Coolant overflow and loss - (bizarre?) observations

[spoolin01]

For a few months, 2003 FX4 4.0L manual,160K mi Ranger has been losing coolant. The reservoir can lose up to a quart after a hundred or two miles, and sometimes the radiator is a cup or two low as well. Have been topping with plain water. Coolant was drained and refilled with 50:50 a year ago January, now it's 4000 miles later, and the coolant has barely any color. When refilled, I didn't do anything about bleeding the system. About a month ago, the rock-steady temp gauge started reading way low, and the ScanGauge showed 160 - 170F. Previously always around 190-195, so the thermostat was replaced, without draining the coolant, and that problem was solved. Doubt these two are related, but that's the info.

Have spent some fair time looking around above and below with good lighting for leaks or signs of past leaks, but nothing. Nothing on the cardboard underneath while parked. Oil is not milky, tailpipe doesn't steam except when starting on cold days. Two days ago after a 100 mile trip, for the first time, the reservoir was now maybe a quart *higher* than when last checked (I don't drive this at the moment, but driver says this was not added water), and the radiator was about a pint low, also an extreme.

Got a pressure kit from Autozone, pumped to 16 psi (what the cap says), and pressure dropped about 1 psi every few minutes, at least at first. Pumped to 20 psi and after 10 minutes or so it was down to 15. Looked underneath and there were drops off the transverse frame member, between center line and the passenger wheel. Maybe a drop or two every minute, not a lot. From underneath, it looked like it might be running down the engine side of the alternator, which sits up tight against the front of the block, next to the thermostat housing. From up top, there was a bit of dampness where the thermostat housing sits on top of the water pump, but again not a lot.

Took off the pressure kit and the radiator was another near pint low. There was not nearly a pint on the ground - just a damp patch about a foot in diameter after a half hour of pressure on the radiator. Was concerned if it were possible to have pushed water through a head gasket leak into a cylinder, so I just touched the starter a couple of times first, but no problem then when I started it. Back at the tailpipe, after warming up there was a couple cc's of water at the end of the pipe, but little dripping, no steam even at first start.

Assuming the report is believable that the reservoir was only half full at the start of the trip and 3 inches higher at the end, while the radiator was a pint low, what could explain that? How about the pressure test forcing the radiator level down a pint, when there's not nearly that much on the ground? The radiator cap is probably original (I've owned it since 17K miles), but the seal looks OK to my amateur eyes. After 4000 miles, could there still be voids in the system from when it was drained and refilled? Would a compression test be the next thing to do (prior to possibly changing the thermostat housing)? This one exceeds my meager understanding.

 

[2011Supercab]

First thing I would try is a new radiator cap.

A bad cap could let air back into the system when the engine cools down instead of sucking coolant back from the reservoir

 

[sgtsandman]

There is also the possibility that there is some pin hole leaks in the radiator or one of the tank seals is failing. If you have old hoses, look at the ends of the hoses for coolant weeping out. Hoses can sometimes crack internally and weep coolant out through the cords that reenforce the hoses.

Depending on what engine you have, the short hoses between the water pump and other parts of the engine can have hidden leaks as well.

 

[spoolin01]

Will look again when I see the truck next, and try a new cap. Probably do a compression test too. I'm inclined to just manage it until it becomes clear where the problem is.