Coolant Leak on the Overflow/Fill Tank - ’92 Ranger

Hey, all.

I have a bunch of small coolant leaks on the ’92 Ranger (4.0, A4LD) that I picked up a month ago. I plan to take pics for the more detailed ones later, but to start with, I noticed that the bottom of the cap on the coolant overflow/fill reservoir is soaking wet with coolant after a drive. I read somewhere that it’s not a pressurized reservoir, but shouldn’t there still be some kind of an o-ring or something on the cap where it presses down onto the tank neck? If not and if I’m getting this kind of leak, does it mean that there’s excessive pressure in the system somewhere? My heat works great by the way, so the core shouldn’t be plugged.

Also, is there a safe flushing agent out there for the coolant system? I’m in the process of changing every fluid in the truck and am just wondering if there’s a good product out there, or if I’ll just be using gallons of straight distilled water like I usually do to flush things (I use this instead of tap water, since it contains less minerals). Thanks!

Overflow tank on this type of system needs to breath.

As coolant warns up in the engine and radiator it expands in volume
This is what creates the pressure in the cooling system
If rad cap rating is 14lbs pressure then at 15lbs it is push open and hot coolant flows out the radiator to overflow tank and comes in at the BOTTOM of the tank, bottom is important.

When engine is shut off and cools down coolant shrinks in volume, so the 14psi pressure goes down, if hot coolant was sent over to overflow tank then pressure in the cooling system will drop below 0psi, and Second smaller valve in rad cap will be pulled open and coolant in overflow tank will be sucked back in to radiator.
This keeps radiator topped up to the very top.

If you pull off rad cap on cold engine and coolant is not at the very very top then there is a problem in this system

The radiator cap is at a high point in the cooling system, as coolant circulates any air will make its way to the top of the radiator and stay there.
So when coolant expands enough to push open rad cap the AIR will be the first thing pushed out into the overflow tank
The hose from rad to overflow tank must come in at the bottom of the tank so any air will bubble up to the top of the tank.
Then as engine/coolant cools down coolant will be sucked back in from bottom of tank not air, this makes the system self purging of any air in radiator, and why tank needs to be vented, to release the air.

If you have cooling system leaks then places where coolant comes out can also SUCK AIR IN as engine cools down.
It is much easier to pull air in from the outside than to suck coolant back from the overflow tank, coolant is heavy, air is light no contest, lol, air will be sucked in at 0psi.

If this is the case then there will be air in the cooling system every time you start the engine, and when coolant heats up and rad cap opens overflow tank would be bubbling like crazy as it purged the air first.
So top of overflow tank would get splashed with coolant every time.
If you fix engine/radiator coolant leaks the splashing will stop

In normal operation coolant flows out to overflow tank and then is pulled back in, no splashing in the tank, so dry cap.

Purging or bubbling of air in the overflow tank would only happen after engine was fully warmed up, so takes a good 10 minutes +
If you see bubbling within a minute or two after cold start then you have a blown head gasket, or cracked head

Sometimes vehicles will bubble after shut down, this means there is air in the engine, obviously, and that there are pockets of air in the heads, when you shut off the engine you also shut off the water pump, the heads are still hot, if there is air in the heads then it will flash to steam against hot head metal, this bumps up pressure in the system above the 14psi and pushes out the other air at the top of the radiator

@RonD: What an incredibly solid and detailed explanation! Thank you so much!!!

Truck runs like a champ, has great power, and no oil in the coolant (or vice-versa), so I really don’t think there are any issues with the heads or gaskets. I’ll take some pics of the known leaks in the system and do a little research. Nice to know that when I fix them, it may solve the problem of the wet tank top. Much appreciated!!

Thank you, and your welcome

You can rent a cooling system pressure tester for $20-$25, check auto parts store and tool rental places
It is basically a radiator cap with a hand pump and gauge attached
You fill up radiator on cold engine
Put on pressure cap and pump system up to 20psi
Now look for leaks and pressure to drop

It is way way easier to find leaks on a COLD engine, hot coolant doesn't evaporate on hot parts so you can usually trace it back to the source, and see it dripping on the ground

I’ll definitely plan on doing that, RonD. Now that you’ve explained things so well, I think I understand why the temperature needle barely moves above “COLD,” even though I have great heat (possibly air preventing hot coolant from properly reaching the temp sending unit sensor).

I do know of one leak (possibly two) right off the bat. One is at this doohickey:

https://drive.google.com/file/d/1mXf_BiG6cRs1Z3v-ZxaBbOn70lxfnrBq/view?usp=sharing

Not sure what it is, but it’s on the line coming off of the radiator and running past the fuel pump. It has compression fittings which are leaking, but I have no idea how to replace these.

There is also green fluid coming from here:

https://drive.google.com/file/d/18-AkRGC12Py6TODiqfivkW1lg5EEZZu3/view?usp=sharing

Weird thing is, this appears to be part of the AC system. Any idea why coolant would be coming out? I’m not terribly knowledgeable about AC, since only my last couple of vehicles have had it.

Any help is very much appreciated, folks!

Both of those parts pictured are part of the AC system, and neither of them go anywhere near the fuel pump.

The first one is just a line muffler, and that is a power steering pump it is near.

