I apologize if this is common knowledge, but I could use some assistance. I bought a used 2001 Ford Ranger a year ago, in good condition other than having the radiator replaced at our auto shop. However, I've now had an intermittent issue of pressure in the system when removing the radiator cap when cold. Even after the truck sits for a few days, it still sometimes spits coolant when I remove the radiator cap. Since the issue only happened a handful of times I hoped it would resolve on it's own and thought they perhaps overfilled the coolant. However, it happened again today with a fair amount of coolant spraying and hitting the ground and the car had been sitting since Saturday.

The shop can't check it until the 22nd, so I began looking for anything obvious. Upon closer inspection, I finally noticed the nearly-hidden "cold fill" indicator on the coolant reservoir. It's hard for me to make out, but I believe it looks like the below screenshot. If this is correct, can someone please confirm whether the reservoir coolant should only be to the green line when cold and no higher? Mine was filled to the red line after the radiator replacement, and is now about 1/4"-1/2" under the red line - I assume from these radiator spurts when checking. Is my coolant reservoir overfilled, and if so, would that account for why my radiator cap spits fluid when I check it with the engine cold? I did try checking my manual, but it didn't give any detail regarding the hard-to-make-out fill line and it's very hard to see in my engine.


coolant tank



coolant tank fill line

 

Your coolant level is fine, as long as it's between those 2 lines when cold.

If you shut the truck off when the outside air temp is cold 50 or so, let truck sit for a couple days, then remove cap when outside air temp is warm 80 or so, it's very possible for a little fluid to come out.

How much coolant do you think is coming out ? tablespoon or less ?

 

The truck has lately been shut off when the outside temp is between 70-90 degrees. It is usually checked days later when the temp ranges between 65-90 degrees either in the morning or later in the afternoon once it's been in the shade for quite awhile. Tough to ballpark measurement wise - but I'd guess at least 5tbsp worth of coolant spilled out today. It gushed over my hand, left a puddle on the ground and a lot to be wiped going down the radiator.

 

Only thing I can think of is the vacuum valve part of the cap is faulty.

I'd try a new cap, Gates or Stant are good quality brands, and less than $20

 

Thanks for the input. I did buy a new cap in town but not sure of the brand (from Napa). Happy to try a better brand if we see no improvement.

 

Are you certain about the coolant reservoir level when cold? I just want to be sure, since a lot of what I see online mentions to fill "up to" the cold fill line and I see the following on the Ford website. I believe my reservoir only shows a down arrow to a cold fill line. To my eye, it doesn't seem to indicate a cold range.

And from the manual.

 

How an Overflow Cooling system works
(and yes your radiator cap was bad)

First some general physics
When you heat up a Fluid it expands in volume, i.e. if you fill a pot with cold water to the very top, and then heat up that water, it will overflow the pot edge long before it boils, because the water EXPANDS in Volume as it heats up
A gallon of water becomes 1.1 gallons, then 1.2 gallons, then 1.3 gallons, ect............the hotter it gets, until it turns into Steam and REALLY EXPANDS, lol, they ran great BIG Steam Locomotives on that simple principle, water expands in volume as it gets hotter

Overflow Cooling system has to allow for this expansion of water/coolant as engine heats it up
Assuming cold engine and fluid filled up to top of radiator
Radiator cap has TWO valves in it, a larger main valve with large spring, and a smaller valve in the center of main valve

As running engine heats up coolant it starts to expand and it could just be sent over to the overflow tank, but there is a benefit to not do that at this time, and thats PRESSURE
Engines best operating temperature is 190-200degF, best MPG and best oil cleaning

At sea level water boils at 212DegF, 50/50 coolant mix boils at 226deg so 12degF more, but not great
In Denver water boils at 203degF because its 5,000ft above sea level so less air pressure, so 50/50 would boil at 217degF in Denver
Low pressure lowers boiling point, and Higher pressure moves the boiling point higher
1psi of pressure raises boiling point about 2.5degF

