I recently made a 3,000 mile road trip in our 1996 3.9L five-speed without a problem on the highway (now has 299,000 miles and purchased new by my father). In recent weeks, I have added almost a gallon of coolant/water two different times in the radiator and overflow bottle. I borrowed a coolant system pressure tester from AutoZone and it seemed to hold pressure at 16 psi. I have never seen a leak anywhere either. Where is this liquid going? I have not noticed any decrease in engine performance or drive-ability either. This is my daily driver, so I need to fix this problem before it balloons into something major. Thank you.

 

There is a bypass hose under the a/c compressor, that likes to rot out, and leak, but, the coolant pools on top of the timing cover, and boils off, before you see much of a drip on the ground. Though, one would think, the pressure tester "Should" catch that one.... Another option is a failing head gasket..... You can get a test kit for that at your local parts store......

 

I also agree with a failed head gasket, if in correct place will simply leak into the cylinder and get burned with the fuel .... you might see some white smoke (steam) from the exhaust.
Also possible you might pull the spark plugs, and if you find one that looks brand new ..... is getting steam cleaned from the coolant.

I bought a used Toyota years ago that had this issue .... It was obvious the car was towed behind a motor home and just used as a get around car when they got to location.

It ran great when I bought it, no idea a head gasket was leaking .... I was just going to change the plugs because it was a used car new to me.
From the conditions, the plug was fused in the head so hard it broke trying to remove it ..... The fix was removing the head and drilling it out and inserting a helicoil.

I'm just suggesting if you check the plugs .... you find one that is being very difficult .... you might want to skip it until you are ready to pull the heads and fix it.
With 300K on the engine you might want to consider a replacement new or used engine.

 

Update: Pulled the spark plugs. The most difficult to unthread was from cylinder number 5 (the one at an angle in the photo). That same plug had what appeared to have just a hint of moisture/wetness on the porcelain under the tip, but hardly noticeable. Could be just my imagination playing tricks on me. The porcelain of the two center plugs (#3 and #4) was whiter in color than the four corner plugs. In conclusion, we still don't know anything new or definitive. I'm still hesitant to drive anywhere.

 

That bottom middle plug looks like it was running lean.....porcelain looks awful clean there....

 

Update:I noticed the lean plug tool

I did a compression test on cylinder #5 (135) and #3 (115). Then I replaced the plugs and ran the motor for about 3 minutes to check for smoke or vapor at the tail pipe. Did not notice anything at that end. After I shut off the ignition, I went out to shut the hood. However, I heard some kind of noise like hissing or sizzling. After about 60 seconds the hissing or whatever it was stopped.
So I started the motor a second time, ran it for only about a minute, and then shut it off. I heard the same sound again, and again after about 60 seconds it went away. i could not determine where it was coming from. I put my ear near the overflow tank where I could still hear the noise, but when I looked inside the tank I did not see any gurgling or bubbles or any "activity" inside at all. The sun began to burn me up, so I quit and came inside for a soda. I guess this might be considered progress, if that obvious noise leads us to the problem eventually.

 

Coco brown plugs means you have a really nice clean burning engine. Looks like those plugs have quite a few miles on them .... due to be replaced.

The middle plug in the bottom row is far to clean, a brilliant white .... that is not normal. I'm afraid you do have a head gasket leaking. Does explain where your coolant is going.

Now the question of what to do about it?
The engine looks to be running really well .... if you can change the head gaskets yourself, worth the cost of materials to do it.

If you need to take it to a shop and pay labor to have it done, will be a large labor bill .... And with 277K miles on the engine ??? Hard to say.

Swapping in a rebuilt long block would cost the price of the rebuilt engine, plus the labor to install it and would be good for another 200-300K miles.

You're going to pay about the same in labor to swap head gaskets as swapping the engine .... you will be left with a engine that still has almost 300K on it and near the end of it's life cycle.

 

The hissing is likely just vacuum from the reservoir bleeding off..... Maybe one of the check valves isn't doing its job properly. I really don't think it is anything to worry about.

Think I would be testing the coolant for combustion byproducts though.......

 

The blown or soon to blow head gasket comments are looking more and more like what we will end up finding in the long run. That white plug does have lower compression than the adjacent cylinder, which would indicate a problem there.

 

Get underneath and look at the bottom of the water pump. Is there a green stain around the weep hole on the bottom? I'd also grab the fan and try to rock it side to side. If there is any staining, it's wet underneath, or the fan will rock the shaft side to side, you may just need a water pump. Could your head gasket be blown? Yes. Is it blown? Check simpler (and cheaper) things first.