Coolant Disappearing - No One Knows Why

2013 F150 5.0 FX4 - 145,000 miles - over the past couple months I have noticed my engine running a little hot some times. What went from missing a little coolant and adding some every once in awhile has turned into every 10 days I have to add about a gallon to get it back to the correct level. A month or so ago I went ahead and replaced the water pump and thermostat along with the gaskets/seals/o-rings in most of the hose connections especially at the problematic T/Y connector. I have also gone through and replaced any fittings that seemed like they were failing.

I work in heavy Civil construction (roads and bridges) and I am always either hauling or towing along with driving a TON of miles. I am set up with my company’s vehicle maintenance program that is pretty good. Every 5000 I get the oil changed fluids replaced etc… The other day I had them run a pressure test and it held for 30 minutes at 16 psi which is what it is supposed to be. No external leaks were found after looking over it. I ran an engine block test with a kit I have and the blue fluid did change to a yellow/blue-yellow color indicating that there is some combustion gases getting into the system. But it wasn’t pure yellow - can something like a head gasket/cracked cylinder cause me to loose that much coolant without any trace? No leaks/pooling on the ground or in the engine compartment…

Did you check the transmission fluid for coolant contamination?



If the engine sucks in 1 cubic millimeter of coolant on each stroke in one cylinder, it would take about 21 hours of run time to remove one gallon. If a gasket breach pulls in 10 cubic millimeters per stroke, that gallon would disappear out the tail pipe in 2 hours. This assumes you average 3000 rpm while driving.

This is just to show that a small amount of coolant lost through a hole in head gasket Adds up fast at typical engine speeds.


Probably a bad gasket. Whether it’s lower intake or head gasket. If it’s a head gasket, white smoke will likely be blowing out of your tail pipe. If it’s lower intake gasket, you’ll notice brownish oil. Good luck.

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If it’s a lower intake gasket leak, you will have contamination of the oil without any combustion gases leaking into the coolant. Since the “block check” fluid changed color, we can assume that the head gasket(s) are defective, or there is a crack in one of the heads. A defective head gasket or cracked cylinder head will open up more at operating temperature, so the amount of leakage might be too low to measure when the engine is at room temperature in your garage.

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Y’all are quick with the replies! Haha

Jtsanders - incredible thought process on it. That definitely puts it in perspective. I appreciate that.

Tester- I had my lead frame replaced in the transmission the other week due to a recall about the Output Speed Sensor shorting out and the dealer technician said everything look good when they dropped the pan.

When i got my oil changed I had them show me a sample of the old oil - didn’t look milky/frothy only relatively dark since it had been a little past my due back mileage.

Haven’t seen any white smoke on start up or while driving. I have seen some fluid drip out of the tailpipe sometimes but never really thought of checking it. You can definitely smell coolant once the truck has been running a good bit.

Think it would be good to remove the spark plugs and check the cylinder heads with a bore scope?

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Some head casket leaks only leak when cylinder pressures go up under load like climbing hills or towing.

No, but it would be worth removing and inspecting the spark plugs to see which of them look different from the rest.

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I have done that after pressurizing the cooling system for an hour to look for drops of coolant that has seeped passed the head gasket.

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In that case, your company’s mechanics may not be very good

If you can smell coolant, and these guys can’t find the cause of the problem . . . :frowning_face:


Unfortunately it’s not company mechanics - they would probably fix the issue but they sub all the work out to vendors like Firestone, Pep Boys, etc… I usually use Firestone just because I’ve had good experiences with them so far over the past year I started using the program.

Find a real Air conditioner shop .

There are obviously exceptions to what I’m going to say next . . .

Places like Firestone, Sears and Pep Boys are just stepping stones, in my opinion

The guys that aren’t cut out to be good mechanics eventually wash out and wind up doing something else entirely

And the guys that turn out to be good mechanics learn what they can, and then move on to bigger and better things, such as working at a fleet, opening their own shop or what have you


The cooling system was pressure tested and held at 16 PSI. Did they not pressure test the cap?



Find a real Air conditioner shop .


A/C shops get involved with Engine Coolant loss?

hvac . . . heating ventilation air conditioning

Definitely related to the cooling system, I’d say

Check for wet under your floormat. Your heater core could be leaking. Also the radiator hoses could be leaking when you are driving.

Sounds like a blown head gasket. If you have combustion gas contamination in your coolant, that is your answer.

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Did you check the coolant for contamination? Seeing if it turned to blue to indicate a leak?