Engine coolant in reservoir is disappearing and this is what I have checked:
1. Reservoir is not leaking
2. Heater is not leaking
3.Radiator is not leaking
4.Radiator hoses are not leaking
5.Water pump is not leaking
6. No moisture is on the oil check stick
7. Oil is not "milky"
8. The oil fill cap is totally clean on underside---no residue
9. No colored exhaust or noticeable odor.
10. Tested radiator air for engine combustion CO2---Negative
11. Check for air in engine block, lines and heater. No air burped out.

Just completed a driving test of 30 days, 428 miles, all locally & 95% at less than 60 mph. Reservoir coolant was down by 20 oz.
Engine never misses and runs smoothly at all speeds.

What am I missing here?

 

Its most likely to be an rad/engine leak since you don't see coolant on the ground, it is not leaking until system has pressure which is after coolant gets hot, so coolant is leaking at a place where it is evaporating before it can drip to the ground

Couple of ways to find it
You can rent a pressure tester, basically a Rad cap with a gauge and a hand pump
You pump up the COLD cooling system to 20-25psi and watch for pressure to start dropping, and if it does then look for the coolant leaking out and dripping to the ground

UV(ultra violet) Dye, you can get UV dye kit with UV flashlight, you add the dye to the coolant and run the engine for a few days, then in the dark use the UV flashlight to see if there is a dye trail leading to the leak point.
The UV Dye is safe for engine, AC or oil systems, and can not be seen without UV light so no visible marks or trails

Check the Overflow/reservoir hose, mine had a small crack, where it ran on top of rad, and leaked only when hot, and coolant on rad evaporated pretty fast

 

I had a similar situation a few years ago....with my 03 4.0...
Turned out to be the lower plastic thermostat housing.......Very common deal....
It would huff the contents of the overflow bottle in a100 miles......and was difficult to spot the leak...because hot engine evaporated leaked coolant
Bought a Stant replacement parts kit from Rock auto....about $35......I think?
Can be done...without taking the intake off.....Was a bit fiddly.....but not too bad...

 

check thermostat housing

 

Mine, too.

At about 85000 on the clock, I noticed my coolant level low. I don't see anything coming out. I filled up the rez and drove off. A week later, same.

With a pressure test, I Found my housing was leaking and going into the valley and probably evaporating as the engine got hot.

I took off my intake because I hate being restricted LOL

New housing and thermostat. I'm good.

Got a new knock sensor while I was there, too

 

I just completed the pressure test of the radiator. I searched all water connections, connections to engine, thermostat housing and radiator and the only thing I found was an oil leak on the top of the engine. I was able to get a sample on a small long dowel rod (hard to reach) and it was definitely slick with oil. After 14 minutes the pressure gauge had dropped from 20 psi to 19 psi. I suppose that small pressure drop could be within the test kit system. Although the overflow is a no pressure system, I blocked the top of the tank and put a light pressure on the end of the hose and could not detect a leak.
No more ideas except to buy more antifreeze.

 

Was your test done cold...or at operating temp?

 

Yes, you will have to wait until leak point gets bigger

If you still have the pressure tester you could test for a head gasket leak
Disable coil, unplug connector
Put in 5psi pressure in cooling system
Crank engine over 3 or 4 times, check pressure, if its higher than 5PSI then you have a cracked head or blown head gasket, so coolant is being burned

That should show up on the 20psi pressure test, but never know

Without pressure tester you can do the "Glove Test" search for it here I have posted it many times

 

Thanks RonD. I understand "disable coil". But "unplug connector"? I still have the test kit. Will test again when weather permits and will post results.

Yes, test was with cold engine.

 

Try your pressure test again....with engine HOT....
The lower T-stat housing is plastic......and hot coolant will cause crack to expand and leak.
Mine did not leak when cold...

 

Two more test this am.
1. Engine hot. pressure test. No leaks found including at the lower thermostat housing.

2. Disconnected wires from coil. Set tester at 5 psi. cranked engine over 5 different times. Pressure did not increase.
3. Pressure actually dropped to 3.5 to 4 psi. My guess is the drop in pressure was cause by water pump circulating water out of radiator.
4. Checked reservoir again. No leak.

