I too am loosing coolant. I cannot figure it out.......this has been a battle with this truck for the last 3 years or 100k, I am currently at 350k. I have replaced the radiator, water pump, all hoses & fittings, elliminated the rear heater and cooling lines to rear, and best of all replaced the heater when that went sour on me. And new intake gaskets as well. Each of these areas have been areas with small leaks, corrosion, dry rot, ext....replaced for wear not just because. But it still loses coolant, and best of all I have no heat anymore either, from breaking the heat control box valves inside the dash.
My best guess is it must be a head gasket where I lose a small amount of coolant over time, but not bad enough to show any "real problems" of smoke, lack of power ext?

 

I ended up disconnecting the rear heater lines entirely because the pipes back by the heater core were rusted almost completely through. Im not really sure why it hadn't ruptured before then.

 

The coolant bypass tube under the lower intake manifold may be leaking and the heat of the block burns it off.

 

Have tried a stop leak product yet. Maybe your radiator as a pin hole leak. Have checked the oil cooler lines over by the oil filter?

 

I also have similar symptoms on my 3.4L as OP. Slowly using coolant without any obvious leaks. I have used a combustion leak tester/radiator sniffer (free loaner at Autozone), to eliminate the head gasket. I suspect a leak but not seeing anything obvious. Kinda hoping to hear what the solution is here to give me another clue.

 

See my post a few above....coolant bypass tube possibly.
Heater core ok?

 

I left it running and used a strong flashlight to shine on all the likely spots top & bottom. No drips or wet spots, and my carpets aren't wet. The rear heater hoses look dry. That lower bypass hose looks dry. That still leaves several hidden spots including inside the heated part of the throttle body.

One difficulty with the sniffer is you can't pull much air thru it unless you actually have a leak, although it is pretty sensitive. I figure the few bubbles I did see over a 10 minute test would have been enough to make the fluid change color, since when I held it near the tailpipe it only took a few seconds.

1 drop per second is around a gallon a day, which I'd think would be noticeable no matter where it leaked. I'll have to check to see how much I lose each day.

 

Update:
I began tracking the water loss. In the beginning, I was losing maybe half the reservoir amount in one week of commuting. As time went by I seemed to be losing less. Now a week has gone by and I don't seem to have lost any. The only thing I can think of that has changed over that time is the weather got warmer and I used the heater less and less.

I guess I need to leave the heater on and examine those rear heater lines more closely.

 

certainly check your rear heater lines. They can pump out your coolant without you ever knowing it. I simply disconnected mine by bypassing the rear heater core in the engine compartment .

 

Solved. I found that my radiator cap had cracked and was not holding pressure. Once I replaced that, it was obvious my 2yr old radiator had a pinhole leak up near the top. Once I replaced that, my lower radiator hose started dripping. Once I replaced the hoses, that seems to have taken care of the leak. It took awhile but I am relieved it was not head gaskets.

 

Excellent.



Andreas