Coolant becoming low, engine overheating

On our Mercury Monterey the engine overheated today, the display showing the message and the gauge shooting to H. The radiator tank was completely empty. No message on the dash about coolant low. filled almost 1.5 gallons of 50/50 coolant. Drove the vehicle for 10 minutes and temp remained steady at less than halfway. When checked the tank almost half gallon was lost. Any idea what could be happening? There are no leaks to the ground or at any visible location. Can someone suggest how and what to check? Vehicle is driven almost 70/80 miles daily and should it be driven in this case? mileage around 148k. Thanks!

1st guess would be a head gasket leak. However a hose could have a small leak that does not drip when cold but shoots a stream of coolant out under pressure, but not enough to produce a steam cloud you would see when driving. Same with the radiator. Find a local mechanic and tell them the symptoms, they can pressure check the radiator and see if there is a leak. They can also test for a head gasket leak.

Agree with @SteveCBT Head gasket. What you’ve described is the classic symptom.

No visible leak on the ground doesn’t mean it’s always a head gasket. Could only leak when running (pump or hose due to pressure), and any leak when running would just evaporate away.

Thanks for the reply. The car was not driven today for work and I had idled it in the morning for 10/15 minutes. there is some drop in the level. would you recommend adding any sealer to the radiator? May get a radiator pressure test tomorrow. Thanks!

“adding any sealer to the radiator…”

Why waste your time and money. If you don’t see coolant on the ground or around the engine bay, you likely do not have a cooling system leak. Save the money for the head gasket repair…

Engine (number of cylinders, displacement)?

The Monterey is 2005, V6 4.2L

Some possibilities for rapid unexplained coolant loss with no external leaks

  • As mentioned above, head gasket breech is allowing coolant to flow into the combustion chamber and out the tailpipe. Usually there is some white smoke out the tailpipe visible, but not always.

  • Coolant is moving into the crankcase oil. Head gasket or oil-cooler leaks are common ways.

  • Coolant is moving into the automatic transmission fluid. The radiator cools the automatic transmission, so a leak could occur inside the radiator allowing coolant to flow into the transmission fluid.

  • Coolant can leak onto something hot and evaporate. Usually that kind off leak is a pretty small one. Losing a cup a day or less. A faulty radiator cap is one place this can happen. B/c your leak is so much, I doubt this last one is the case for your vehicle OP.

A cooling system pressure test is a good start. Good time to also visually assess the condition of the engine oil and transmission fluid for signs coolant is leaking into those places. And make sure all the engine cooling fans are doing their thing. If inconclusive, a chemical test of the coolant is probably next, testing for high levels of exhaust products (carbon dioxide etc) in the coolant. Cylinder leak-down tests can be diagnostic also.

I would buy a new radiator cap at the car parts store and replace the old one when the engine is stone cold.

You say the added coolant is going down. Are you adding it in the radiator directly, or in the overflow tank?

I don’t think most cars have any warning light or message for low coolant levels, that’s one you need to visually inspect weekly or biweekly when you check your oil and other fluids

Most of my cars have that, (several GM products). Additionally, low oil level indication when/if level gets about a quart low.

Interesting…:thinking: I’ve never had a vehicle that did (to my knowledge, I check myself anyway!), including an early 2000s Corolla, 94 Saturn sedan, 09 Focus SE, 05 Town & Country. I definitely know the T&C didn’t have a warning light because my wife had no warning when (likely) a stone poked a hole in the bottom of the radiator and coolant rapidly disappeared, just the temp gauge going through the roof! None of them had a low oil level light, just the low pressure warning light (again, though, I check those levels regularly too!).

Yep, but I think GM has stopped both low coolant and low oil sensing for some reason (liability if the sensor fails?).

More likely because it costs money and most car buyers don’t care.