Any other engine block test I can do?

 

After the last post. I thought more about item #3 in last post. Maybe drop in pressure was caused by water being sucked into a cylinder?

 

Static engine compression is above 150PSI in a cylinder so would pump up the 5psi pressure with even a small leak, I think your first conclusion was more accurate, the water pumps circulation moved coolant in the heads and heater hoses to lower overall pressure, 5psi is just not enough to move coolant, 20psi is for the overall pressure test.
A cracked head or head gasket issue does suck in coolant but also increases overall pressure in the system more than it would lose pressure, I would say no cracked head or head gasket issue on your engine

You could try 25psi test, coolant is going somewhere

 

RonD, thanks for your analysis and opinion. I will conduct another test at 25 psi. You are right. Coolant is going somewhere.

 

Crank the engine again, with coil disabled, see if 25psi drops like 5psi did

Wondering if it could be losing coolant at water pump bearing, so pressure drops when it spins
Usually that weep hole should show that but?

 

Found my first drops of engine coolant today. I pressurized the system to 25 psi, spun the engine with coil disconnected. No evidence of leak at water pump weep hole. The pressure did fall but only slightly faster than the 20 psi test. The leak is on the right side of the engine directly above the first freeze plug.. The freeze plug is recessed into the block and there is definitely no moisture around the plug itself. There is a wet trail to the rear of the freeze plug about 2" and to the front of the block and cylinder head. I do not believe that the coolant is coming from the valley on the top right of the engine because I can't find any moisture even near the thermostat housing. There is an oil leak reward in the valley probably from the cylinder head gasket. . Because of the test results up to this point, I believe the bottom of the cylinder head gasket is broken/damaged to the outside of the block and not to a cylinder. If all of this is correct, I see big bucks for this repair. Do I have any options other than these.

1. Continue to drive it and keep check on oil conditions for water/oil mix? Hopefully damage to the gasket would not extend to a cylinder.
2. Is there any type of sealer that would seal this leak without creating problems throughout the cooling system?
3. Get a bank loan for $1000--$1500?
4. Other?

 

Well the higher the pressure the faster it will leak, so at 16psi, your caps rating, it will leak coolant faster after it warms up

Your engines operating temp should be 190-220degF
50/50 coolant/water mix boils at 223degF with no pressure
You get approx, 2.5deg increase in boiling point for each pound of pressure
So 16psi cap makes it 263degF boiling point, which is on the extra safe side, lol, but there is a reason for it
I think the lowest cap rating you could find would be 7psi, which would boil at 240.5degF, so 23deg lower than now, but 40deg HIGHER than expected at normal operating temps

The reason for the margin of error is older cooling system parts, i.e. radiators and water pumps, circulation in the heads(hottest spots) can slow down so extra heat is generated but you would see that now with temp gauge running a bit higher than "normal"

So a 7psi rad cap should slow it down by 1/2 or so

Coolant "stop leaks" can't hurt a good cooling system, in fact on some of the GM engines you had to add GM stop leak as a factory additive when you changed coolant, lol
This was back in the day when Aluminum heads were first used on cast iron blocks, aluminum expands much faster when heated than cast iron, so the heads would RUB the standard composite material head gaskets as they heated up and cooled down, so after 30-50k miles you got blown head gaskets
Multi-layered steel(MLS) head gaskets came into use then, they allowed the aluminum heads to "slide" as they expanded and contracted, but these couldn't seal cooling system when engine got cold, so in a frosty climate, they leaked coolant when cold, lol.

OK, back to the stop leak stuff, it should be fine to use it.
If your radiator or heater core is already partially blocked then the particles in stop leak products can clog them up more, but if the system has been maintained then it can NOT block it up, it would be impossible, the particle sizes used are not big enough.

You situation will require you to use this all the time, the leaks from the head gasket area don't stay sealed because they expand and contract much more than radiator or heater core, this is the issue GM ran into, so you will want to leave the stuff in the coolant full time.