So as the Coolant expands its not released, and this pressure increases the Boiling point of the coolant
Most systems use 14psi radiator cap, that's the rating of the larger valve in the cap
So at 15psi the valve is pushed open, and HOT coolant flows to the Overflow tank, and it comes into the tank at the BOTTOM, this is important
When pressure is at 14psi valve closes
14psi means boiling point is raise by 35deg, so 226 + 35 = 261degF boiling point, so that's a bit better for a safety margin

So why not run 18psi or 22psi rad caps, some systems do, but...........then you need better hoses and connections because of the higher pressures or you will be "blowing" things apart
And for general vehicle applications there is no need for the added expense, 260degF is a big margin over 200degF

OK, back to the hot coolant being sent over to overflow tank
Anything at the top of the radiator will be sent over to the overflow tank, so if there was any AIR at the top of rad it would be sent over to the overflow tank first
Overflow tank is NOT SEALED, it's open to the outside air, it NEEDS to be vented to work, so check that it is still clear, able to vent
So any air in the radiator is sent to overflow tank at the BOTTOM connection, and it bubbles up and is gone thru the Vent

After you shut off the engine the coolant starts to cool down and shrink in volume, so the 14psi starts to go down, 10psi, 5psi, 0psi, -1psi................
If any coolant was sent over to the overflow tank then pressure in the radiator will drop to -1psi, so negative pressure, this is when the smaller valve in the main valve is SUCKED OPEN, it then pulls in coolant from overflow tank, from the BOTTOM, so no air to keep the radiator topped up
So even if there was air in the radiator on warm up, it would be Purged and coolant would be pulled back in to replace it


As for pressure remaining in the system after FULL cool down, bad radiator cap would be first guess
But could be blocked overflow hose, or tank at the bottom, so pressure can't equalize

 

Assuming your new radiator has same MINI fill neck as the original?
Here's the 16psi Cap for that radiator, FordMotorCraft# e9sz-8100-a\rs-76.

 

Thank you both for the wealth of information. I put a new Napa radiator cap on for the moment. I've tested it once today without issue, but since it was an intermittent problem I will continue to do so as the truck gets driven. I will also order a higher quality radiator cap. If the issue continues, we will begin checking the hoses and reservoir tank.

 

I don't believe that the new radiator cap resolved the issue of pressure in the system when removing the radiator cap. I gave the truck a good drive yesterday, parked it and checked it this morning. Seemed fine at that point. My plan was to keep checking to see if the issue was resolved before my auto appointment next week, so, after leaving it parked, I also checked it again this afternoon and it spit coolant when I removed the radiator cap this time.

It was at least 70+ degrees outside when I checked it this morning and only about 82 degrees now when it spit coolant. The car was not driven or started in between the two checks so this doesn't seem normal to me. Does anyone have any idea why it might be happening? I never had this issue with any of my other cars, and I've never had to preventatively put cardboard and a drip pan down before checking the radiator "just in case". I've also been using a hand towel when removing the cap so I can try to catch most of the coolant before it drips everywhere, but here are some photos to give you a rough idea of the mess and the coolant level afterwards. Any ideas or input is very much appreciated.

 

Coolant coming out means there was still positive pressure inside the engine side of the system, well under the 14psi of a running engine but still positive pressure

There would be no common reason for that to occur
Instead of opening the cap just squeeze the upper rad hose to feel if there is still pressure in the system, if its hard to squeeze then there is positive pressure inside so don't open the cap

Pressure inside the engine side can only come from 3 things
Warm coolant, it hasn't cooled off enough to match outside tempertures
Leaking pressure from a cylinder creating a false higher pressure inside the system, i.e. blown head gasket or cracked head

