Mysterious Coolant Leak

[sirben711]

I have owned three cars in my life. 1998 Mustang, 2017 Mustang and my current 2021 Mustang.

My 98 mustang had constant problems losing coolant. Head gasket failure. Multiple hose failures. But it was to be expected as an old car. The 2017 never leaked - but she was also very young and tragically lost her life :( before she could develop any of those problems.

When I purchased my 2021, from the factory the coolant level was below minimum. I thought, that's weird - but maybe they have a pre-fill amount and never doubled checked things. So within the first few weeks I buy more coolant and fill it just below the Max line. This afternoon I was inspecting the car - cleaning up under the hood and noticed the level had dropped to exactly the Minimum line in a little over a year. I have not seen any evidence of coolant on the ground and have never smelled anything sweet coming from the car either.

Any thoughts on an extraordinarily slow leak?

*I doubt it is due to the temperature - this car is packed in a garage that is currently 50 degrees.

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[Ecto1]

That seems fishy. Do you monitor the coolant temp while driving? Does it spend any time in the red? Unless the engine is getting very hot you shouldn't be losing any coolant. Especially on a 2 year old car. Maybe a head gasket issue, maybe not. But since the car is still covered by the warranty the best thing to do is to take it to the ford dealer and have it checked out. I'm sure they'll claim "No Problem Found" but ... you'll have a record of the problem during the warranty period if the engine goes bust after the warranty period expires. Maybe the coolant loss is to blame, maybe not. But the burden of proof will shift from you to ford. (As long as you keep the report from the dealer regarding the issue.)

Let us know if you learn anything more.

p.s. There were issues with the heater core failures in earlier 6G 'stangs. I doubt that yours is effected but check the carpets under the dash for water/coolant. Especially near the trans tunnel. It can't hurt.

 

[Mike Pfeifer]

Maybe I’m totally wrong here, but this would not concern me in the slightest. In my 23 years as a technician, I have never seen a car that doesn’t “lose” coolant exactly as you describe. I’m constantly adding a little coolant every time I have a car in my bay, most commonly in the colder months. I’m not sure exactly where it goes, but I have some theories.

 

[sirben711]

Maybe I’m totally wrong here, but this would not concern me in the slightest. In my 23 years as a technician, I have never seen a car that doesn’t “lose” coolant exactly as you describe. I’m constantly adding a little coolant every time I have a car in my bay, most commonly in the colder months. I’m not sure exactly where it goes, but I have some theories.

What are your theories???

My 2017 never lost coolant for years... well, until I was turned into a car crash sandwich - then it lost a lot at once.

 

[Scootsmcgreggor]

If you end up getting misfires at startup it’s likely the head gasket. On these motors you can get headgasket leaks so small/slow that they even pass a leakdown test. I did.

 

[BlackandBlue]

My theory has always been some cars lose coolant(mostly water) vapors at some point cooling down. I smell coolant from time to time in the garage after driving my car. Not every time just intermittently. It has lost about and inch of coolant in 25k miles. I put a marker dot on the coolant level when completely full years ago. I have had a few cars over the years the I smell coolant around when cooling off and they all used a tiny bit of coolant. Other cars I never smelled it around never used any. Antifreeze smell is unmistakable.

I figured a new radiator cap would fix it but I don’t care enough.

 

[sirben711]

So... weird question. What does a misfire on one of these engines sound like?

I am now even more worried because of the mysteriously disappearing coolant (Filled to the max line and a year later just above the min line) and when the car idles at start up - the exhaust note isn't 100% smooth

 

[Mike Pfeifer]

What are your theories???

My 2017 never lost coolant for years... well, until I was turned into a car crash sandwich - then it lost a lot at once.

For one, coolant level varies with temperature. So unless you measured it at the same ambient temp and engine temp as last time, your measurement may be very different. Also, the cap has a pressure relief valve - it is possible that during spirited driving, or severe hot temps that the pressure went above the relief valve and some vapor was allowed to escape. I have also heard of the rubber seals and hoses absorbing a small amount over time. A sealed system is not truly sealed 100%, and some evaporation is expected as well. I would be surprised that, at a molecular level, there would be zero possibility of small amounts being able to slowly seep past seals or o-rings, likely very dependant on hot/cold cycles.

A quick google search gave some results of normal coolant loss to be at .25% every 4-6 months, generically speaking, and that could explain the exact amount you are seeing.

 

[Scootsmcgreggor]

Misfire at idle is an ever so slight hiccup/mild pop as a cylinder misses a combustion cycle. Some misfires in direct injection engines is normal and the Ecu allows for a certain count before a code but keep an ear out for them and see if frequency is increasing over time. Also if your headgasket is leaking enough the threads of the spark plug for affected cylinders will be black/dark as the coolant in the cylinder wicks up the threads as the engine cools then burns off next time you start the engine.

 

[BlackandBlue]

Just send the oil for analysis when you change it. They will tell you if the car is burning coolant. Short of tearing it down it’s pretty hard to know where it is going.

If it is still under warranty just buy the extended warranty from someone. It’s cheap insurance against worry. 10 year 100k warranty for 1300.

 

[Radiation Joe]

Just send the oil for analysis when you change it. They will tell you if the car is burning coolant. Short of tearing it down it’s pretty hard to know where it is going.

If it is still under warranty just buy the extended warranty from someone. It’s cheap insurance against worry. 10 year 100k warranty for 1300.

My car showed coolant in the oil long before the car showed lower coolant level. Get an oil analysis done ASAP. If it shows coolant in the oil, bring the car to the dealer and document the oil analysis. I'm currently in an ugly legal battle with Ford over my engine failure which I brought to Ford's attention while it was still under warranty. The misfire and major head gasket failure didn't happen until I was out of warranty.

 

[sirben711]

I am due for an oil change in a couple hundred miles.. Ill look at the oil - see if it is milkshakey. Ill also pull the spark plugs and take a look with my borescope camera into the cylinders

I thought they corrected the design issue of the block/head gasket flexing under load and leaking for the +2020s?

 

[Radiation Joe]

You definitely need a legit oil analysis. Mine showed no symptoms for a year after positive indication from Blackstone Labs. I had three positive indications before symptoms showed.

 

[BlackandBlue]

FYI Blackstone is like 30 bucks. Stupid cheap.

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