You can try the GM tablets they are not expensive and made for similar issue:
Parts stores should also have these, or Bars Leak Gold powder, same thing

 

. RonD, Thank you so much. I'll try one of the sealants that has a lot of positive feedback. I purchased the truck about 2 years ago so I don't know the heater core condition, but I did install a new radiator about 1 1/2 years ago.
Thanks again.

 

Just as a heads up, always swap around heater hoses at the firewall anytime you service coolant, or any time you think about it, lol.

Heater cores have no "direction" of flow, both connections are in/out doesn't matter, but what does matter is that "stuff'/debris can flow IN but then can't get out, if you reverse the hoses then that 'stuff' will be pushed out the way it came in, this will make heater core on any vehicle last longer

 

That's pretty much where I found my first sign of coolant on the engine...

I dosed it with a 1/2 bottle of Risolene block seal.....and it bought me about a year....

Before you start yanking cylinder heads.....I'd roll the dice and change out that upper and lower thermostat housing...

The kit I bought was from Stant,as I recall......and was less than $40.00....

There's a good vid on youtube...that walks you through it....

 

Thanks for the posts on V6~4.0L~SOHC engines & TStat housings; good reference info.
Replaced the old leaking FRPlastic TStat Housing with a new Aluminum TStat Housing
in 2019Aug on a 2001 Ford ExSportTrac; no leaks as of 2020Mar after ~5K more miles.
My full write up with pics on that repair:
http://2001fordexplorersporttrac.fandom.com/Coolant_ThermoStat_Housing

 

I delayed the repair as long as I could and finally took it to a shop. Shop confirmed that I had an oil leak at one valve cover gaskets. Also a minor coolant leak at the thermostat housing that I could never find in all of test. Makes me question why I couldn't find it. If I had found it, I doubt that I was prepared to replace valve cover gaskets anyway. All is well.

 

I have all the same symptoms in my '90 4.0L. I have been dealing with it for years. Four years ago a rad hose popped off and got quite hot...had to have the head gasket replaced, costly. Since then (i think) I have been having to add coolant often, depending on how often I drive it. Does my Ranger have a "lower plastic thermostat housing" I'm not sure what that is. My Ranger sucks coolant out of the reservoir after shutting down. I have done an exterior check after running for 20 minutes, can't see any signs of a leak. No puddles ever! Driving me up a wall, but i want to fix so I can let my son use truck without worrying....Also I recently pressure tested via the radiator cap, no signs of a leak. I pumped to 13, because the cap said 13 on it....should I have pumped up more? Please help....

 

Welcome to the forum

Yes, usually you pump it up to 20-25psi and watch for pressure to drop, it shouldn't drop at all, if it does then you have a leak, you just need to find it

Rangers all used a recovery/overflow cooling system(except for the 2.3l Duratec)
So as the engine coolant heats up it expands in volume and that's what builds up pressure in the system
When pressure gets to 14psi the larger valve in rad cap opens and sends some hot coolant over to the overflow tank, reference HOT mark on tank
After shut down the coolant cools down a shrinks in volume, and it any coolant was sent over to overflow tank it needs to be pulled back in to rad, the smaller valve in rad cap will such coolant back in when pressure in rad drops to -1psi, reference COLD mark on tank

So every time the engine is fully warmed up and cooled down coolant should flow TO and then FROM the overflow tank, every time

This also purges air from radiator, since air would collect at the top of rad(high point), when engine fully warmed up the air would be sent over to overflow tank and bubble to the top, gone, then on cool down coolant would be sucked back in to replace the air
When you remove rad cap on a cold engine it should be filled top the top, NO AIR at all, if it has this type of system

Check the overflow hose, if it has a crack/leak it can leak hot coolant on its way to the overflow tank and it can evaporate on hot rad before it drips to the ground, and this won't show up on pressure test since its not in that side of the system

 

Thanks Ron. I will try pumping the radiator pressure up to 20-25 vs 13. I do the test with the engine cold, right? I read another post where the guy recommended to do the test hot, that seems unusual ....also Does my Ranger have a "lower plastic thermostat housing" I'm not sure what that is. If it does have a lower plastic thermostat housing where is it? I have checked and even replaced the over flow hose a year ago thinking it may have been leaking a bit