External air pressure changing i.e. a storm, Barometric pressure(outside air pressure) changes with weather, Falling Barometric Pressure outside would mean pressure inside the system would change to positive pressure
The cap doesn't equalize high pressure in the engine side if its under cap rating 14psi(or 16psi) if only equalize low pressure on the engine side

You have a unique issue, I would be curious as too the reason, lol, I know not much help
But one good thing, you FOR SURE have no coolant leaks in the engine side...............glass half full


Just for "mess control" you could get a Rad cap with Ranger specs but one that also has the Pressure release lever on top
This lever would release any remaining pressure and coolant would flow to the overflow tank, NOT all over the rad and floor

 

Thank you for the input and information. The headgasket is my bigger fear, although I don't see any of the main symptoms I read about online. The truck drives and behaves as expected other than this one issue, and all dash indicators look spot on while driving. At this point I plan to keep my auto appointment next week and hope they can figure it out. Long term, I have no issue using a cap with a pressure release lever, but since I've never needed that on any other vehicle I want to make sure that I'm not masking a bigger problem with a bandaid since I have no mechanical knowledge.

Any other input is still appreciated. If I have any luck at the mechanic I'll be sure to update on what they find.

 

Not a band-aid, its a "mess preventer", that's all, doesn't change anything in the way the system works

Yes, I would be curious about their diagnosis, as well

If it is the start of a head gasket, or cracked head, issue then there is nothing "in the wide wide world of sports" you can do about it at this time except to redo the heads
Its not one of those "well good thing we caught it early" issues
So if that's what "they" say it is, it will not hurt anything to "wait and see", because if that IS what it is then it will get worse, and you will have other signs

 

Why do you need to constantly remove the cap? This is a sealed system and the only place you need to check is the overflow reservoir. If operating correctly, there will always be coolant at the radiator cap that will leak as that is the way it is designed in order to remove any air from the system. Put a new cap on it and leave it alone so it will work correctly.

 

We've always checked the fluid levels on all of our vehicles every few weeks, including removing the radiator cap to make sure there is coolant there. That was the way I was taught when I was younger. I've never been told that it was incorrect, and none of my vehicles over 20+ years have spit coolant from the radiator cap when checking them this way. Just to be clear, it's not "leaking" a bit of coolant, it's actually under pressure when cold and spitting quite a bit out.

With this vehicle in particular, it had a coolant leak when I bought it a year ago and immediately had the radiator replaced. I've been checking it periodically ever since to make sure the issue was resolved, but since it still sometimes spits coolant when checking I'm worried that something is still amiss. It was my understanding that the system should not be under pressure once it has been allowed to cool, especially after sitting for a few days.

 

You are correct, there should be no pressure in the cooling system once engine is at outside temp

Older systems were Degas setups, no overflow tank
There would be a FULL line on the radiator, so there was ALWAYS air in the radiator
The coolant could expand as it heated up into that air space, creating the same pressure but because there was air to be compressed no coolant would come out the "overflow hose" UNLESS you overfilled the radiator

On these you would check the levels every now and then because there was no external way to check the level

Some newer vehicles still use Degas system but have a Degas "Bottle", white/clear bottle, that has the air and coolant level visible, there would be a FULL line visible
This is NOT like an overflow tank, it HAS same pressure as radiator inside when engine is warmed up, so cap should NOT be removed until engine is cold
These systems often have a rad cap that has no valve its a true cap just there to seal the hole, no other function

Most with overflow tank system just check level in overflow tank to see if any coolant has been lost
But there is no reason at all you can't do that by opening rad cap


And just a thought...............open Overflow tank cap FIRST, then open rad cap, wondering if pressure was in the overflow tank, not radiator at all
Overflow tank should be vented 0psi all the time

 

Thanks. I will check the coolant tomorrow and will be sure to open the overflow tank first. It's just the flip top cap on the overflow tank, as shown in my first post on this thread. Not sure if that matters but I'll report back. And I do still have the drop off on Wednesday as well to have them fully check everything over.