibly a busted water pump that is preventing warm coolant from circulating to the coolant temperature sensor.

If it were me, I’d replace the thermostat and pray that somehow fixes everything.

If you are losing antifreeze, as soon as the level in the radiator drops below the coolant sensor your temp will read low and the light will come on. Have your system pressure tested to find out where it is going.