The green stuff is AC dye.

Those are not compression fittings. This is a compression fitting:
https://www.plumbingsupply.com/images/compression-brass-male-to-female.jpg

Those are garter spring lock couplers, they are common on automotive AC and fuel lines.

adsm08 said:

Both of those parts pictured are part of the AC system, and neither of them go anywhere near the fuel pump.

The first one is just a line muffler, and that is a power steering pump it is near.

The green stuff is AC dye.

Those are not compression fittings. This is a compression fitting:
https://www.plumbingsupply.com/images/compression-brass-male-to-female.jpg

Those are garter spring lock couplers, they are common on automotive AC and fuel lines.

Click to expand...

My typo. I meant to write “PS pump,” not fuel pump. Compression fitting was also a mistake. Just the first word that came to mind (long day). Any idea how one goes about removing/replacing spring lock couplers? Is a line muffler even necessary? Sounds like I’m going to end up having to have the AC system recharged, since at least two seals are leaking. The AC works currently, so I guess I’ll just wait until it stops working.

However, I am having a weird idle problem where the engine races, then drops off over and over again. Every time it races, the AC pump also kicks on briefly. I’m currently looking at the AIC unit, but I’m wondering if having leaks in the AC system could possibly be causing the truck to call for the AC pump to turn on, thus resulting in the idle kicking up in order to handle the upcoming power draw by the AC pump? I know it probably doesn’t work that way; just spitballing here...

If you have Defrost selected on climate control then AC will cycle on and off to "dry" the air blowing onto the windshield.

When AC compressor comes on the computer should bump up the idle because of the load, that is normal.

The IAC(idle air control) Valve is what the computer uses to set idle levels
Cold engine idle is 1,100rpms at least
After 5 to 8 minutes of warm up idle should have dropped to 650 for manual trans, 750 for automatics

ECT(engine coolant temp) sensor is used by the computer to set idle based on engine temp, this is a TWO WIRE sensor, on the 4.0l it is just above the thermostat housing on lower intake.

There is also an ECT SENDER, next to the ECT sensor, the sender has 1 wire in a '92 Ranger, it is only used for the temp gauge in the dash.
These are not interchangeable.

If your dash temp gauge is reading low then ECT SENSOR might also be reading low, and this would cause computer to set a higher idle.

ECT sensor has a cone shaped tip
ECT sender a cylinder shaped tip
If you ask an auto parts guy for a SENDER, he will always bring you a sensor the first time, lol, they are not the same thing, just FYI


An AC system has two pressure switches, low pressure and high pressure switches.
The power to turn on the AC compressor runs thru both switches to get to the compressor.

12volt---------------low pressure switch-------------high pressure switch-------------------compressor

So compressor will not get power, will not turn on, if AC system pressure is too low(fluid is low) or too high(passages are blocked)

@RonD:

Thanks so much for your helpful insights into this issue. Went out to test a few of your theories before running errands yesterday. Moved the climate controls from defroster to floor and disconnected the power to the AC compressor pump (breaking the plastic retaining clip in the process, of course). Started her up and she idled right at 800-900 RPMs, just like she should (instead of the 1300-1700 that she had been). Plugged the AC back in and everything stayed fine, with the clutch no longer cycling on and off like before. But by the time I reach the first stop sign a few miles away, she was back to idling at 17K.

While I was in the parking lot at the laundromat, I disconnected the IAC wires and she went back down to 800-900. With the IAC disabled, I drove the truck home and noticed several improvements. She shifted a little sooner (a good thing) and much smoother. The problem I’ve been having with her kicking down through several gears at the last minute (10-20 feet) when I pulled up to a stop sign was also taken care of. I thought, “Great! I’ll just pick up a new IAC and at least this problem will be solved!” Then I pulled into the driveway and the idle started out at 15K, before slowly dropping down to 13K and bouncing between 11K and 13K before shutting her down. Today, she ran just great to and from work, with idle varying between 750 and 1100 RPMs (not bouncing, just idling at different speeds at different stops). Maybe not ideal, but it’s running a thousand times better today than it did before I started looking into the IAC.

So I’m thinking that maybe the IAC is bad, but I have an additional vacuum leak somewhere or an issue with the ECT. Or maybe it just took a day for the computer to adjust to not having the IAC hooked up. I went ahead and ordered this Hitachi IAC from Advance: https://shop.advanceautoparts.com/p/...Air+Controller

I know it’s not a Motorcraft, but it’s about half the price and I just have so many other parts to buy for this truck. Anyone bought one before? Again, thanks so much, RonD.

Oddly, the coolant leaks seem to have slowed to a trickle. Haven’t topped her off in about a month and she was only down a couple of cups today. Thinking about changing out the coolant soon and I see that Advance has a flushing agent made by Blue Devil. The guys at the store had no feedback to offer on it, so does anyone here have an opinion? I’ve read a few horror stories here about folks that did a thorough flush, only to have problems arise (water pumps leaking afterward, etc).

The truck has awesome heat, so I don’t think there are any plugging issues. Just old coolant that’s starting to look a little crappy.

Any time you flush (clean out) a system and it breaks the dirt is what was holding it together, so it was going to break soon anyway.