Truck: 2000 F150 w/ 4.2 V6
149,500 trouble free miles (except for having the radiator flushed out)
About a month or so back I noticed my coolant level had dropped about an inch or so in the defoaming tank, or overflow tank, whatever you want to call it.
I have never had to add coolant before.
The early runs of the 4.2 had problems with premature gasket failure but this was corrected in, I believe, Jan of '98, so I am way past that issue with my truck. I know of no abnormally high failure rates of gaskets in 2000 or later model year 4.2.
So here is some more info..
The truck has been very well maintained.
I seem to drop about a pint of coolant every week or so.
I never have any problems starting, NO indication of coolant getting into a cylinder while the truck is sitting idle (not running).
No miss, stumble, rough idle, nothing. The engine runs as smoothly as it did when new.
I have looked high and low, but no sign of a leak.
No indications of burning coolant - no steam, smoke, smell - nothing.
My coolant shows no oily film. No sign of oil contamination.
My oil level never rises and no sign of ANY contamination by coolant.
No coolant leak or smell in the cab so I don't suspect the heater core.

It's just like the coolant is disappearing.

I plan to pull the plugs and check them when I get some time.
I plan to get a UOA done next oil change to check for glycol / water contamination.

I have had a head gasket fail in a boat that I once owned w/ a 351. It was obvious, with all the expected symptoms.

I have NO symptoms of ANY kind.
I suspect I could have a very minor head gasket leak that only shows up when the engine is hot and coolant pressure builds. That is the only thing I can think of.

Given that, what are your thoughts???
Would it be worth trying one of those high-end gasket-fix-in-a-bottle products? I have never used one, but if I do have a very minor leak, maybe that would solve the problem.

Maybe a leak down test of each cylinder, or pressure test of the coolant system is in order.

Thanks in advance for all input!

 

try pressure testing the system , with that milage it could possibly be the heater core leaking . you'l know from the test

 

I've never dealt with a problem like this before, but the tests you suggested sound like a good start.

I have one thing that came to mind, however. You suggested you think it happens when the engine is hot and coolant pressure builds up. Try letting the truck idle hot for about an hour. If it's really leaking, running for an hour should produce some kind of drip, and maybe you could track it down from there.

Good Luck!

 

Sounds like getting some dye would be a good idea.

This rig wouldn't happen to have a frost plug?

 

Hey Clint,
Thanks for the response. Yep, I believe all the 4.2 motors have freeze plugs.
I have really looked the engine over and can't find any sign of a leak. I park in places where any drip could be easily noticed.
I'm stumped.....

 

My old Ranger had a small split in the plastic radiator. It lost about a pint every week or two as well. When it eventually started leaking more I finally figured out the coolant dripped down the side of the radiator and into a channel in the frame. It was almost impossible to see the water stains even when using a flashlight because of where the leak was. Never was able to see it actually leak nor find any puddles on the ground and I parked in the same spot at home and at work.

The dye or a pressure test should expose a leak.

I drove about 60,000 miles(2 years) before I got the radiator replaced and even then it was more for peace of mind- I didn't want to be hundreds of miles from home if/when the pin hole leak expanded and all the coolant leaked out.

Tim

 

The advantage of using a pressure tester is being able to look, listen and feel for leaks without heating up the engine. I'd have to agree with captchaz and others who recommended pressure testing the system first. Like the poster above me, I have a tiny occassional leak in the radiator on my Ranger. Seems to leak at a fitting where the tranny line screws into the side of the radiator about 2" below the filler cap. It's done this for almost 2 years now and it's never leaked enough to measure. The only way I found it was the dried antifreeze stains on the side of the radiator.

 

Well brian, an awful lot of good advice has been posted so i'm not goin to repeat it , one thing though is your question about adding a stop leak additive, do not use them, it will not fix the problem only delay finding the cause, sometimes with bad results. let us know the results of the dye and compression tests,,,,,

 

My brother has a 4.2 that had an unexplained coolant loss. We suspected the intake gasket, so we started to tear into it when i noticed something on the upper intake. One of the runners at the rear was washed clean and had coolant in it. At the top of that intake runner (rear right of upper intake) is a heated pcv passage. We bypassed the whatever its called and havent had a problem since. I'm not 100% sure that was the problem, but its been ok so far.

 

if the engine isn't leaking, then I would suspect that the radiator cap isn't holding pressure. I had a problem like this in a 95 4.6 tbird not holding pressure, a new cap fixed that.

You can tell if the cap is holding pressure by opening it right after you shut the motor off, you should hear the pressure bleed off.

But then again, i don't know how the coolant setup is on a 4.2 motor, but thats what i would do if it were a v8.

 

Update:
I found the problem.
Eventually enough dried green crust built up so that I could see the leak.
The hose attached to the thermostat housing - very slight leak that appears only to occur when the engine is hot and running.
Never even enough to drip. Just "oozes" a little and dries in-place.
I clean it all off and tighten the clamp good.

 

Get a new hose. It probably leaks because the hose is going bad. The inside often separates from the webbing, letting the coolant seep out through the webbing. Tightening the clamp will only be a patch.

Glad to hear it is a minor issue.

George

 

I agree, considering how cheap hoses are, I think it's worth the few bucks to know your truck won't die and leave you stranded.

But, whatever you choose, it's good to hear you found the problem!

 

Well, I was wrong - at least a little wrong. The hose was leaking at the thermostat housing. (The hose from heater core). That has not leaked since I addressed that problem. But, I am still loosing coolant.
This is very puzzling.
The truck shows NO sign of a leaking head gasket or intake manifold gasket. This has been going on for two months and I never have any problems cranking the truck - never an indication that a cylinder is admitting coolant while not running.
No steam or smoke in exhaust. No idle problems. No miss, stumble, hesitation,etc.
No O2 sensor problems or any OBD codes.
Nothing - except that I lose about a quart of coolant every week or so.
I now have 153,300 miles and the truck runs good enough that I can't believe something is wrong internaly. I keep looking for a leak, convinced that has to be the answer, but I find no leak.

help....

 

You wouldn't by chance have a Auto tranny? You "could" have an internal leak that "may" be leaking into the tranny fluid. High levels of tranny fluid would indicate this.

Sounds like it's time to get it hooked up to a pressure tester, or time to try some dye.